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[Alexei A. Belik](https://orcid.org/0000-0001-9031-2355)

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[The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System](https://mdr.nims.go.jp/datasets/ca016c92-1355-440a-b332-d810166f8122)

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The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 SystemAcademic Editors: Jianhua Han,Yuyu Su and Xuanling LiuReceived: 25 February 2025Revised: 12 March 2025Accepted: 14 March 2025Published: 18 March 2025Citation: Belik, A.A. The ConversionPolymorphism of Perovskite Phases inthe BiCrO3–BiFeO3 System. Inorganics2025, 13, 91. https://doi.org/10.3390/inorganics13030091Copyright: © 2025 by the author.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ArticleThe Conversion Polymorphism of Perovskite Phases in theBiCrO3–BiFeO3 SystemAlexei A. BelikResearch Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan; alexei.belik@nims.go.jpAbstract: Perovskite-type materials containing Bi3+ cations at A sites are interesting from theviewpoints of applications and fundamental science as the lone pair of Bi3+ cations often sta-bilizes polar, ferroelectric structures. This can be illustrated by a lot of discoveries of differentnew functionalities in bulk and thin films of BiFeO3 and its derivatives. In this work, weinvestigated solid solutions of BiCr1−xFexO3 with 0.1 ≤ x ≤ 0.4 prepared by a high-pressure(HP) method and post-synthesis annealing at ambient pressure (AP). HP-BiCr1−xFexO3 modi-fications with 0.1 ≤ x ≤ 0.3 were mixtures of two phases with space groups C2/c and Pbam,and the amount of the C2/c phase decreased with increasing x. The amount of the C2/cphase was also significantly decreased in AP-BiCr1−xFexO3 modifications, and the C2/c phasealmost disappeared in AP-BiCr1−xFexO3 with 0.2 ≤ x ≤ 0.3. Fundamental, strong reflectionsof HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 were almost unchanged; on the other hand, weaksuperstructure reflections were different and showed clear signs of strong anisotropic broaden-ing and incommensurate positions. These structural features prevented us from determiningtheir room-temperature structures. On the other hand, HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3showed high-temperature structural phase transitions to the GdFeO3-type Pnma modifica-tion at Tsrt = 450 K (x = 0.1), Tsrt = 480 K (x = 0.2), Tsrt = 510 K (x = 0.3), and Tsrt = 546 K(x = 0.4). Crystal structures of the GdFeO3-type Pnma modifications of all the samples were inves-tigated by synchrotron powder X-ray diffraction. Magnetic properties of HP-BiCr1−xFexO3 andAP-BiCr1−xFexO3 were quite close to each other (HP vs. AP), and the x = 0.2 samples demon-strated negative magnetization phenomena without signs of the exchange bias effect.Keywords: multiferroics; doped BiFeO3; BiCrO3; high-pressure synthesis; irreversibletransformations; conversion polymorphism; incommensurate structures1. IntroductionBi3+ cations have a lone electron pair similar to Pb2+ cations and, therefore, are oftenconsidered as a replacement of toxic lead in ferroelectric and piezoelectric materials. BiFeO3perovskite has been known since the late 1950s [1–4]. However, it has received tremendousinterest in the 2000s [5,6] after the discovery that thin films of BiFeO3 can have goodferroelectric properties and are effective and model multiferroic materials [7–9]. Since then,thousands of publications have been devoted to BiFeO3 perovskite and its derivatives [6]. Alot of fundamental science and practical discoveries have recently been made using BiFeO3perovskite [10–14], for example, related to manipulations of charge domain walls, chiralspin transport, and spin cycloids, the formation of skyrmion lattices, and so on.BiFeO3 crystallizes in space group R3c below its ferroelectric transition temperatureTFE = 1100 K, and it shows a number of high-temperature structural transitions [6]. Itsfirst transition at TFE is to a GdFeO3-type Pnma phase [15], not to a centrosymmetric pairInorganics 2025, 13, 91 https://doi.org/10.3390/inorganics13030091https://doi.org/10.3390/inorganics13030091https://doi.org/10.3390/inorganics13030091https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/inorganicshttps://www.mdpi.comhttps://orcid.org/0000-0001-9031-2355https://doi.org/10.3390/inorganics13030091https://www.mdpi.com/article/10.3390/inorganics13030091?type=check_update&version=1Inorganics 2025, 13, 91 2 of 20(space group R-3c) of space group R3c as one would expect. The ferroelectricity of BiFeO3originates from the activity of the lone electron pair of Bi3+ cations [6]. The pure antifer-romagnetic (AFM) transition occurs at the Néel temperature TN = 643 K and originatesfrom superexchange interactions between magnetic Fe3+ cations [6]. The absence of anynet ferromagnetic (FM) moments in BiFeO3 originates from long-period incommensuratespin ordering, which averages the total net moment to zero [6]. BiFeO3 perovskite is theonly compound among other simple BiMO3 perovskites that can be prepared in bulk formunder ambient pressure (AP) conditions.BiCrO3 perovskite has been known since the middle of the 1960s [16–18], but itreceived renewed interest in the 2000s [19–31] as a counterpart to BiFeO3. It crystallizes inspace group C2/c at room temperature (RT) and shows a structural phase transition to aGdFeO3-type Pnma modification above Tstr = 420 K [23,24]. BiCrO3 demonstrates an AFMtransition at TN = 112 K to a G-type AFM structure with small spin canting (and, therefore,with the appearance of weak FM properties) [23–26,30,31], and there is a spin-reorientationtransition near 72–75 K with an increased weak FM moment. BiCrO3 perovskite can onlybe stabilized in bulk form under high-pressure (HP) synthesis conditions [32].Other simple BiMO3 perovskites have been studied less because they need an HPhigh-temperature method for their preparation in bulk form [32]. It is interesting thatthere are many structural variations in simple BiMO3 perovskites [27,32]: samples withM = Sc, Cr, and Mn crystallize in space group C2/c; M = Al and Fe samples—in polar spacegroup R3c; the M = Co sample—in polar space group P4mm; the M = Ni sample—in spacegroup P-1; the M = Ga sample—in a pyroxene-type (non-perovskite) Pcca structure; and theM = Rh sample—in space group Pnma [32]. Therefore, different BiM11−xM2xO3 solidsolutions can show complex structural variations and transformations as a function ofcomposition x and temperature. Such solid solutions have received a tremendous amountof interest in the literature [33–51]. For example, the appearance of polar R3c phases wasobserved in the solid solution system between centrosymmetric BiCrO3 and BiGaO3 [47,50].In the solid solutions between BiFeO3 and BiCrO3, one composition has been investigated alot, namely Bi2FeCrO6, after the first-principle predictions [52] that this composition (withordered arrangements of Fe3+ and Cr3+ cations) should show large ferroelectric polarizationand large magnetization [53–58]. In particular, thin films of Bi2FeCrO6 have attracted alot of attention from the application point of view as photovoltaic materials [57,58]. Thenumber of investigations focusing on properties of bulk BiCr1−xFexO3 solid solutions isvery limited because such solid solutions need an HP method for their preparation [59–64],and the majority of such studies are focused on