# Fileset
[Journal of Materials Chemistry C-Unexpected p-type thermoelectric transport arising from magnetic Mn substitution in Fe2V1-xMnxAl Heusler compounds.pdf](https://mdr.nims.go.jp/filesets/c0e3ef70-97bd-469b-aaa8-4f5e6a3fd72e/download)
## Creator
[Rajveer Jha](https://orcid.org/0000-0002-9481-8705), [Naohito Tsujii](https://orcid.org/0000-0002-6181-5911), Fabian Garmroudi, Sergii Khmelevskyi, Ernst Bauer, [Takao Mori](https://orcid.org/0000-0003-2682-1846)
## Rights
[Creative Commons BY-NC Attribution-NonCommercial 4.0 International](https://creativecommons.org/licenses/by-nc/4.0/)
## Other metadata
[Unexpected p-type thermoelectric transport arising from magnetic Mn substitution in Fe2V1−xMn xAl Heusler compounds](https://mdr.nims.go.jp/datasets/5226ad7d-072b-4919-8e65-4bd5b539c951)
## Fulltext
Unexpected p-type thermoelectric transport arising from magnetic Mn substitution in Fe2V1−xMnxAl Heusler compoundsThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8861Cite this: J. Mater. Chem. C,2024, 12, 8861Unexpected p-type thermoelectric transportarising from magnetic Mn substitutionin Fe2V1�xMnxAl Heusler compounds†Rajveer Jha,a Naohito Tsujii, *a Fabian Garmroudi,b Sergii Khmelevskyi, cErnst Bauer b and Takao Mori *adp-Type Fe2VAl-based thermoelectrics have been much less investigated compared to their respectiven-type counterparts. Thus, it is crucial to identify novel doping strategies to realize enhanced p-typeFe2VAl Heusler compounds. In the current study, the effect of Mn substitution in Fe2V1�xMnxAl isinvestigated with respect to temperature-dependent electronic transport as well as temperature- andfield-dependent magnetic properties. We find an anomalous and unexpected p-type Seebeck coefficientfor nominally n-doped Fe2V1�xMnxAl over an extremely large range of concentrations up to x = 0.6.Using density functional theory (DFT) calculations, this is traced back to distinct modifications of theelectronic structure, i.e., localized magnetic defect states (m = 2.43mB) at the valence and conductionband edges, and a concomitant pinning of the Fermi level within the pseudogap. Furthermore, we wereable to further optimize the thermoelectric properties by co-doping Al antisites in off-stoichiometricFe2V0.9Mn0.1Al1+y, yielding sizeable values of the power factor, PF = 2.2 mW K�2 m�1 in Fe2V0.9Mn0.1Al1.1at 350 K, and figure of merit, ZT B 0.1 for highly off-stoichiometric Fe2V0.9Mn0.1Al1.5 at T = 500 K.Our work underlines the prospect of engineering Fe2VAl-based Heusler compounds via magneticdoping to realize enhanced p-type thermoelectris and encourages studies involving other types of co-substitution for Mn-substituted Fe2V1�xMnxAl.IntroductionThermoelectric (TE) materials can convert waste heat intoelectric energy, or vice versa, by utilizing the Seebeck effect,thus representing a viable option for generating eco-friendlyenergy to address the current energy crisis.1–4 A crucial task,however, lies in further improving the conversion efficiency ofthermoelectric materials.5,6 The efficiency of TE materials isdetermined by a dimensionless figure of merit (ZT) = (S2/rk) �T, where S is the Seebeck coefficient, r the electrical resistivity,k = (kelec+ kph) the thermal conductivity, consisting of electronand lattice contributions, and T the temperature. These keyproperties have tradeoffs that limit progress in enhancing ZT,and it is necessary to develop new enhancement principles.7–9Among several material classes, the Heusler-type intermetalliccompound Fe2VAl is a promising candidate for thermoelectricpower generation near room temperature.10,11 This alloy has thecubic L21-ordered crystal structure and exhibits a high thermo-electric power factor PF = S2/r near room temperature.12 Fe2VAlshows a semiconductor-like temperature dependence of the elec-trical resistivity in a wide temperature range up to 1200 K or above.10The electronic structure of stoichiometric Fe2VAl suggests a deeppseudogap at the Fermi level.13 Stoichiometric Fe2VAl is a nonmag-netic, low carrier, and compensated semimetal.13–18 Recently, it wasshown that rational co-substitution can tune the band gap andtherefore significantly improve the thermoelectric properties, yield-ing high power factors up to around 10.3 