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[Alexei A. Belik](https://orcid.org/0000-0001-9031-2355), [Ran Liu](https://orcid.org/0000-0002-1659-2325), [Kazunari Yamaura](https://orcid.org/0000-0003-0390-8244)

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[Phase Separation Phenomena in Lightly Cu-Doped A-Site-Ordered Quadruple Perovskite NdMn7O12](https://mdr.nims.go.jp/datasets/5e338aa1-e7eb-45e9-8c47-1a3e2bf486e4)

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Phase Separation Phenomena in Lightly Cu-Doped A-Site-Ordered Quadruple Perovskite NdMn7O12Academic Editor: Takashiro AkitsuReceived: 29 October 2025Revised: 21 November 2025Accepted: 25 November 2025Published: 26 November 2025Citation: Belik, A.A.; Liu, R.;Yamaura, K. Phase SeparationPhenomena in Lightly Cu-DopedA-Site-Ordered Quadruple PerovskiteNdMn7O12. Molecules 2025, 30, 4561.https://doi.org/10.3390/molecules30234561Copyright: © 2025 by the authors.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/).ArticlePhase Separation Phenomena in Lightly Cu-DopedA-Site-Ordered Quadruple Perovskite NdMn7O12Alexei A. Belik 1,* , Ran Liu 1,2,3 and Kazunari Yamaura 1,21 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan; liu.ran@sanken.osaka-u.ac.jp (R.L.);yamaura.kazunari@nims.go.jp (K.Y.)2 Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku,Sapporo 060-0810, Hokkaido, Japan3 Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1,Ibaraki 567-0047, Osaka, Japan* Correspondence: alexei.belik@nims.go.jpAbstractA-site-ordered quadruple perovskite manganites, AMn7O12, show many interesting physi-cal phenomena, including orbital and spin modulations, spin-induced multiferroic prop-erties, and competitions between different magnetic ground states. Doping with Cu2+can result in colossal magnetoresistance properties, ferrimagnetism, and additional struc-tural modulations producing electric–dipole helicoidal textures. Many previous workshave focused on large-concentration doping, reaching ACu3Mn4O12 compositions. Small-concentration doping has been investigated in a limited number of systems, e.g., inBiCuxMn7−xO12. In this work, we investigated solid solutions of NdCuxMn7−xO12 withx = 0.1, 0.2, and 0.3, prepared at 6 GPa and 1500 K. Specific heat measurements detectedthree magnetic transitions at x = 0 (at TN3 = 9 K, TN2 = 12 K, and TN1 = 84 K) and twotransitions at x = 0.1 (at TN2 = 10 K and TN1 = 78 K), while only one transition was foundat x = 0.2 (TN1 = 72 K) and x = 0.3 (TN1 = 65 K). Differential scanning calorimetry (DSC)measurements showed sharp and strong peaks near TOO = 664 K at x = 0, corresponding toan orbital-order (OO) structural transition from I2/m to Im-3 symmetry. DSC anomalieswere significantly broadened and their intensities were significantly reduced at x = 0.1–0.3,and structural transitions were observed near TOO = 630 K at x = 0.1, TOO = 600 K at x = 0.2,and TOO = 570 K at x = 0.3. The x = 0.1 sample clearly showed double-peak features onthe DSC curves near TOO because of the presence of two close phases. High-resolutionsynchrotron powder X-ray diffraction studies gave strong evidence that phase separationphenomena took place in the x = 0.1–0.3 samples, where two I2/m phases with an ap-proximate ratio of 1:1 were present (e.g., a = 7.47143 Å, b = 7.36828 Å, c = 7.46210 Å, andβ = 90.9929◦ for one phase and a = 7.46596 Å, b = 7.37257 Å, c = 7.45756 Å, and β = 90.9328◦for the second phase at x = 0.3). The Curie–Weiss temperature changed from negative(for x = 0, 0.1, and 0.2) to positive (for x = 0.3). TOO, TN1, the Curie–Weiss temperature, andmagnetization (at 5 K and 70 kOe) changed almost linearly with x.Keywords: A-site-ordered quadruple perovskites; orbital order; phase separation1. IntroductionClassical perovskite-structure materials with the stoichiometry of ABO3 have largeA cations and smaller B cations [1–5]. The stability of the perovskite structure is de-Molecules 2025, 30, 4561 https://doi.org/10.3390/molecules30234561https://doi.org/10.3390/molecules30234561https://doi.org/10.3390/molecules30234561https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/moleculeshttps://www.mdpi.comhttps://orcid.org/0000-0001-9031-2355https://orcid.org/0000-0002-1659-2325https://orcid.org/0000-0003-0390-8244https://doi.org/10.3390/molecules30234561https://www.mdpi.com/article/10.3390/molecules30234561?type=check_update&version=2Molecules 2025, 30, 4561 2 of 17termined by the