studies of Fe-rich samples [60–62].Therefore, in this work we investigated samples from the Cr-rich side of bulkBiCr1−xFexO3 solid solutions with 0.1 ≤ x ≤ 0.4. We found structural phase transitions tothe Pnma modification with systematic changes in the phase transition temperature. Wefound that the as-synthesized samples do not transform back into their initial state aftercooling from high temperatures, therefore realizing the conversion polymorphism phe-nomenon [41]. Weak incommensurate structural modulations were found in BiCr1−xFexO3solid solutions with 0.1 ≤ x ≤ 0.4. Detailed magnetic properties are reported.2. Results and DiscussionAll BiCr1−xFexO3 samples contained small amounts of Bi2O2CO3 impurity. Thex = 0.1 and 0.2 samples additionally contained very small amounts of Cr2O3 impurity; theamount of Cr2O3 impurity was at the background level in the x = 0.3 and 0.4 samples.HP-BiCr1−xFexO3 samples with x = 0.1, 0.2, and 0.3 consisted of two perovskite-typemodifications: one modification had the C2/c symmetry as found in undoped BiCrO3 [24],and the second modification had the PbZrO3-type structure (space group Pbam (No. 55)Inorganics 2025, 13, 91 3 of 20and a√2ap × 2√2ap × 2ap superstructure, where ap (≈3.95 Å) is the parameter of the cubicperovskite subcell [62,65,66]). The amount of the BiCrO3-type C2/c modification rapidlydecreased with increasing x from about 75% (x = 0.1) to about 50% (x = 0.2) and about5% (x = 0.3) in the HP samples (Figures 1 and 2). The amount of the BiCrO3-type C2/cmodification also rapidly decreased with increasing x from about 40% (x = 0.1) to traceamounts (x = 0.2) to undetectable amounts (x = 0.3) in the AP samples (Figures 1 and 2).Both HP- and AP-BiCr0.6Fe0.4O3 contained only a perovskite phase with the PbZrO3-typestructure (Figure 2b).Inorganics 2025, 13, x FOR PEER REVIEW 4 of 20   Rwp (%) 5.48 7.18 6.39 7.09 Rp (%) 4.07 5.28 4.81 5.18 RI (%) 3.18 4.17 4.26 4.16 RF (%) 2.51 3.81 4.35 4.56 Impurities:     Bi2O2CO3 1.1 wt. % 0.8 wt. % 1.6 wt. % 1.6 wt. % Cr2O3 0.6 wt. % 0.9 wt. % – – Crystal system: orthorhombic. Space group: Pnma (No. 62); Z = 4. Source: synchrotron powder X-ray diffraction (λ = 0.420138 Å). d-space range used in the refinements: 0.497–11.563 Å. Fractional coordinates: Bi: 4c (x, 0.25, z), Cr/Fe: 4b (0, 0, 0.5), O1: 4c (x, 0.25, z), and O2: 8d (x, y, z). Occupation factors, g, of the Bi and O sites are 1. The occupation factor of the Cr/Fe site is mixed based on the nominal compositions.  Figure 1. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.9Fe0.1O3 and (b) BiCr0.8Fe0.2O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 550 K—by blue lines, and the AP modifica-tions at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (C2/c, Pnma, and Pbam) and Bi2O2CO3 and Cr2O3 impurities. The characteristic 3.4 4.4 5.4 6.4 7.4 8.4 9.4297 K, HP550 K, Pnma297 K, after 550 K, AP3.4 4.4 5.4 6.4 7.4 8.4 9.4297 K, HP550 K, Pnma297 K, after 550 K, AP2θ  (deg): λ = 0.420138 Å Intensity (ar. un.) Bi2O2CO3 Cr2O3 C2/c ****# # # Intensity (ar. un.) # # # Bi2O2CO3 Cr2O3 Pbam # Pnma Pnma (b) BiCr0.8Fe0.2O3 (a) BiCr0.9Fe0.1O3 # # * ***Figure 1. Magnified fragments of experimental synchrotron powder X-ray diffraction patternsof (a) BiCr0.9Fe0.1O3 and (b) BiCr0.8Fe0.2O3. Patterns for the as-synthesized HP modifications atT = 297 K are shown by black lines, for the Pnma modifications at T = 550 K—by blue lines, and theAP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflection positionsfor the main perovskite phases (C2/c, Pnma, and Pbam) and Bi2O2CO3 and Cr2O3 impurities. Thecharacteristic reflections of the C2/c modification are additionally marked by black/red octothorps.The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars.Inorganics 2025, 13, 91 4 of 20Inorganics 2025, 13, x FOR PEER REVIEW 5 of 20   reflections of the C2/c modification are additionally marked by black/red octothorps. The character-istic reflections of the Bi2O2CO3 impurity are additionally marked by green stars.  Figure 2. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.7Fe0.3O3 and (b) BiCr0.6Fe0.4O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 600 K—by blue lines, and the AP modifica-tions at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (Pnma and Pbam) and Bi2O2CO3 impurity. The characteristic reflections of the C2/c modification are marked by black octothorps. The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars. (hkl) indices of some superstructure reflections of the Pbam-related modification are given. 3.4 4.4 5.4 6.4 7.4 8.4 9.4297 K, HP600 K, Pnma297 K, after 600 K, AP3.4 4.4 5.4 6.4 7.4 8.4 9.4297 K, HP600 K, Pnma297 K, after 600 K, AP2θ  (deg): λ = 0.420138 Å Intensity (ar. un.) Bi2O2CO3 * ***Intensity (ar. un.) # Bi2O2CO3 Pbam Bi2O2CO3 Bi2O2CO3 Pbam Pnma Pnma (b) BiCr0.6Fe0.4O3 (a) BiCr0.7Fe0.3O3 * ***021 111 110 130 210 Figure 2. Magnified fragments of experimental synchrotron powder X-ray diffraction patternsof (a) BiCr0.7Fe0.3O3 and (b) BiCr0.6Fe0.4O3. Patterns for the as-synthesized HP modifications atT = 297 K are shown by black lines, for the Pnma modifications at T = 600 K—by blue lines, andthe AP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflectionpositions for the main perovskite phases (Pnma and Pbam) and Bi2O2CO3 impurity. The characteristicreflections of the C2/c modification are marked by black octothorps. The characteristic reflections ofthe Bi2O2CO3 impurity are additionally marked by green stars. (hkl) indices of some superstructurereflections of the Pbam-related modification are given.Figure 3 shows the results of the DSC measurements. All samples demonstrated areversible phase transition with sharp peaks on the DSC heating curves at Tstr = 450 K(x = 0.1), 480 K (x = 0.2), 510 K (x = 0.3), and 546 K (x = 0.4), where Tstr stands for thestructural phase transition temperature. The first heating curve was slightly differentfrom the second and third heating curves; all cooling curves were almost identical. Thesmall difference between the heating curves can be explained by the fact the HP mod-ification transforms to the AP modification after the first DSC run. The synchrotronXRD data (Figures 1 and 2; blue curves) clearly showed that the DSC anomalies corre-Inorganics 2025, 13, 91 5 of 20spond to a structural phase transition to the Pnma modification. With Tstr = 420 K for un-doped BiCrO3 [23,24], Tstr almost linearly increases with increasing x in the BiCr1−xFexO3solid solutions with 0.0 ≤ x ≤ 0.4. All the samples showed comparable enthalpies(about 7.0–7.9 J/g) of the structural phase transition (Figure 3). However, a systematicincrease in enthalpy with increasing x can be seen.Inorganics 2025, 13, x FOR PEER REVIEW 6 of 20    Figure 3. Differential scanning calorimetry (DSC) curves of (a) BiCr0.9Fe0.1O3, (b) BiCr0.8Fe0.2O3, (c) BiCr0.7Fe0.3O3, and (d) BiCr0.6Fe0.4O3 on heating (the left-hand axes) and cooling (the right-hand axes). Three DSC runs are given for each sample. Temperatures of peak positions on the heating curves are given. The peak areas (in J/g) are also given for the first heating curve, for the second and third heating curve (an average value), and for the first, second, and third cooling curves (an average value).  Figure 4. Fragments (between 2° and 25°) of experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of BiCr0.9Fe0.1O3 at T = 550 K in the Pnma modification. The tick marks show possible Bragg reflection positions for the main phase (black) and Bi2O2CO3 (blue) and Cr2O3 (green) impurities from top to bottom. The inset shows a magnified fragment.  -0.19-0.15-0.11-0.07300 400 5000.060.100.140.181st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.22-0.18-0.14-0.10-0.06300 400 5000.060.100.140.180.221st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.27-0.23-0.19-0.15-0.11-0.07300 400 5000.060.100.140.180.220.261st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.24-0.20-0.16-0.12-0.08-0.04300 400 5000.060.100.140.180.220.261st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling(a) BiCr0.9Fe0.1O3 Temperature (K) Heat flow (W/g) (b) BiCr0.8Fe0.2O3 (c) BiCr0.7Fe0.3O3 (d) BiCr0.6Fe0.4O3 450 K 480 K 510 K 546 K heating heating heating heating cooling cooling cooling cooling 7.17 J/g 6.97 J/g 7.02  J/g 7.08 J/g 7.13 J/g 7.21 J/g 7.18 J/g 7.77 J/g 7.60 J/g 7.67 J/g 7.63 J/g 7.86 J/g  -0.20.00.20.40.60.82 7 12 17 220.000.020.043.4 4.4 5.4 6.4 7.4 8.4Intensity (counts/106 ) 2θ  (deg): λ = 0.420138 Å BiCr0.9Fe0.1O3 T = 550 K space group: Pnma Figure 3. Differential scanning calorimetry (DSC) curves of (a) BiCr0.9Fe0.1O3, (b) BiCr0.8Fe0.2O3,(c) BiCr0.7Fe0.3O3, and (d) BiCr0.6Fe0.4O3 on heating (the left-hand axes) and cooling (the right-handaxes). Three DSC runs are given for each sample. Temperatures of peak positions on the heatingcurves are given. The peak areas (in J/g) are also given for the first heating curve, for the secondand third heating curve (an average value), and for the first, second, and third cooling curves(an average value).The Pnma modification of all the samples had sharp reflections without any anisotropicbroadening or asymmetry. Therefore, structure parameters of the Pnma modifications couldbe readily refined from synchrotron XRD data. Refined structural parameters of the Pnmamodifications for all the samples are summarized in Table 1, and Figure 4 shows fragmentsof experimental, calculated, and difference synchrotron XRPD data after the Rietveld fit at550 K for the x = 0.1 sample as an example.Figure 5 shows the compositional dependence of the lattice parameters and unit cellvolume of the Pnma modifications at 550 K (Table S1 provides numerical data). Nearlylinear increases in all parameters were observed with the increase in the Fe content inagreement with the larger ionic radius of Fe3+ cations (rVI = 0.645 Å) in comparison to Cr3+cations (rVI = 0.615 Å) [67], confirming the formation of the solid solutions.On the other hand, superstructure reflections of the PbZrO3-type Pbam modificationsin all the samples showed strong anisotropic broadening, and some reflections showedasymmetry from the high-angle side of the reflections (Figures 1 and 2 and Figure S1). Forexample (Figure 2b), the (021) reflection had the same width as the fundamental, mainreflections, and it was symmetrical. The (111) reflection was very broad, and it was nearlysymmetrical. The (110) reflection had intermediate broadening, and it showed asymmetry.Moreover, some reflections of the AP modification showed noticeable shifts from theirexpected, commensurate positions (for example, the (110), (130), and (210) reflections). Suchshifts suggest the presence of incommensurate modulations. Such shifts were also presentInorganics 2025, 13, 91 6 of 20in the HP modifications, but the shifts were less pronounced. Therefore, the presence ofincommensurate modulations and significant anisotropic broadening prevented us fromdetermining the precise structural parameters of the HP and AP modifications at RT.Table 1. Structure parameters of BiCr1−xFexO3 at high temperatures from synchrotron powder X-raydiffraction data.x 0.1 0.2 0.3 0.4T (K) 550 550 600 600a (Å) 5.55595 (3) 5.56458 (3) 5.57383 (2) 5.58329 (2)b (Å) 7.77499 (6) 7.78492 (6) 7.80159 (3) 7.81360 (3)c (Å) 5.44100 (3) 5.44552 (3) 5.45751 (2) 5.46389 (2)V (Å3) 235.037 (3) 235.899 (3) 237.318 (2) 238.366 (2)ρcal (g/cm3) 8.743 8.721 8.680 8.653x (Bi) 0.04309 (6) 0.04275 (8) 0.04304 (7) 0.04281 (8)z (Bi) 0.99574 (14) 0.99616 (23) 0.99618 (18) 0.99613 (21)Biso (Bi) (Å2) 1.173 (7) 1.364 (11) 1.528 (10) 1.643 (11)Biso (Cr/Fe) (Å2) 0.55 (2) 0.72 (3) 0.76 (2) 0.79 (3)x (O1) 0.4800 (9) 0.4871 (12) 0.4852 (11) 0.4860 (12)z (O1) 0.0823 (10) 0.0747 (13) 0.0847 (12) 0.0824 (13)Biso (O1) (Å2) 0.62 (13) 0.10 (17) 0.88 (16) 0.55 (18)x (O2) 0.2915 (9) 0.2921 (15) 0.2945 (11) 0.2994 (13)y (O2) 0.0385 (6) 0.0413 (11) 0.0374 (8) 0.0381 (10)z (O2) 0.7068 (9) 0.7071 (15) 0.7030 (11) 0.7041 (13)Biso (O2) (Å2) 1.06 (10) 2.4 (2) 1.51 (13) 1.97 (17)Rwp (%) 5.48 7.18 6.39 7.09Rp (%) 4.07 5.28 4.81 5.18RI (%) 3.18 4.17 4.26 4.16RF (%) 2.51 3.81 4.35 4.56Impurities:Bi2O2CO3 1.1 wt. % 0.8 wt. % 1.6 wt. % 1.6 wt. %Cr2O3 0.6 wt. % 0.9 wt. % – –Crystal system: orthorhombic. Space group: Pnma (No. 62); Z = 4. Source: synchrotron powder X-ray diffraction(λ = 0.420138 Å). d-space range used in the refinements: 0.497–11.563 Å. Fractional coordinates: Bi: 4c (x, 0.25, z),Cr/Fe: 4b (0, 0, 0.5), O1: 4c (x, 0.25, z), and O2: 8d (x, y, z). Occupation factors, g, of the Bi and O sites are 1. Theoccupation factor of the Cr/Fe site is mixed based on the nominal compositions.Inorganics 2025, 13, x FOR PEER REVIEW 6 of 20    Figure 3. Differential scanning calorimetry (DSC) curves of (a) BiCr0.9Fe0.1O3, (b) BiCr0.8Fe0.2O3, (c) BiCr0.7Fe0.3O3, and (d) BiCr0.6Fe0.4O3 on heating (the left-hand axes) and cooling (the right-hand axes). Three DSC runs are given for each sample. Temperatures of peak positions on the heating curves are given. The peak areas (in J/g) are also given for the first heating curve, for the second and third heating curve (an average value), and for the first, second, and third cooling curves (an average value).  Figure 4. Fragments (between 2° and 25°) of experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of BiCr0.9Fe0.1O3 at T = 550 K in the Pnma modification. The tick marks show possible Bragg reflection positions for the main phase (black) and Bi2O2CO3 (blue) and Cr2O3 (green) impurities from top to bottom. The inset shows a magnified fragment.  -0.19-0.15-0.11-0.07300 400 5000.060.100.140.181st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.22-0.18-0.14-0.10-0.06300 400 5000.060.100.140.180.221st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.27-0.23-0.19-0.15-0.11-0.07300 400 5000.060.100.140.180.220.261st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling-0.24-0.20-0.16-0.12-0.08-0.04300 400 5000.060.100.140.180.220.261st heating2nd heating3rd heating1st cooling2nd cooling3rd coolling(a) BiCr0.9Fe0.1O3 Temperature (K) Heat flow (W/g) (b) BiCr0.8Fe0.2O3 (c) BiCr0.7Fe0.3O3 (d) BiCr0.6Fe0.4O3 450 K 480 K 510 K 546 K heating heating heating heating cooling cooling cooling cooling 7.17 J/g 6.97 J/g 7.02  J/g 7.08 J/g 7.13 J/g 7.21 J/g 7.18 J/g 7.77 J/g 7.60 J/g 7.67 J/g 7.63 J/g 7.86 J/g  -0.20.00.20.40.60.82 7 12 17 220.000.020.043.4 4.4 5.4 6.4 7.4 8.4Intensity (counts/106 ) 2θ  (deg): λ = 0.420138 Å BiCr0.9Fe0.1O3 T = 550 K space group: Pnma Figure 4. Fragments (between 2◦ and 25◦) of experimental (black crosses), calculated (red line), anddifference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of BiCr0.9Fe0.1O3at T = 550 K in the Pnma modification. The tick marks show possible Bragg reflection positions forthe main phase (black) and Bi2O2CO3 (blue) and Cr2O3 (green) impurities from top to bottom. Theinset shows a magnified fragment.Inorganics 2025, 13, 91 7 of 20Inorganics 2025, 13, x FOR PEER REVIEW 7 of 20   Figure 5 shows the compositional dependence of the lattice parameters and unit cell volume of the Pnma modifications at 550 K (Table S1 provides numerical data). Nearly linear increases in all parameters were observed with the increase in the Fe content in agreement with the larger ionic radius of Fe3+ cations (rVI = 0.645 Å) in comparison to Cr3+ cations (rVI = 0.615 Å) [67], confirming the formation of the solid solutions.  