mW K�2 m�1.19–22However, the lattice thermal conductivity of Fe2VAl is about oneorder of magnitude larger than that of other high-performancethermoelectric materials, resulting in much smaller ZT valuescompared to the best-performing systems at room temperature likeBi2Te3 or Mg3Sb2-based compounds.6,23–28In order to further increase ZT of Fe2VAl-based systems,various strategies have been employed already, including ele-mental substitution at all lattice sites, tuning the microstruc-ture and grain size as well as thin film deposition.29–50Particularly, B. Hinterleitner et al. reported an ultrahigh powera Research Center for Materials Nanoarchitectonics (MANA), Nanomaterials Field,National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047,Japan. E-mail: tsujii.naohito@nims.go.jp, mori.takao@nims.go.jpb Institute of Solid State Physics, TU Wien, A-1040 Vienna, Austriac Vienna Scientific Cluster Research Center, TU Wien, A-1040 Vienna, Austriad Graduate School of Pure and Applied Sciences, University of Tsukuba,Tennodai 1-1-1, Tsukuba 305-8671, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc00779dReceived 27th February 2024,Accepted 22nd May 2024DOI: 10.1039/d4tc00779drsc.li/materials-cJournal ofMaterials Chemistry CPAPERhttps://orcid.org/0000-0002-6181-5911https://orcid.org/0000-0001-5630-7835https://orcid.org/0000-0001-7376-5897https://orcid.org/0000-0003-2682-1846http://crossmark.crossref.org/dialog/?doi=10.1039/d4tc00779d&domain=pdf&date_stamp=2024-06-03https://doi.org/10.1039/d4tc00779dhttps://doi.org/10.1039/d4tc00779dhttps://rsc.li/materials-c8862 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 2024factor, above 45 mW K�2 m�1, and ZT around 5 at 350 K forFe2V0.8W0.2Al metastable thin films.50 Also, doping of Fe2VAlp-type Heusler compounds with various elements, such as Tiand Zr, has been studied to understand the effects of differentdopants on the thermoelectric performance.40,51,52 However,the performance of p-type Fe2VAl-based thermoelectrics remainsstill far below those of the best n-type compounds. Therefore, it iscrucial to identify novel p-type candidate materials and dopingelements.Furthermore, it has been demonstrated that spin fluctua-tions can contribute significantly to optimize the power factor(PF = 1.2 mW K�2 m�1 at 400 K) in weakly ferromagneticFe2V0.9Cr0.1Al0.9Si0.1 and Fe2.2V0.8Al0.6Si0.4.53 That being said,likewise to spin fluctuations, the usage of magnetism indifferent forms to enhance the Seebeck coefficient is alsorapidly increasing; for example, spin Seebeck effect, anomalousNernst effect, magnon drag, paramagnon drag, spin entropy,etc.54–61 are most recently studied instances.Thus, magnetic doping can be a useful strategy in improvingthermoelectric properties. In the case of Fe2VAl-based com-pounds, however, the effect of magnetic element doping withrespect to thermoelectric properties, has not been well studied.This motivated us to substitute V/Mn in Fe2V1�xMnxAl.We observed a unusual hole-dominated Seebeck coefficient innominally n-doped Fe2V1�xMnxAl, which increases with increas-ing Mn content up to x = 0.2. Employing first principles electronicstructure calculations within the framework of the density func-tional theory62 and the coherent potential approximation,63 usedto model the effects of the substitutional atomic disorder, we tracethis back to a severe modification of the electronic structure,wherein magnetic impurity states are formed by Mn atoms,leading to a pinning of the Fermi energy near the valence bandedge and within the pseudogap.Moreover, we additionally co-substituted Al antisites at theFe and V sites in Al-rich Fe2V0.9Mn0.1Al1+y samples to furtheroptimize the thermoelectric performance. Since p-type Fe2VAlHeusler compounds have been reported to a lesser contentcompared to n-type systems, we systematically studied the effectof combined Al off-stoichiometry and Mn co-substitution in p-typeFe2V1�xMnxAl1+y. As an extra finding, the thermal conductivity issubstantially suppressed already in single-substituted Fe2V1�x-MnxAl and even further by introducing additional Al off-stoichiometry in Fe2V1�xMnxAl1+y. Moreover, the latter shows asizeable enhancement of the thermopower and a shift of themaximum towards higher