relative sizes of A and B cations and is usually understood in termsof the Goldschmidt tolerance factor [2]. The perovskite structure (at least, in the oxideform) is quite flexible and can adapt (and form ordered structures) to situations wheresome A cations (defined as A′ and A′′) have similar sizes to B cations. Such adaptationstake place through the a+a+a+ tilt (in Glazer’s notation [6]) in the case of A-site-orderedquadruple perovskites, AA′3B4O12 [7–13], and through the a+a+c− tilt in the case of A-sitecolumnar-ordered quadruple perovskites, A2A′A′′B4O12 [13]. Strong octahedral tiltingproduces square planar coordination (as the first coordination sphere) around the A′ sitesin both subfamilies.If A′ = B = Mn in A-site-ordered quadruple AA′3B4O12 perovskites, an interestingclass of perovskite manganites is formed, AMn3Mn4O12 or AMn7O12 in short form [12].Depending on the oxidation state of the A cation, Mn cations can take different oxidationstates (at the B sites) from +3.5 for A+ [14,15] to +3.25 for A2+ [16] to +3 for A3+ [17–34],while Mn cations usually take the +3 oxidation state at the A′ site. The average oxidationstate of Mn at the B sites determines the structural and magnetic behaviors of AMn7O12.All AMn7O12 compounds crystallize in the parent structure of the AA′3B4O12 quadru-ple perovskites with space group Im-3 at high temperatures. However, with decreasingtemperature, different structural distortions take place. For example, all members of thetrivalent subfamily A3+Mn7O12 (except BiMn7O12 [20–22]) crystallize in the monoclinicspace group I2/m at room temperature (RT). The Im-3 to I2/m transition is driven by theorbital ordering (OO) of Jahn–Teller active Mn3+ cations at the B sites.The average oxidation state of Mn at the B sites can be continuously changed in differ-ent solid solutions, such as A+Mn7O12–A2+Mn7O12 and A2+Mn7O12–A3+Mn7O12 [35–37].The average oxidation state of Mn at the B sites can also be controlled through Cu2+ doping,producing ACuxMn7−xO12 solid solutions. Cu2+ cations are usually selected as dopantsbecause Cu2+ cations occupy square planar sites similar to Mn3+ cations and do not dis-turb/dilute Mn in the B sublattice [38–55]. With Cu2+ doping, the Mn3+/Mn4+ ratio isonly changed in the B sublattice. Large Cu2+ doping, e.g., reaching ACu3Mn4O12 [46–55],produces ferrimagnetic (FiM) properties with high magnetic transition temperatures aboveRT. Such A2+Mn7O12 and A3+Mn7O12 manganites also show good catalytic properties [10].It was recently found that light Cu2+ doping of BiMn7O12 can introduce electric-dipole helicoidal textures [56], mesoscopic helices of polar domains [57], and complextemperature–composition phase diagrams [58,59]. Light Cu2+ doping of other members ofthe trivalent subfamily A3+Mn7O12 has been poorly investigated. LaCuxMn7−xO12 solidsolutions were only studied at small doping levels [60]. However, the structural evolutionat RT was only reported as a function of x without detailed physical properties [60]. Itwas found that the monoclinic I2/m structure of the parent LaMn7O12 was realized atx = 0.12–0.32, the R-3 structure, observed in Ca2+Mn7O12 [16], was realized at x = 0.52–0.84,and the cubic Im-3 parent structure of AA′3B4O12 quadruple perovskites was realized atx = 1–3 [60]. Detailed effects of the low doping levels of Cu2+ were mainly investigatedin CaMn7O12 [61–65], where incommensurate crystal and magnetic structures are highlysensitive to such doping. We emphasize that previous structural studies with neutrondiffraction [44,51,53,56,58,59,61] showed that Cu2+ cations are always localized at the squareplanar A′ site due to the strong Jahn–Teller effect of Cu2+.Therefore, in this work, we prepared and investigated NdCuxMn7−xO12 solid solu-tions at small doping levels of x = 0.1, 0.2, and 0.3. We found that the NdCuxMn7−xO12solid solutions preserve the monoclinic structure of the parent NdMn7O12. However, thephase separation phenomenon was observed, where two close monoclinic I2/m phaseswere present in NdCuxMn7−xO12 solid solutions. The structural divergence betweenthe two phases monotonically increases with increasing x. The Curie–Weiss temperatureMolecules 2025, 30, 4561 3 of 17changes from negative (for x = 0, 0.1, and 0.2) to positive (for x = 0.3). The structuraltransition temperature (Tstr = TOO), the first magnetic transition temperature (TN1), theCurie–Weiss temperature, and magnetization (at 5 K and 70 kOe) change