Figure 5. Compositional dependence of the lattice parameters of the Pnma modifications of the BiCr1−xFexO3 solid solutions at T = 550 K. (a) The a and b lattice parameters, (b) the c lattice parameter and unit cell volume. Data on heating are used for x = 0.1, 0.2, and 0.3 and on cooling—for x = 0.4. On the other hand, superstructure reflections of the PbZrO3-type Pbam modifications in all the samples showed strong anisotropic broadening, and some reflections showed asymmetry from the high-angle side of the reflections (Figures 1, 2 and S1). For example (Figure 2b), the (021) reflection had the same width as the fundamental, main reflections, and it was symmetrical. The (111) reflection was very broad, and it was nearly symmet-rical. The (110) reflection had intermediate broadening, and it showed asymmetry. More-over, some reflections of the AP modification showed noticeable shifts from their ex-pected, commensurate positions (for example, the (110), (130), and (210) reflections). Such shifts suggest the presence of incommensurate modulations. Such shifts were also present in the HP modifications, but the shifts were less pronounced. Therefore, the presence of incommensurate modulations and significant anisotropic broadening prevented us from determining the precise structural parameters of the HP and AP modifications at RT. BiFeO3 has an incommensurate AFM structure, but BiFeO3 has a well-defined com-mensurate crystal structure [6]. Incommensurate structural modulations were only ob-served in the rare-earth-doped samples, Bi1−xRxFeO3, where R3+ is a rare-earth element, and at very limited compositional regions [40,68–73]. To the best of our knowledge, incom-mensurate structural modulations found in BiCr1−xFexO3 samples, especially in the AP modifications, have never been observed before in the only-transition-metal-doped BiFe1−xMxO3, where M is a transition metal element. Electron diffraction will be essential to understand incommensurate structural modulations of BiCr1−xFexO3 samples. Figure 6a shows the temperature dependence of the lattice parameters of BiCr0.6Fe0.4O3 on heating and cooling (Table S2 provides numerical data). Drastic changes in the fundamental perovskite reflections were observed during the transition from the Pbam modification to the Pnma modification. The unit cell volume also drops by −1.4% at 550 K, where the two phases coexist, suggesting the first-order structural phase transition. It is interesting that the a lattice parameter of the Pnma modification decreased with in-creasing temperature, suggesting anisotropic thermal expansion. All other parameters of the Pnma modification and all parameters of the Pbam modification increased with increas-ing temperature as expected. The fundamental lattice parameters of the HP and AP 5.5555.5655.5750.1 0.2 0.3 0.47.777.787.797.807.81ab5.4405.4500.1 0.2 0.3 0.4235236237238cVT = 550 K space group: Pnma a (Å) c (Å) b (Å) V (Å3) x x (a) (b) Figure 5. Compositional dependence of the lattice parameters of the Pnma modifications of the BiCr1−xFexO3solid solutions at T = 550 K. (a) The a and b lattice parameters, (b) the c lattice parameter and unit cell volume.Data on heating are used for x = 0.1, 0.2, and 0.3 and on cooling—for x = 0.4.BiFeO3 has an incommensurate AFM structure, but BiFeO3 has a well-defined commen-surate crystal structure [6]. Incommensurate structural modulations were only observed inthe rare-earth-doped samples, Bi1−xRxFeO3, where R3+ is a rare-earth element, and at verylimited compositional regions [40,68–73]. To the best of our knowledge, incommensuratestructural modulations found in BiCr1−xFexO3 samples, especially in the AP modifica-tions, have never been observed before in the only-transition-metal-doped BiFe1−xMxO3,where M is a transition metal element. Electron diffraction will be essential to understandincommensurate structural modulations of BiCr1−xFexO3 samples.Figure 6a shows the temperature dependence of the lattice parameters of BiCr0.6Fe0.4O3on heating and cooling (Table S2 provides numerical data). Drastic changes in the fun-damental perovskite reflections were observed during the transition from the Pbam mod-ification to the Pnma modification. The unit cell volume also drops by −1.4% at 550 K,where the two phases coexist, suggesting the first-order structural phase transition. It isinteresting that the a lattice parameter of the Pnma modification decreased with increasingtemperature, suggesting anisotropic thermal expansion. All other parameters of the Pnmamodification and all parameters of the Pbam modification increased with increasing tem-perature as expected. The fundamental lattice parameters of the HP and AP modificationswere very close to each other at RT (Figure 6). However, the superstructure reflections wereslightly different as discussed above. Therefore, we can discuss the different modifications,namely the HP and AP modifications, and the observation of the conversion polymorphismphenomenon [41] in the BiCr1−xFexO3 system similar to the conversion polymorphismphenomenon in the BiFe1−xScxO3 system [41] and the BiFe1−xMnxO3 system [36,49].Because of the presence of some structural differences between the HP and AP modifi-cations, we investigated the effects of the structural differences on the magnetic properties.Temperature-dependent magnetization curves of the HP and AP modifications are shownin Figures 7–10, and the results of M versus H measurements are shown in Figures 11–14(Figures S2–S5 show magnified parts of Figures 11–14). The HP and AP modificationsof BiCr0.9Fe0.1O3 showed very similar magnetic properties with one detectable magnetictransition at TN = 100 K despite their different phase compositions (a different ratio of thePbam and C2/c phases). Undoped BiCrO3 has TN = 112 K and shows a spin reorientationtransition at TN2 = 72 K [23]. No signs of a spin reorientation transition were found inBiCr0.9Fe0.1O3. A noticeable weak FM moment appeared below TN = 100 K at H = 100 Oeand H = 10 kOe (Figure 7). The transition with the weak FM moment with TN = 100 Kcould correspond to the behavior of the C2/c phase. The Pbam phase has a much weakerFM moment (see properties of the x = 0.3 and 0.4 samples); therefore, its contribution tomagnetic properties could be hidden in the x = 0.1 sample.Inorganics 2025, 13, 91 8 of 20Inorganics 2025, 13, x FOR PEER REVIEW 8 of 20   modifications were very close to each other at RT (Figure 6). However, the superstructure reflections were slightly different as discussed above. Therefore, we can discuss the dif-ferent modifications, namely the HP and AP modifications, and the observation of the conversion polymorphism phenomenon [41] in the BiCr1−xFexO3 system similar to the con-version polymorphism phenomenon in the BiFe1−xScxO3 system [41] and the BiFe1−xMnxO3 system [36,49].  Figure 6. (a) The temperature dependence of the normalized lattice parameters of BiCr0.6Fe0.4O3 on heating (full symbols) and cooling (empty symbols). (b) The temperature dependence of the nor-malized unit cell volumes (V/Z). αV is the volumetric coefficient of thermal expansion. Because of the presence of some structural differences between the HP and AP mod-ifications, we investigated the effects of the structural differences on the magnetic proper-ties. Temperature-dependent magnetization curves of the HP and AP modifications are shown in Figures 7–10, and the results of M versus H measurements are shown in Figures 11–14 (Figures S2–S5 show magnified parts of Figures 11–14). The HP and AP modifica-tions of BiCr0.9Fe0.1O3 showed very similar magnetic properties with one detectable mag-netic transition at TN = 100 K despite their different phase compositions (a different ratio of the Pbam and C2/c phases). Undoped BiCrO3 has TN = 112 K and shows a spin reorien-tation transition at TN2 = 72 K [23]. No signs of a spin reorientation transition were found in BiCr0.9Fe0.1O3. A noticeable weak FM moment appeared below TN = 100 K at H = 100 Oe and H = 10 kOe (Figure 7). The transition with the weak FM moment with TN = 100 K could correspond to the behavior of the C2/c phase. The Pbam phase has a much weaker FM moment (see properties of the x = 0.3 and 0.4 samples); therefore, its contribution to mag-netic properties could be hidden in the x = 0.1 sample. HP-BiCr0.8Fe0.2O3 clearly showed one transition at TN = 90 K (Figure 8) because of the relatively large fraction of the C2/c phase. On the other hand, AP-BiCr0.8Fe0.2O3 clearly showed two transitions, the first one at TN = 90 K from traces of the C2/c phase and the second one at TN = 74 K from the majority of the Pbam phase. Traces of the C2/c phase could still be detected as this phase had a much larger FM moment. It is interesting that both HP- and AP-BiCr0.8Fe0.2O3 showed a negative magnetization phenomenon [74] when the FCC curves were measured at H = 100 Oe and 1 kOe. Negative magnetization 59.459.659.860.060.2290 390 490 590V/ZV/Z, PnmaV/Z, coolingV/Z, Pnma,cooling3.853.873.893.913.933.95290 390 490 590a bc a, Pnmab, Pnma c, Pnmaa, cooling b, coolingc, cooling a, Pnma, coolingb, Pnma, cooling c, Pnma, coolingLattice parameters (Å) Temperature (K) a/√2(Pnma) b/2(Pnma) a/√2(Pbam) b/2√2(Pbam) c/2(Pbam) c/√2(Pnma) BiCr0.6Fe0.4O3 V/Z (Å3 ) (a) (b) αV = 1.8(2)×10−5 K−1 295–540 K αV = 3.4(2)×10−5 K−1, 560–580 K Figure 6. (a) The temperature dependence of the normalized lattice parameters of BiCr0.6Fe0.4O3on heating (full symbols) and cooling (empty symbols). (b) The temperature dependence of thenormalized unit cell volumes (V/Z). αV is the volumetric coefficient of thermal expansion.Inorganics 2025, 13, x FOR PEER REVIEW 9 of 20   phenomena are often observed in different RCr1−xFexO3 or RFe1−xCrxO3 solid solutions, where R3+ is a rare-earth element [75,76].  Figure 7. Magnetic properties of HP-BiCr0.9Fe0.1O3 (circles) and AP-BiCr0.9Fe0.1O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe.  Figure 8. Magnetic properties of HP-BiCr0.8Fe0.2O3 (circles) and AP-BiCr0.8Fe0.2O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The main inset on panel (a) shows the FCC curves H = 1 kOe. 0.00.10.10.20.20.30 50 100 150 200HP, ZFC, 100 OeHP, FCC, 100 OeAP, ZFC, 100 OeAP, FCC, 100 Oe0.0040.0050.0060.0070.0080 50 100 150 200HP, ZFC, 10 kOeHP, FCC, 10 kOeAP, ZFC, 10 kOeAP, FCC, 10 kOeχ (emu×mol−1 ×Oe−1 ) Temperature (K) BiCr0.9Fe0.1O3 TN = 100 K (a) (b)    0.0000.0030.0060.0090 50 100 150 200 250 300 350100150200250300HP, ZFC, 10 kOeHP, FCC, 10 kOeAP, ZFC, 10 kOeAP, FCC, 10 kOe-0.30-0.25-0.20-0.15-0.10-0.050.000.050 50 100 150 200 250 300HP, ZFC, 100 Oe HP, FCC, 100 OeAP, ZFC, 100 Oe AP, FCC, 100 Oe-0.025-0.020-0.015-0.010-0.0050.0000.0050.0100 50 100 150 200HP, FCC, 1 kOeAP, FCC, 1 kOeχ (emu×mol−1 ×Oe−1 ) χ (emu×mol−1 ×Oe−1 ) Temperature (K) Temperature (K) BiCr0.8Fe0.2O3 TN = 90 K (a) (b) χ−1 (emu−1×mol×Oe) 0.0040.0080.0120 30 60 90 120HP, ZFC, 100 OeAP, ZFC, 100 OeTN = 90 K TN = 74 K Figure 7. Magnetic properties of HP-BiCr0.9Fe0.1O3 (circles) and AP-BiCr0.9Fe0.1O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shownat (a) H = 100 Oe and (b) H = 10 kOe.Inorganics 2025, 13, 91 9 of 20Inorganics 2025, 13, x FOR PEER REVIEW 9 of 20   phenomena are often observed in different RCr1−xFexO3 or RFe1−xCrxO3 solid solutions, where R3+ is a rare-earth element [75,76].  Figure 7. Magnetic properties of HP-BiCr0.9Fe0.1O3 (circles) and AP-BiCr0.9Fe0.1O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe.  Figure 8. Magnetic properties of HP-BiCr0.8Fe0.2O3 (circles) and AP-BiCr0.8Fe0.2O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The main inset on panel (a) shows the FCC curves H = 1 kOe. 0.00.10.10.20.20.30 50 100 150 200HP, ZFC, 100 OeHP, FCC, 100 OeAP, ZFC, 100 OeAP, FCC, 100 Oe0.0040.0050.0060.0070.0080 50 100 150 200HP, ZFC, 10 kOeHP, FCC, 10 kOeAP, ZFC, 10 kOeAP, FCC, 10 kOeχ (emu×mol−1 ×Oe−1 ) Temperature (K) BiCr0.9Fe0.1O3 TN = 100 K (a) (b)    0.0000.0030.0060.0090 50 100 150 200 250 300 350100150200250300HP, ZFC, 10 kOeHP, FCC, 10 kOeAP, ZFC, 10 kOeAP, FCC, 10 kOe-0.30-0.25-0.20-0.15-0.10-0.050.000.050 50 100 150 200 250 300HP, ZFC, 100 Oe HP, FCC, 100 OeAP, ZFC, 100 Oe AP, FCC, 100 Oe-0.025-0.020-0.015-0.010-0.0050.0000.0050.0100 50 100 150 200HP, FCC, 1 kOeAP, FCC, 1 kOeχ (emu×mol−1 ×Oe−1 ) χ (emu×mol−1 ×Oe−1 ) Temperature (K) Temperature (K) BiCr0.8Fe0.2O3 TN = 90 K (a) (b) χ−1 (emu−1×mol×Oe) 0.0040.0080.0120 30 60 90 120HP, ZFC, 100 OeAP, ZFC, 100 OeTN = 90 K TN = 74 K Figure 8. Magnetic properties of HP-BiCr0.8Fe0.2O3 (circles) and AP-BiCr0.8Fe0.2O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shownat (a) H = 100 Oe and (b) H = 10 kOe. The main inset on panel (a) shows the FCC curves H = 1 kOe.The secondary inset shows the ZFC curves H = 100 Oe. The right-hand axis on panel (b) gives theinverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).Inorganics 2025, 13, 91 10 of 20Inorganics 2025, 13, x FOR PEER REVIEW 10 of 20   The secondary inset shows the ZFC curves H = 100 Oe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).  Figure 9. Magnetic properties of HP-BiCr0.7Fe0.3O3 (circles) and AP-BiCr0.7Fe0.3O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).     0.00350.00450.00550.00650.00750 50 100 150 200 250 300 350130180230HP, ZFC, 10 kOeHP, FCC, 10 kOeAP, ZFC, 10 kOeAP, FCC, 10 kOe0.0000.0100.0200.0300 50 100 150HP, ZFC, 100 OeHP, FCC, 100 OeAP, ZFC, 100 OeAP, FCC, 100 Oeχ (emu×mol−1 ×Oe−1 ) χ (emu×mol−1 ×Oe−1 ) Temperature (K) Temperature (K) BiCr0.7Fe0.3O3 TN = 92 K (a) (b) χ−1 (emu−1×mol×Oe) Figure 9. Magnetic properties of HP-BiCr0.7Fe0.3O3 (circles) and AP-BiCr0.7Fe0.3O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shownat (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).Inorganics 2025, 13, 91 11 of 20Inorganics 2025, 13, x FOR PEER REVIEW 11 of 20    Figure 10. Magnetic properties of HP-BiCr0.6Fe0.4O3 (circles) and AP-BiCr0.6Fe0.4O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).    0.00350.00450.00550 50 100 150 200 250 300 350160200240HP, ZFC, 100 OeHP, FCC, 100 OeAP, ZFC, 100 OeAP, FCC, 100 Oe0.000.010.020.030.040 50 100 150 200HP, ZFC, 100 OeHP, FCC, 100 OeAP, ZFC, 100 OeAP, FCC, 100 Oeχ (emu×mol−1 ×Oe−1 ) χ (emu×mol−1 ×Oe−1 ) Temperature (K) Temperature (K) BiCr0.6Fe0.4O3 TN = 122 K (a) (b) χ−1 (emu−1×mol×Oe) Figure 10. Magnetic properties of HP-BiCr0.6Fe0.4O3 (circles) and AP-BiCr0.6Fe0.4O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shownat (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).Inorganics 2025, 13, 91 12 of 20Inorganics 2025, 13, x FOR PEER REVIEW 12 of 20    Figure 11. Comparison of magnetic properties of HP-BiCr0.9Fe0.1O3 and AP-BiCr0.9Fe0.1O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures un-der H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.  