almost linearlywith x.2. Results and Discussion2.1. Magnetic PropertiesThe χ versus T curves of NdMn7O12 clearly showed three magnetic anomalies(Figure 1) at TN3 = 9 K, TN2 = 12 K, and TN1 = 84 K [33,34]. A transition at TN1 = 84 Kshows sharp increases in the χ values (especially at H = 100 Oe). Then, there are addi-tional small increases in the χ values at TN2 = 12 K and H = 10 kOe and small dropsin the χ values at TN3 = 9 K and H = 10 kOe. The behavior of the χ versus T curves ofNdMn7O12 at H = 10 kOe agrees well with magnetic structures found by neutron diffrac-tion studies [33]. Below TN1, a FiM transition takes place on the Mn sublattice at the A′ site(with one Mn3+ uncompensated moment per unit cell), while an antiferromagnetic (AFM)order is realized on the Mn sublattice at the B site. The presence of one uncompensatedsite leads to a strong rise in χ values below TN1. Below TN2, small spin canting of theAFM spins and incommensurate spin modulation appear at the B site—this leads to anadditional small increase in χ values below TN2. Below TN3, the Nd3+ sublattice is orderedwith moments being antiparallel to the FiM moments of Mn3+ at the A′ site—this leads tosmall drops in χ values at TN3 = 9 K. 0102030405060700 20 40 60 80 100 120 140 160 180 2000.00.30.60.91.2ZFC, 100 OeFCC, 100 OeZFC, 10 kOeFCC, 10 kOe0510150 50 100 150 200 250 300 350 (emumol1 Oe1 ) at 100 Oe  (emumol 1Oe1) at 10 kOe Temperature (K) 1  (emu1 molOe) eff = 13.561(0)B  = 63.3(0) K calc = 13.426B NdMn7O12 Temperature (K) Figure 1. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves(χ = M/H) of NdMn7O12 measured at H = 100 Oe (black symbols; the left-hand axis) and H = 10 kOe(red symbols; the right-hand axis). The inset shows the 10 kOe FCC χ−1 versus T curve with theCurie–Weiss fit (black line) between 200 K and 350 K; the parameters of the fit (µeff and θ) are givenin the figure.The χ versus T curves of NdCu0.1Mn6.9O12 (Figure 2) showed the first strong increasesin the χ values below TN1 = 78 K. However, the additional upturn (observed near 12 Kin NdMn7O12) nearly disappeared, while the small drops survived (near 10 K). There-fore, we can suggest that an incommensurate ordering (observed below TN2 = 12 K inNdMn7O12) is suppressed, and only magnetic transitions, corresponding to TN1 and TN3in NdMn7O12, survived. The Nd sublattice remained undisturbed; therefore, the Nd sub-lattice can behave similarly in NdMn7O12 and NdCu0.1Mn6.9O12. Cu2+ cations should beMolecules 2025, 30, 4561 4 of 17located at the A′ sites; however, Cu2+ doping introduces Mn4+ cations at the B sites ofNdCu0.1Mn6.9O12. Therefore, the incommensurate ordering and spin canting at the B sitescould be strongly affected by Cu2+ doping. In comparison with NdMn7O12, TN2 disappearsin NdCu0.1Mn6.9O12. However, we use sequential numbering of magnetic phase transitionswith TN2 = 10 K and TN1 = 78 K for NdCu0.1Mn6.9O12. -5515253545550 20 40 60 80 100 120 140 160 180 2000.00.30.60.91.2ZFC, 100 OeFCC, 100 OeZFC, 10 kOeFCC, 10 kOe04812160 50 100 150 200 250 300 350 (emumol1 Oe1 ) at 100 Oe  (emumol 1Oe1) at 10 kOe Temperature (K) 1  (emu1 molOe) eff = 13.125(13)B  = 33.6(6) K calc = 13.314B NdCu0.1Mn6.9O12 Temperature (K) Figure 2. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H)of NdCu0.1Mn6.9O12 measured at H = 100 Oe (black symbols; the left-hand axis) and H = 10 kOe(red symbols; the right-hand axis). The inset shows the 10 kOe FCC χ−1 versus T curve with theCurie–Weiss fit (black line) between 200 K and 350 K; the parameters of the fit (µeff and θ) are givenin the figure.The low-temperature drops are further suppressed in NdCu0.2Mn6.8O12 (Figure 3) andcompletely disappear in NdCu0.3Mn6.7O12 in all magnetic fields (Figure 4). The transitiontemperatures are summarized in Table 1, and the first transition temperature (TN1) almostlinearly decreases with increasing x in the NdCuxMn7−xO12 solid solutions (Figure 5a). -551525354555650 20 40 60 80 100 120 140 160 180 2000.00.30.60.91.21.5ZFC, 100 OeFCC, 100 OeZFC, 10 kOeFCC, 10 kOe0510150 50 100 150 200 250 300 350 (emumol1 Oe1 ) at 100 Oe  (emumol 1Oe1) at 10 kOe Temperature (K) 1  (emu1 molOe) eff = 12.977(11)B  = 9.7(5) K calc = 13.200B Temperature (K) NdCu0.2Mn6.8O12 Figure 3. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H)of NdCu0.2Mn6.8O12 measured at H = 100 Oe (black symbols; the left-hand axis) and H = 10 kOe(red symbols; the right-hand axis). The inset shows the 10 kOe FCC χ−1 versus T curve with theCurie–Weiss fit (black line) between 200 K and 350 K; the parameters of the fit (µeff and θ) are givenin the figure.Molecules 2025, 30, 4561 5 of 17 -551525354555650 20 40 60 80 100 120 140 160 180 2000.00.30.60.91.21.51.8ZFC, 100 OeFCC, 100 OeZFC, 10 kOeFCC, 10 kOe0510150 50 100 150 200 250 300 350 (emumol1 Oe1 ) at 100 Oe  (emumol 1Oe1) at 10 kOe Temperature (K) 1  (emu1 molOe) eff = 12.561(8)B  = +21.9(3) K calc = 13.086B Temperature (K) NdCu0.3Mn6.7O12 Figure 4. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H)of NdCu0.3Mn6.7O12 measured at H = 100 Oe (black symbols; the left-hand axis) and H = 10 kOe(red symbols; the right-hand axis). The inset shows the 10 kOe FCC χ−1 versus T curve with theCurie–Weiss fit (black line) between 200 K and 350 K; the parameters of the fit (µeff and θ) are givenin the figure.Table 1. Temperatures of structural transitions (Tstr) and magnetic anomalies (TN) and param-eters of the Curie–Weiss fits and M versus H curves at T = 5 K for NdCuxMn7−xO12 withx = 0, 0.1, 0.2, and 0.3.x Tstr (K) TN (K) µeff (µB/f.u.) µcalc (µB/f.u.) θ (K) MS (µB/f.u.) MR (µB/f.u.) HC (kOe/f.u.)0 664 9, 12, 84 13.651 13.426 −63.3 4.74 1.04 ~0.40.1 ~630 10, 78 13.125 13.314 −33.6 5.78 1.19 ~1.00.2 ~600 72 12.977 13.200 −9.7 6.64 1.53 ~1.50.3 ~570 65 12.561 13.086 +21.9 7.58 1.64 ~3.3The Curie–Weiss fits were performed between 200 and 350 K using the FCC χ−1 versus T data at 10 kOe. MS isthe magnetization value at T = 5 K and H = 70 kOe. MR is the remnant magnetization value at T = 5 K. HC is thecoercive field at T = 5 K. µeff is an experimental effective magnetic moment. µcalc is a calculated effective magneticmoment based on the formal oxidation states. TN values were determined from peaks on the 100 Oe and 10 kOeFCC d(χT)/dT versus T curves. Tstr values were determined from peak positions on the heating (or cooling forx = 0.3) DSC curves. Tstr corresponds to a transition to the Im-3 modification.At high temperatures, the inverse magnetic susceptibilities followed the Curie–Weisslaw. To extract the Curie–Weiss parameters, we fitted the field-cooled inverse magneticsusceptibilities (at H = 10 kOe) between 200 K and 350 K. The fitting parameters aresummarized in Table 1 [66]. The Curie–Weiss temperature changes from negative (forx = 0, 0.1, and 0.2) to positive (for x = 0.3), and it follows a nearly linear change with x(Figure 5b). The increase in Cu2+ doping increases the concentration of Mn4+ at the Bsites. Therefore, the concentration of ferromagnetic (FM) interactions through the double-exchange mechanism between Mn3+ and Mn4+ cations [67] also increases. This is reflectedin the changes in the Curie–Weiss temperature.Molecules 2025, 30, 4561 6 of 175605806006206406606800 0.1 0.2 0.3607080x T str = TOO (K) TN1  (K) 70503010100 0.1 0.2 0.345678Curie–Weiss temperature (K) Magnetization at 5 K and 70 kOe (B /f.u.) Tstr TN1 C–W Temp. MS (a) (b)     Figure 5. (a) Compositional dependence of the structural transition temperature (Tsrt = TOO;black, the left-hand axis) and the first magnetic transition temperature (TN1; red, the right-handaxis) in the NdCuxMn7−xO12 solid solutions. (b) Compositional dependence of the Curie–Weisstemperature (black, the left-hand axis) and the magnetization values (at T = 5 K and H = 70 kOe)(red, the right-hand axis).Isothermal magnetization curves (M versus H) at T = 5 K are given on Figure 6.These are typical for ferrimagnets with well-defined hysteresis near the origin and grad-ual continuous increases in magnetization at higher magnetic fields due to the AFM Bsublattice. There were monotonic increases in the remnant magnetization and coer-cive fields with x and a nearly linear increase in the magnetization values, MS (at T = 5 Kand H = 70 kOe), with x (Figure 5b). Some parameters of the M versus H curves are sum-marized in Table 1.Molecules 2025, 30, 4561 7 of 17 -2-1012-8 -6 -4 -2 0 2 4 6 8Cu0.0Cu0.1Cu0.2Cu0.3-8-6-4-202468-80 -60 -40 -20 0 20 40 60 80Cu0.0Cu0.1Cu0.2Cu0.3Magnetization  (B / f.u.) (a) (b) Magnetization  (B / f.u.) Magnetic Field (kOe) T = 5 K Figure 6. (a) M versus H curves of NdCuxMn7−xO12 with x = 0 (black), 0.1 (red), 0.2 (blue), and0.3 (green), measured at T = 5 K. (b) The same M versus H curves zoomed in on near the origin.The results of the specific heat measurements of NdCuxMn7−xO12 with x = 0, 0.1, 0.2,and 0.3 at different magnetic fields are shown in Figure 7. In agreement with