Figure 12. Comparison of magnetic properties of HP-BiCr0.8Fe0.2O3 and AP-BiCr0.8Fe0.2O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves were measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7 T, 5 KAP, ZFC, 5 KAP, FCC-7 T, 5 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 60 KHP, FCC-7 T, 60 KAP, ZFC, 60 KAP, FCC-7 T, 60 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 90 KHP, FCC-7 T, 90 KAP, ZFC, 90 KAP, FCC-7 T, 90 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 120 KAP, ZFC, 120 KBiCr0.9Fe0.1O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d)    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 120 KAP, ZFC, 120 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 80 KHP, FCC-7T, 80 KAP, ZFC, 80 KAP, FCC-7T, 80 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 50 KHP, FCC-7T, 50 KAP, ZFC, 50 KAP, FCC-7T, 50 K-0.09-0.06-0.030.000.030.060.09-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7T, 5 KAP, ZFC, 5 KAP, FCC-7T, 5 KBiCr0.8Fe0.2O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d) Figure 11. Comparison of magnetic properties of HP-BiCr0.9Fe0.1O3 and AP-BiCr0.9Fe0.1O3: M versusH curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 120 K. M versus H curves are measuredunder the ZFC procedure, when samples are cooled from 300 K to measurement temperatures underH = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 Kto measurement temperatures under H = 70 kOe.Inorganics 2025, 13, x FOR PEER REVIEW 12 of 20    Figure 11. Comparison of magnetic properties of HP-BiCr0.9Fe0.1O3 and AP-BiCr0.9Fe0.1O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures un-der H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.  Figure 12. Comparison of magnetic properties of HP-BiCr0.8Fe0.2O3 and AP-BiCr0.8Fe0.2O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves were measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7 T, 5 KAP, ZFC, 5 KAP, FCC-7 T, 5 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 60 KHP, FCC-7 T, 60 KAP, ZFC, 60 KAP, FCC-7 T, 60 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 90 KHP, FCC-7 T, 90 KAP, ZFC, 90 KAP, FCC-7 T, 90 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 120 KAP, ZFC, 120 KBiCr0.9Fe0.1O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d)    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 120 KAP, ZFC, 120 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 80 KHP, FCC-7T, 80 KAP, ZFC, 80 KAP, FCC-7T, 80 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 50 KHP, FCC-7T, 50 KAP, ZFC, 50 KAP, FCC-7T, 50 K-0.09-0.06-0.030.000.030.060.09-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7T, 5 KAP, ZFC, 5 KAP, FCC-7T, 5 KBiCr0.8Fe0.2O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d) Figure 12. Comparison of magnetic properties of HP-BiCr0.8Fe0.2O3 and AP-BiCr0.8Fe0.2O3: M versusH curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves were measuredunder the ZFC procedure, when samples are cooled from 300 K to measurement temperatures underH = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 Kto measurement temperatures under H = 70 kOe.Inorganics 2025, 13, 91 13 of 20Inorganics 2025, 13, x FOR PEER REVIEW 13 of 20   H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.  Figure 13. Comparison of magnetic properties of HP-BiCr0.7Fe0.3O3 and AP-BiCr0.7Fe0.3O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures un-der H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe. The magnetic properties of both HP- and AP-BiCr0.7Fe0.3O3 were nearly identical when measured at high magnetic fields, such as H = 10 kOe (Figure 9b), with one AFM-like anomaly at TN = 92 K. A very weak FM contribution (from spin canting) started emerg-ing below about 20 K (Figures 9b and 13a). An extremely weak FM contribution also ap-peared just below TN = 92 K at weak magnetic fields, such as H = 100 Oe (Figure 9a), and HP-BiCr0.7Fe0.3O3 had a larger FM contribution in comparison to AP-BiCr0.7Fe0.3O3. The weak FM contribution in HP- and AP-BiCr0.7Fe0.3O3 at H = 100 Oe was about 10 times smaller than that of HP- and AP-BiCr0.9Fe0.1O3. Magnetic properties of both HP- and AP-BiCr0.6Fe0.4O3 were nearly identical when measured at high magnetic fields, such as H = 10 kOe (Figure 10b), with one AFM-like anomaly at TN = 122 K. No weak FM contributions appeared at lower temperatures in comparison with HP- and AP-BiCr0.7Fe0.3O3 at H = 10 kOe. At a small magnetic field of H = 100 Oe, HP-BiCr0.6Fe0.4O3 had a larger FM contribution in comparison to AP-BiCr0.6Fe0.4O3. However, the weak FM contributions were extremely small, meaning that they could not be detected on the M versus H curves, which showed linear behavior in the vicinity of the origin between about −20 kOe and 20 kOe (Figure 14).    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 120 KAP, ZFC, 120 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 80 KAP, ZFC, 80 K-0.09-0.06-0.030.000.030.060.09-80 -40 0 40 80HP, ZFC, 50 KHP, FCC-7T, 50 KAP, ZFC, 50 K-0.10-0.050.000.050.10-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7 T, 5 KAP, ZFC, 5 KAP, FCC-7 T, 5 KBiCr0.7Fe0.3O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d) Figure 13. Comparison of magnetic properties of HP-BiCr0.7Fe0.3O3 and AP-BiCr0.7Fe0.3O3: M versusH curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves are measuredunder the ZFC procedure, when samples are cooled from 300 K to measurement temperatures underH = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 Kto measurement temperatures under H = 70 kOe.Inorganics 2025, 13, x FOR PEER REVIEW 14 of 20    Figure 14. Comparison of magnetic properties of HP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 150 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures un-der H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe. At high temperatures, inverse magnetic susceptibilities follow the Curie–Weiss law for all the samples (Figures 8b, 9b and 10b), and parameters of the Curie–Weiss fits are summarized in Table 2. The experimental effective magnetic moments were close to the expected, calculated ones. The Weiss temperature varied between −230 K and −280 K, re-sulting in a moderate frustration index of about 2.3 to 2.9. It is interesting that some M versus H curves showed noticeable upturn deviations from the linear behavior (at high magnetic fields), suggesting the presence of gradual field-induced transitions, for exam-ple, at T = 5 K for x = 0.4 (Figure 14a), T = 50 K for x = 0.3 (Figure 13b), T = 5 K and 50 K for x = 0.2 (Figure 12a,b), and T = 5 K for x = 0.1 (Figure 11a). Table 2. Temperatures of structural transitions (Tstr) and magnetic anomalies (TN) and parameters of the Curie–Weiss fits and M versus H curves at T = 5 K for BiCr1−xFexO3. x Tstr (K) TN (K) μeff (μB/f.u.) μcalc (μB/f.u.) θ (K) MS (μB/f.u.) 0.1 (HP) 450 100 3.995 4.123 −247 0.075 0.1 (AP) 450 98 4.061 4.123 −259 0.073 0.2 (HP) 480 90 4.068 4.359 −240 0.086 0.2 (AP) 478 90, 74 3.934 4.359 −227 0.087 0.3 (HP) 511 92 4.269 4.583 −264 0.099 0.3 (AP) 510 92 4.305 4.583 −273 0.099 0.4 (HP) 546 122 4.293 4.796 −279 0.083 0.4 (AP) 546 122 4.321 4.796 −279 0.083    -0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 150 KAP, ZFC, 150 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 90 KHP, FCC-7 T, 90 KAP, ZFC, 90 K-0.08-0.040.000.040.08-80 -40 0 40 80HP, ZFC, 60 KHP, FCC-7 T, 60 KAP, ZFC, 60 KAP, FCC-7 T, 60 K-0.09-0.06-0.030.000.030.060.09-80 -40 0 40 80HP, ZFC, 5 KHP, FCC-7 T, 5 KAP, ZFC, 5 KAP, FCC-7 T, 5 KBiCr0.6Fe0.4O3 Magnetization  (μB / f.u.) Magnetic Field (kOe) (a) (b) (c) (d) Figure 14. Comparison of magnetic properties of HP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3: M versusH curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 150 K. M versus H curves are measuredunder the ZFC procedure, when samples are cooled from 300 K to measurement temperatures underH = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 Kto measurement temperatures under H = 70 kOe.HP-BiCr0.8Fe0.2O3 clearly showed one transition at TN = 90 K (Figure 8) because ofthe relatively large fraction of the C2/c phase. On the other hand, AP-BiCr0.8Fe0.2O3 clearlyInorganics 2025, 13, 91 14 of 20showed two transitions, the first one at TN = 90 K from traces of the C2/c phase and thesecond one at TN = 74 K from the majority of the Pbam phase. Traces of the C2/c phase couldstill be detected as this phase had a much larger FM moment. It is interesting that both HP-and AP-BiCr0.8Fe0.2O3 showed a negative magnetization phenomenon [74] when the FCCcurves were measured at H = 100 Oe and 1 kOe. Negative magnetization phenomena areoften observed in different RCr1−xFexO3 or RFe1−xCrxO3 solid solutions, where R3+ is arare-earth element [75,76].The magnetic properties of both HP- and AP-BiCr0.7Fe0.3O3 were nearly identicalwhen measured at high magnetic fields, such as H = 10 kOe (Figure 9b), with one AFM-likeanomaly at TN = 92 K. A very weak FM contribution (from spin canting) started emergingbelow about 20 K (Figures 9b and 13a). An extremely weak FM contribution also appearedjust below TN = 92 K at weak magnetic fields, such as H = 100 Oe (Figure 9a), and HP-BiCr0.7Fe0.3O3 had a larger FM contribution in comparison to AP-BiCr0.7Fe0.3O3. The weakFM contribution in HP- and AP-BiCr0.7Fe0.3O3 at H = 100 Oe was about 10 times smallerthan that of HP- and AP-BiCr0.9Fe0.1O3.Magnetic properties of both HP- and AP-BiCr0.6Fe0.4O3 were nearly identical whenmeasured at high magnetic fields, such as H = 10 kOe (Figure 10b), with one AFM-likeanomaly at TN = 122 K. No weak FM contributions appeared at lower temperatures incomparison with HP- and AP-BiCr0.7Fe0.3O3 at H = 10 kOe. At a small magnetic field ofH = 100 Oe, HP-BiCr0.6Fe0.4O3 had a larger FM contribution in comparison toAP-BiCr0.6Fe0.4O3. However, the weak FM contributions were extremely small, meaningthat they could not be detected on the M versus H curves, which showed linear behavior inthe vicinity of the origin between about −20 kOe and 20 kOe (Figure 14).At high temperatures, inverse magnetic susceptibilities follow the Curie–Weiss lawfor all the samples (Figures 8b, 9b and 10b), and parameters of the Curie–Weiss fits aresummarized in Table 2. The experimental effective magnetic moments were close to theexpected, calculated ones. The Weiss temperature varied between −230 K and −280 K,resulting in a moderate frustration index of about 2.3 to 2.9. It is interesting that someM versus H curves showed noticeable upturn deviations from the linear behavior (at highmagnetic fields), suggesting the presence of gradual field-induced transitions, for example,at T = 5 K for x = 0.4 (Figure 14a), T = 50 K for x = 0.3 (Figure 13b), T = 5 K and 50 K forx = 0.2 (Figure 12a,b), and T = 5 K for x = 0.1 (Figure 11a).Table 2. Temperatures of structural transitions (Tstr) and magnetic anomalies (TN) and parameters ofthe Curie–Weiss fits and M versus H curves at T = 5 K for BiCr1−xFexO3.x Tstr (K) TN (K) µeff(µB/f.u.)µcalc(µB/f.u.) θ (K) MS(µB/f.u.)0.1 (HP) 450 100 3.995 4.123 −247 0.0750.1 (AP) 450 98 4.061 4.123 −259 0.0730.2 (HP) 480 90 4.068 4.359 −240 0.0860.2 (AP) 478 90, 74 3.934 4.359 −227 0.0870.3 (HP) 511 92 4.269 4.583 −264 0.0990.3 (AP) 510 92 4.305 4.583 −273 0.0990.4 (HP) 546 122 4.293 4.796 −279 0.0830.4 (AP) 546 122 4.321 4.796 −279 0.083The Curie–Weiss fits are performed between 250 and 350 K using the FCC χ−1 versus T data at 10 kOe. MS is themagnetization value at T = 5 K and H = 70 kOe. µcalc is calculated using 5.916µB for Fe3+ and 3.873µB for Cr3+.TN values are determined from peaks on the 100 Oe FCC d(χT)/dT versus T curves. Tstr values are determinedfrom peak positions on the heating DSC curves, where the anomalies on the first heating curves are assigned to theHP modification and the anomalies on the second and third heating curves are assigned to the AP modification.Tstr corresponds to a transition to the GdFeO3-type Pnma modification.Inorganics 2025, 13, 91 15 of 20Figure 15 summarizes the temperature–composition phase diagram of the BiCr1−xFexO3system using the results of the current study and the literature data for the BiFeO3 [6],BiCrO3 [23,24,31], and BiCr1−xFexO3 systems with high Fe content [63]. On the scale ofFigure 15, TN remains nearly the same for 0.0 ≤ x ≤ 0.4 and then monotonically increasesfrom x = 0.4 to x = 1. Tstr increases gradually for 0.0 ≤ x ≤ 0.4 and then more rapidly fromx = 0.4 to x = 1.Inorganics 2025, 13, x FOR PEER REVIEW 15 of 20   The Curie–Weiss fits are performed between 250 and 350 K using the FCC χ−1 versus T data at 10 kOe. MS is the magnetization value at T = 5 K and H = 70 kOe. µcalc is calculated using 5.916µB for Fe3+ and 3.873µB for Cr3+. TN values are determined from peaks on the 100 Oe FCC d(χT)/dT versus T curves. Tstr values are determined from peak positions on the heating DSC curves, where the anomalies on the first heating curves are assigned to the HP modification and the anomalies on the second and third heating curves are assigned to the AP modification. Tstr corresponds to a transition to the GdFeO3-type Pnma modification. Figure 15 summarizes the temperature–composition phase diagram of the BiCr1−xFexO3 system using the results of the current study and the literature data for the BiFeO3 [6], BiCrO3 [23,24,31], and BiCr1−xFexO3 systems with high Fe content [63]. On the scale of Figure 15, TN remains nearly the same for 0.0 ≤ x ≤ 0.4 and then monotonically increases from x = 0.4 to x = 1. Tstr increases gradually for 0.0 ≤ x ≤ 0.4 and then more rapidly from x = 0.4 to x = 1.  Figure 15. The temperature–composition phase diagram of the BiCr1−xFexO3 system. Tstr is the tem-perature of the structural transition to the Pnma modification. TN is the Néel temperature. TSR is the temperature of the spin reorientation transition. 