the χ versusT curves, NdMn7O12 showed two sharp anomalies at TN2 = 12 K and TN1 = 84 K and ashoulder-like anomaly at TN2 = 9 K (the inset of Figure 7a). The double-peak anomaliesnear 10–15 K survived in NdMn7O12 at H = 90 kOe. The specific heat anomaly nearTN2 = 10 K was significantly suppressed in NdCu0.1Mn6.9O12 (Figure 7b) and was com-pletely suppressed in NdCu0.2Mn6.8O12 (Figure 7c) and NdCu0.3Mn6.7O12 (Figure 7d).On the other hand, the magnetic entropy that was released near TN2 in NdMn7O12 andNdCu0.1Mn6.9O12 moved to higher temperatures (up to about 45 K) in NdCu0.2Mn6.8O12and NdCu0.3Mn6.7O12 (the inset of Figure 7d). The Cp/T values of NdCu0.2Mn6.8O12 andNdCu0.3Mn6.7O12 were nearly the same between 2 K and 60 K.Molecules 2025, 30, 4561 8 of 17 0.00.51.01.52.00 50 100 1500 Oe: Cu0.270 kOe: Cu0.20120 50 100Cu0.2Cu0.10.00.51.01.52.00 50 100 1500 Oe: Cu0.370 kOe: Cu0.30120 50 100Cu0.0Cu0.1Cu0.2Cu0.30.00.51.01.52.00 50 100 1500 Oe: Cu0.090 kOe: Cu0.00120 5 10 15 20 250.00.51.01.52.00 50 100 1500 Oe: Cu0.170 kOe: Cu0.1Cp / T (JK2 mol1 ) Temperature (K) (a) (b) (c) (d) 0.00.30.60.91.21.50 5 10 15 20 25TN2 = 12 K TN2 = 10 K TN3 = 9 K Figure 7. Cp/T versus T curves of the NdCuxMn7−xO12 solid solutions measured at H = 0 Oe(blue curves) and 90 kOe (or 70 kOe) (red curves) during cooling for (a) x = 0, (b) x = 0.1, (c) x = 0.2,and (d) x = 0.3. The insets on panels (a,b) show the same curves below 25 K. Arrows give the magnetictransition temperatures. The inset on panel (c) compares Cp/T versus T curves at H = 0 Oe for x = 0.1and x = 0.2. The inset on panel (d) compares Cp/T versus T curves at H = 0 Oe for all samples.2.2. High-Temperature Structural TransitionsThe high-temperature behavior of NdCuxMn7−xO12 with x = 0, 0.1, 0.2, and 0.3 wasinvestigated with differential scanning calorimetry (DSC). NdMn7O12 showed very sharpand strong DSC anomalies near a structural (str) phase transition with Tstr = 684 K(defined from peak positions on heating curves) [34]. The phase transition temperature re-mained nearly the same during cycling (three runs) (Figure 8a,b). This structural transitioncorresponds to the symmetry change from I2/m (below Tstr) to Im-3 (above Tstr) and isrelated to the orbital order (OO) of Mn3+ cations at the B sites below Tstr [12,34].Molecules 2025, 30, 4561 9 of 17 -0.40-0.30-0.20-0.10400 450 500 550 600 6501st run2nd run3rd run0.080.180.28400 450 500 550 600 6501st run2nd run3rd run-0.13-0.12-0.11-0.10-0.09-0.08400 450 500 550 600 6501st run2nd run3rd run0.100.110.120.130.140.15400 450 500 550 600 6501st run2nd run3rd runHeat flow (W/g) Temperature (K) (a) heating NdMn7O12 (b) cooling NdMn7O12 (c) heating NdCu0.1Mn6.9O12 (d) cooling NdCu0.1Mn6.9O12 Figure 8. Differential scanning calorimetry (DSC) curves of (a,b) NdMn7O12 and (c,d) NdCu0.1Mn6.9O12during (a,c) heating and (b,d) cooling, shown between 400 K and 680 K. Three DSC runs are shown.The anomalies near 470 K on the cooling curves are instrumental artifacts.DSC anomalies were already significantly broadened in NdCu0.1Mn6.9O12 (Figure 8c,d),and their intensities were strongly suppressed in comparison with NdMn7O12. In addition,the first heating DSC curve showed three peaks, while the second and third heatingDSC curves showed two peaks. The cooling curves were reproducible and showed twopeaks. The appearance of two peaks could be explained by the phase separation discussedin the next part. The difference between the first and subsequent DSC heating curvessuggests the presence of “annealing” effects, when metastable states, obtained throughquenching at a high pressure of 6 GPa, transform to more stable states. The behaviorshown in the first and subsequent heating DSC curves was observed, for example, inBiMn7O12 [22].DSC anomalies became even broader in NdCu0.2Mn6.8O12 (Figure 9a,b). Therewas irreproducibility between the first and subsequent heating curves, while the cool-ing DSC curves were almost reproducible. In the case of NdCu0.3Mn6.7O12, no clearDSC anomalies were observed in the heating curves (Figure 9c), while the cooling curvesenabled very broad anomalies starting from about 570 K to be detected (Figure 9d).The structural phase transition temperatures decreased almost linearly with x in theNdCuxMn7−xO12 solid solutions (Figure 5a).Molecules 2025, 30, 4561 10 of 17    0.090.100.11400 450 500 550 600 6501st run2nd run3rd run-0.17-0.15-0.13-0.11400 450 500 550 600 6501st run2nd run3rd run0.100.110.120.130.14400 