3. Materials and Methods The as-synthesized HP modifications of BiCr1−xFexO3 solid solutions with x = 0.1, 0.2, 0.3, and 0.4 were prepared from stoichiometric mixtures of Bi2O3 (Rare Metallic Co., To-kyo, Japan, 99.9999%), Fe2O3 (Rare Metallic Co., Tokyo, Japan, 99.999%), and Cr2O3 (Rare Metallic Co., Tokyo, Japan, 99.9%). The synthesis was performed at about 6 GPa and about 1600 K for 1 h in sealed Pt capsules using a belt-type HP instrument. After annealing at 1600 K, the samples were cooled down to room temperature by turning off the heating current, and the pressure was slowly released. The AP modifications of BiCr1−xFexO3 solid solutions with x = 0.1, 0.2, 0.3, and 0.4 were obtained by heating HP-BiCr1−xFexO3 in air at AP at 623 K for 10 min (with a heating/cooling rate of 10 K/min). We note that at x ≥ 0.5, a different modification with space group R3c is formed [62] (that is confirmed by us); there-fore, compositions with x ≥ 0.5 were not included in the present work. Tstr TN TN [63] TSR x Temperature (K) R3c Pbam Pnma C2/c (partial) decomposition C2/c+Pbam 020040060080010000.0 0.2 0.4 0.6 0.8 1.0Figure 15. The temperature–composition phase diagram of the BiCr1−xFexO3 system. Tstr is thetemperature of the structural transition to the Pnma modification. TN is the Néel temperature. TSR isthe temperature of the spin reorientation transition.3. Materials and MethodsThe as-synthesized HP modifications of BiCr1−xFexO3 solid solutions with x = 0.1,0.2, 0.3, and 0.4 were prepared from stoichiometric mixtures of Bi2O3 (Rare Metallic Co.,Tokyo, Japan, 99.9999%), Fe2O3 (Rare Metallic Co., Tokyo, Japan, 99.999%), and Cr2O3(Rare Metallic Co., Tokyo, Japan, 99.9%). The synthesis was performed at about 6 GPa andabout 1600 K for 1 h in sealed Pt capsules using a belt-type HP instrument. After annealingat 1600 K, the samples were cooled down to room temperature by turning off the heatingcurrent, and the pressure was slowly released. The AP modifications of BiCr1−xFexO3solid solutions with x = 0.1, 0.2, 0.3, and 0.4 were obtained by heating HP-BiCr1−xFexO3 inair at AP at 623 K for 10 min (with a heating/cooling rate of 10 K/min). We note that atx ≥ 0.5, a different modification with space group R3c is formed [62] (that is confirmed byus); therefore, compositions with x ≥ 0.5 were not included in the present work.X-ray powder diffraction (XRPD) data were collected at room temperature on aMiniFlex600 diffractometer (Rigaku, Tokyo, Japan) using CuKα radiation (2θ range of8–100◦, a step width of 0.02◦, and scan speed of 2 ◦/min). Synchrotron XRPD data of HP-BiCr1−xFexO3 were collected at 297 K, upon heating to 550 K (x = 0.1 and 0.2) or to 600 K(x = 0.3 and 0.4), and then on cooling to 297 K using the beamline BL02B2 [77,78] of SPring-8,Japan. Intensity data were taken between 2.082◦ and 78.216◦ at a 0.006◦ interval in 2θ usinga wavelength of λ = 0.420138 Å; however, data up to 50◦ were used in the Rietveld analysisas no detectable experimental reflections were observed above 50◦. The measurement timewas 300 s at 297 K and 550 K (or 600 K) and 60 s at other temperatures. The samples wereplaced into open Lindemann glass capillary tubes (with an inner diameter of 0.1 mm),Inorganics 2025, 13, 91 16 of 20which were rotated during the measurements. The Rietveld analysis of all XRPD data wasperformed using the RIETAN-2000 program [79]. The reported weight fractions of all thephases were calculated by the RIETAN-2000 program [79] from the refined scale factors.Magnetic measurements were performed on a SQUID magnetometer (Quantum De-sign MPMS3, San Diego, CA, USA) between 2 and 350 K in different applied fields usingzero-field-cooled (ZFC) and field-cooled on cooling (FCC) procedures. Isothermal magne-tization measurements, M versus H, were performed from 70 kOe to −70 kOe and from−70 kOe to 70 kOe using both ZFC and FCC procedures. In the ZFC procedure for M versusH measurements, the samples were cooled from 300 K to a measurement temperature undera zero magnetic field; in the FCC procedure, the samples were cooled from 300 K to ameasurement temperature under a magnetic field of 70 kOe. The ZFC and FCC proceduresfor M versus H measurements were used to check the presence or absence of the exchangebias effect; no detectable difference was observed on M versus H curves measured in theZFC and FCC procedures, suggesting the absence of the exchange bias effect.Pieces of pellets were used in magnetic measurements. A pellet of eachHP-BiCr1−xFexO3 was first used to obtain magnetic properties of the HP modification; thesame pellet was then transformed to AP-BiCr1−xFexO3 as described above, and it was usedin magnetic measurements to obtain magnetic properties of the AP modification.Differential scanning calorimetry (DSC) curves of powder samples of HP-BiCr1−xFexO3were recorded on a Mettler Toledo DSC1 STARe system between 297 K and maximum573 K in open Al capsules with a heating/cooling rate of 10 K/min. Three DSC runs wereperformed to check the reproducibility.4. ConclusionsIn conclusion, two modifications of the BiCr1−xFexO3 perovskite solid solutions wereprepared. The HP modifications (as-synthesized) were prepared by a high-pressure high-temperature method at 6 GPa. The AP modifications were obtained using a “conversionpolymorphism” strategy after heating at AP above structural phase transition tempera-tures and cooling to room temperature. The HP and AP modifications had subtle struc-tural differences and showed incommensurate structural modulations. Structural phasetransitions to the Pnma modification were observed in all the samples and investigatedin details. Subtle differences in magnetic properties of the HP and AP modificationswere investigated and reported. In particular, the x = 0.2 samples demonstrated negativemagnetization phenomena.Supplementary Materials: The following supporting information can be downloaded at:https://www.mdpi.com/article/10.3390/inorganics13030091/s1, Figure S1: Fragments (between4.4◦ and 7.4◦) of experimental and calculated synchrotron powder X-ray diffraction patterns ofHP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3 at T = 297 K; Figure S2: A magnified part of Figure 11;Figure S3: A magnified part of Figure 12; Figure S4: A magnified part of Figure 13; Figure S5: Amagnified part of Figure 14; Table S1: Numerical data used to plot Figure 5; Table S2: Numerical dataused to plot Figure 6.Funding: This research received no external funding.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: The raw data supporting the conclusions of this article will be madeavailable by the author on request.Acknowledgments: Synchrotron radiation was used at the powder diffraction beamline BL02B2at SPring-8, with permission from the Japan Synchrotron Radiation Research Institute (Proposalhttps://www.mdpi.com/article/10.3390/inorganics13030091/s1Inorganics 2025, 13, 91 17 of 20Number: 2021A1334). The author thanks S. Kobayashi for his help at BL02B2 of SPring-8 and I.S.Soboleva for her preliminary studies of the BiCr1−xFexO3 system. MANA was supported by theWorld Premier International Research Center Initiative (WPI), MEXT, Japan.Conflicts of Interest: The author declares no conflicts of interest.References1. Venevtsev, Y.N.; Zhdanov, G.S.; Solov’ev, S.N.; Bezus, E.V.; Ivanova, V.V.; Fedulov, S.A.; Kapyshev, A.G. Crystal chemical studiesof substances with perovskite structure and special dielectric properties. Sov. Phys. Crystallogr. 1961, 5, 594–599.2. Filip’ev, V.S.; Smolyaninov, N.P.; Fesenko, E.G.; Belyaev, I.N. Synthesis of BiFeO3 and determination of the unit cell. Sov. Phys.Crystallogr. 1961, 5, 913–914.3. Zaslavskii, A.I.; Tutov, A.G. The structure of a new antiferromagnetic, BiFeO3. Dokl. Akad. Nauk SSSR 1960, 135, 815–819.4. Fedulov, S.A. Determination of Curie temperature for BiFeO3 ferroelectric. Dokl. Akad. Nauk SSSR 1961, 139, 1345.5. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1063/1.4999454https://doi.org/10.1107/S1600577520001599https://doi.org/10.4028/www.scientific.net/MSF.321-324.198 Introduction  Results and Discussion  Materials and Methods  Conclusions  References