450 500 550 600 6501st run2nd run3rd run-0.14-0.13-0.12-0.11-0.10-0.09400 450 500 550 600 6501st run2nd run3rd runHeat flow (W/g) Temperature (K) (a) heating NdCu0.2Mn6.8O12 (b) cooling NdCu0.2Mn6.8O12 (c) heating NdCu0.3Mn6.7O12 (d) cooling NdCu0.3Mn6.7O12 Tstr Tstr Figure 9. Differential scanning calorimetry (DSC) curves of (a,b) NdCu0.2Mn6.8O12 and(c,d) NdCu0.3Mn6.7O12 during (a,c) heating and (b,d) cooling, shown between 400 K and 680 K.Three DSC runs are shown. The anomalies near 470 K on the cooling curves and sharp drops on someof the heating curves are instrumental artifacts.2.3. Structural PropertiesThe NdCuxMn7−xO12 samples with x = 0, 0.1, 0.2, and 0.3 did not contain any impurityphases because no impurity peaks could be seen, even for high-resolution, high-intensitysynchrotron powder X-ray diffraction (PXRD) data, indicating the high quality of thesamples. Reflections on laboratory PXRD data could be readily explained/indexed byassuming the presence of one monoclinic phase with the I2/m symmetry similar to un-doped NdMn7O12 [33,34]. However, attempts to fit the synchrotron PXRD data with oneI2/m phase were not successful, as one phase could not explain all reflection splitting(see the inset of Figure 10). All attempts to reduce the symmetry (for example, to triclinicsymmetry) in a one-phase model were also unsuccessful. On the other hand, all reflectionsplitting in all samples could be well explained by assuming the presence of two I2/m phaseswith slightly different lattice parameters (Table S1 in Supplementary Material and Table 2).The DSC results for the x = 0.1 sample (Figure 8c,d) support the conclusion about thepresence of two close phases, as each phase can show slightly different Tstr values. Theintensities of the two DSC peaks in the x = 0.1 sample (on the cooling curves and onthe second and third heating curves) were comparable with each other, in agreementwith the approximate 1:1 ratio of the two monoclinic phases found through synchrotronPXRD. Therefore, in general, the DSC results can provide valuable information aboutphase separation in the absence of high-resolution synchrotron XRPD data. However, inthe x = 0.2 and 0.3 samples, the DSC anomalies became so broad that different Tstr valueswere not resolvable.Molecules 2025, 30, 4561 11 of 17-0.30.00.30.60.96 12 18 24 30 362  (deg):  = 0.65298 Å Intensity (counts/106 ) T = 295 K -0.10.00.120.0 20.1 20.2 20.3 20.4(040) (400) (004) NdCu0.3Mn6.7O12 Figure 10. Fragments of the experimental (black crosses), calculated (red line), and difference (blue lineat the bottom) room-temperature synchrotron X-ray powder diffraction patterns of NdCu0.3Mn6.7O12in a 2θ range of 6◦ and 36◦, analyzed by the Rietveld method. The tick marks show possibleBragg reflection positions for two I2/m phases. The inset shows a zoomed-in part in a 2θ range of20.0◦ and 20.5◦ (shown by a dotted rectangle in the main panel) and emphasizes the splitting of (400),(004), and (040) reflections from phase separation.Table 2. Lattice parameters for NdCuxMn7−xO12 with x = 0, 0.1, 0.2, and 0.3 at room temperature(space group I2/m), determined from high-resolution synchrotron powder X-ray diffraction. Latticeparameters for two I2/m phases (M1 and M2) are given for x = 0.1, 0.2, and 0.3.x Phase a (Å) b (Å) c (Å) β (◦) V (Å3)0 M1 7.49567 (1) 7.35419 (1) 7.49049 (1) 91.2384 (2) 412.814 (1)0.1M1 7.48888 (2) 7.35795 (2) 7.48042 (3) 91.1644 (3) 412.107 (2)M2 7.48677 (3) 7.35966 (3) 7.47774 (3) 91.1437 (2) 411.942 (3)0.2M1 7.48026 (3) 7.36286 (2) 7.47075 (3) 91.0793 (3) 411.387 (2)M2 7.47672 (2) 7.36561 (2) 7.46712 (2) 91.0440 (2) 411.151 (2)0.3M1 7.47143 (2) 7.36828 (2) 7.46210 (2) 90.9929 (3) 410.739 (2)M2 7.46596 (3) 7.37257 (3) 7.45756 (3) 90.9328 (4) 410.434 (3)Figure 11a shows the compositional dependence of the monoclinic lattice parametersof NdCuxMn7−xO12 with x = 0.1, 0.2, and 0.3. In all cases, the second I2/m phase had asmaller monoclinic distortion in comparison with the first I2/m phase in the sense that themonoclinic β angle (of the second I2/m phase) was closer to 90◦, and the difference betweenthe lattice parameters (aM − bM and cM − bM) was smaller for the second I2/m phase. Inaddition, systematic increases in the splitting of the lattice parameters between the sec-ond and first I2/m phases were observed with an increasing x value (Figure 11b). Suchmonotonic changes in the divergence between the two monoclinic phases provide indirectsupport that the observed phase separation is an intrinsic property of the NdCuxMn7−xO12system in the compositional range of x = 0.1–0.3. We found that in the compositionalrange of x = 0.5–0.8, the NdCuxMn7−xO12 solid solutions (prepared in the same condi-tions as the x = 0.1–0.3 samples) had the R-3 symmetry and showed no evidence of phaseseparation (including the DSC results where only one DSC peak was found and high-resolution synchrotron PXRD). The NdCuxMn7−xO12 solid solutions crystallized in thecubic Im-3 structure for x = 1.1–3.0, and we did not see any evidence of phase separation inMolecules 2025, 30, 4561 12 of 17the high-resolution synchrotron PXRD; no phase separation was also found in the litera-ture in this compositional range [50,51,54]. Therefore, the phase separation phenomenondepends on the chemical composition/symmetry and is not an artifact of the preparationconditions, mixing procedures, or other factors. The real reason for the phase separation inthe 0.1–0.3 samples is not clear and will require additional studies in the future. Magneticproperties were not affected by phase separation, and magnetic measurements could notprovide any evidence of phase separation, likely because the magnetic transition temper-atures of the two monoclinic phases were too close to each other and all magnetic andspecific heat anomalies from the two phases severely overlapped. Monoclinic phases withdifferent lattice parameters were observed in the A3+Mn7O12 systems when the oxygencontent was varied (for A = Pr [28]) or small amounts of Mn4+ cations were introducedthrough A-site deficiency (for A = Bi [23,24]). Therefore, one of the driving forces for phaseseparation in the NdCuxMn7−xO12 solid solutions could be small variations in the oxygencontent of the two monoclinic phases. However, the Mn3+/Mn4+ distribution and ratio(which depend on the oxygen content and Cu2+ doping levels) or lattice strain could alsocontribute to the phase separation. We also note that phase separation was reported incubic CdCu3Mn4O12 [45], prepared under high-pressure conditions; however, in this case,the formation of a CuO impurity (and compositional shifts) could have contributed to theeffect. Phase separation was also observed in some of the cubic CaCu3Ti4O12 samples [68],prepared at ambient pressure.0.0060.0040.0020.0000.0020.0040.0060.0 0.1 0.2 0.30.000.010.020.030.040.050.060.077.347.387.427.467.5090.991.091.191.291.3Lattice parameters (Å) x  () aM cM bM M Lattice parameter difference (Å)  angle difference () bM1  bM2 aM1  aM2 cM1  cM2 M1  M2 (a) (b)    Figure 11. (a) Compositional dependence of the monoclinic (M) lattice parameters (the left-hand axis)and monoclinic β angle (the right-hand axis) in NdCuxMn7−xO12 with x = 0, 0.1, 0.2, and 0.3. Thelattice parameters of the first I2/m phase (M1) are shown in red and pink, and the lattice parametersof the second I2/m phase (M2) are shown in blue and navy. (b) Compositional dependence of thedifference in the lattice parameters between phases M1 and M2.Molecules 2025, 30, 4561 13 of 173. Materials and MethodsNdCuxMn7−xO12 samples with x = 0, 0.1, 0.2, and 0.3 were prepared from stoichiomet-ric mixtures of Nd2O3 (Rare Metallic Co., Tokyo, Japan, 99.9%), CuO (Rare Metallic Co.,Tokyo, Japan, 99.9%), MnO2 (Alfa Aesar, Ward Hill, MA, USA, 99.99%), and Mn2O3.Single-phase Mn2O3 was prepared from a commercial MnO2 chemical (Rare Metallic Co.,Tokyo, Japan, 99.99%) by annealing in air at 923 K for 24 h. The synthesis was per-formed at 6 GPa and 1500 K for 2 h in sealed Au capsules using a belt-type, high-pressure HP instrument. After annealing at 1500 K, the samples were cooled down to roomtemperature (RT) by turning off the heating current, and the pressure was slowly released.Powder X-ray diffraction (XRPD) data were collected at RT on a MiniFlex600 diffrac-tometer (Rigaku, Tokyo, Japan) using CuKα radiation (a 2θ range of 5–100◦, a stepwidth of 0.02◦, and a scan speed of 2 ◦/min). RT synchrotron XRPD data were mea-sured on the BL15XU beamline (the former NIMS beamline) of SPring-8 [69] between3.04◦ and 59.33◦ at 0.003◦ intervals in 2θ with a wavelength of λ = 0.65298 Å. The sam-ples were placed into open Lindemann glass capillary tubes (inner diameter: 0.1 mm),which were rotated during the measurements. The Rietveld analysis of all XRPD data andindexing were performed using the RIETAN-2000 program [70].Magnetic measurements were performed on an SQUID magnetometer (QuantumDesign MPMS-XL-7T, San Diego, CA, USA) between 2 K and 350 K in applied fields of100 Oe and 10 kOe under both zero-field-cooled (ZFC) and field-cooled on cooling (FCC)conditions. Magnetic field dependence was measured at T = 5 K between −70 and 70 kOe.The base temperature of the MPMS-XL-7T magnetometer was 10 K, meaning that sampleswere inserted slowly into 10 K, and samples moved through a magnet (with finite trappedmagnetic fields) below their ordering temperatures. This procedure could be the reasonfor negative initial magnetization in some ZFC curves (at H = 100 Oe), even though themagnet reset procedure was applied before each ZFC measurement.Specific heat, Cp, was measured during cooling from 300 K to 2 K at a magneticfield of zero and from 150 K to 2 K at different magnetic fields through a pulse relaxationmethod using a commercial calorimeter (Quantum Design PPMS, San Diego, CA, USA).All magnetic and specific heat measurements were performed using pieces of pellets.Differential scanning calorimetry (DSC) curves of powder samples were recorded ona Mettler Toledo DSC1 STARe system (Columbus, OH, USA) between 297 K and 703 K inopen Al capsules with a heating/cooling rate of 10 K/min. Three DSC runs were performedto check the reproducibility.4. ConclusionsSolid solutions of NdCuxMn7−xO12 with x = 0, 0.1, 0.2, and 0.3 were synthesized by ahigh-pressure, high-temperature method at 6 GPa and 1500 K. The specific heat and mag-netic measurements uncovered three magnetic transitions at x = 0 (at TN3 = 9 K, TN2 = 12 K,and TN1 = 84 K), two transitions at x = 0.1 (at TN2 = 10 K and TN1 = 78 K), and only onetransition at x = 0.2 (at TN1 = 72 K) and x = 0.3 (at TN1 = 65 K). The DSC measurementsshowed sharp and strong peaks near TOO = 664 K at x = 0, while the DSC anomalies weresignificantly broadened and their intensities were significantly reduced in other samples.Structural transitions were detected near TOO = 630 K at x = 0.1, TOO = 600 K at x = 0.2,and TOO = 570 K at x = 0.3. The first magnetic transition temperature, the Curie–Weisstemperature, the structural transition temperature, and magnetization (at 5 K and 70 kOe)changed almost linearly with x. In addition, the Curie–Weiss temperature changed fromnegative (for x = 0, 0.1, and 0.2) to positive (for x = 0.3). The high-resolution synchrotronpowder X-ray diffraction and DSC studies provided strong evidence that a phase separa-Molecules 2025, 30, 4561 14 of 17tion phenomenon took place in the x = 0.1–0.3 samples, where two I2/m phases with anapproximate ratio of 1:1 were present.Supplementary Materials: The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30234561/s1: Table S1: Structure parameters oftwo monoclinic phases in NdCu0.3Mn6.7O12.Author Contributions: Conceptualization, A.A.B.; methodology, A.A.B.; validation, A.A.B.; formalanalysis, A.A.B.; investigation, A.A.B., R.L. and K.Y.; resources, K.Y.; data curation, A.A.B.; writing—original draft preparation, A.A.B.; writing—review and editing, A.A.B.; supervision, A.A.B. and K.Y.;project administration, A.A.B.; funding acquisition, K.Y. All authors have read and agreed to thepublished version of the manuscript.Funding: This work was partially supported by a Grant-in-Aid for Scientific Research (No. JP25K01657)from the Japan Society for the Promotion of Science.Institutional Review Board Statement: Not applicable.Data Availability Statement: The raw data supporting the conclusions of this article will be madeavailable by the author on request.Acknowledgments: The synchrotron radiation experiments were conducted at the former NIMSbeamline (BL15XU) of SPring-8 with the approval of the former NIMS Synchrotron X-ray Station(proposal number: 2020A4501). We thank M. Tanaka and Y. Katsuya for their help at SPring-8. MANAwas supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.Conflicts of Interest: The authors declare no conflicts of interest.References1. Mitchell, R.H. Perovskites: Modern and Ancient; Almaz Press: Thunder Bay, ON, Canada, 2002.2. 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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/10.1088/2053-1591/1/1/016306https://doi.org/10.2109/jcersj2.121.287https://doi.org/10.4028/www.scientific.net/MSF.321-324.198 Introduction  Results and Discussion  Magnetic Properties  High-Temperature Structural Transitions  Structural Properties  Materials and Methods  Conclusions  References