temperatures, together with an un-expectedly low electrical resistivity. It is crucial to focus on andoptimize p-type full-Heusler systems suitable for applications inthermoelectric power generation, because the well-studied n-typematerial must be paired with a p-type counterpart to assembleefficient thermoelectric modules.Experimental & computational detailsHigh-purity elements of Fe (99.98%), Mn (99.99%), V (99.9%),and Al (99.999%), were weighted in the stoichiometric ratio andwere arc-melted in Ar atmosphere on a water-cooled copperhearth. The ingots were turned over on the copper hearth andwere re-melted for four times to ensure homogeneity. Theobtained ingots were loaded to a tungsten carbide jar in theglove box under an Ar atmosphere and milled to a fine powderin a planetary ball mill at a speed of 200 rpm for 5 hours. Theobtained fine powders were loaded in a graphite die andconsolidated by spark plasma sintering (SPS, SPS-1080 System,SPS SYNTEX INC) under a pressure of 40 MPa at 1273 K for10 minutes under static vacuum (2 � 10�2 Pa). We performedpost-annealing for all the samples after the SPS sintering thesamples were sealed in quartz tubes for high-temperatureannealing at 1273 K for 24 hours and then slowly cooled downto 673 K in 48 hours, where the samples were further annealedfor 48 hours and finally naturally cooled to room temperature.Powder X-ray diffraction measurements were performed tocheck the phase purity and lattice parameters of the cubicHeusler compounds using a RINT TTR-3 diffractometer (RigakuCo., Akishima, Tokyo, Japan) and employing Cu Ka radiation.Experimental data were further analyzed by the Rietveld refine-ment method using the program RIETAN-FP.64 Finally, bar-shaped samples were cut out of the disk samples and used tomeasure the Seebeck coefficient and electrical resistivity. Mea-surements were done with a four-probe method on a commer-cial system ZEM3 (Advance Riko Inc.). The thermal conductivityk was calculated by using k = DCpd, where D is the thermaldiffusivity, Cp is heat capacity, and d is density. The thermaldiffusivity coefficient (D) and the heat capacity (Cp) of bulkmaterial were concurrently measured for the disk sample on axenon laser flash system (Netzsch LFA 467, Germany) with apyro Ceram disk as a reference sample. Magnetization mea-surements were performed by using a superconducting quan-tum interference device magnetometer from Quantum Design.Density functional theory calculations of the electronicstructure of Fe2V1�xMnxAl were performed using the coherentpotential approximation (CPA) in the framework of the Kor-ringa–Kohn–Rostoker (KKR) method and the atomic sphereapproximation (ASA).65,66 In our KKR–ASA calculations thepartial wave functions were expanded in a spdf-basis (up tol = 3) and the effects of exchange and correlation are treatedwithin the density functional theory using standard PBEexchange–correlation functionals.62 To model thermal mag-netic disorder above the magnetic ordering temperature, weemployed the disordered local moments (DLM) formalism.67Results and discussionFig. 1(a) and (c) shows the powder X-ray diffraction patternsfor Fe2V1�xMnxAl and Fe2V0.9Mn0.1Al1+y samples, respectively.All the samples crystallize in the Heusler (L21) type structure;no impurity peak or secondary phase has been observed. The(111) peak at around 271 is not very pronounced and is furthersuppressed with increasing Mn or Al concentration in Fe2V1�x-MnxAl or Fe2V0.9Mn0.1Al1+y [see Fig. S1(a) and (b) in the ESI†].The suppressing of the (111) peak suggests that disorder in thePaper Journal of Materials Chemistry CThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8863Fe2VAl compound becomes more significant with doping.Because the 111 peak is mainly due to the difference in thescattering factors of the Al and the V-sites, our findings indicatethat an inter-atomic B2-type disorder exists between the twosites. The (220) peak at around 44.51 for Fe2V1�xMnxAl andFe2V0.9Mn0.1Al1+y is shown in Fig. 1(b) and (d), indicating asteady increase in the lattice parameter with increasing Mn/Alconcentration. The broadening of (220) peak with increasingMn concentration indicates that disorder is increasing inMn-doped Fe2VAl compounds. Also, the grain size reductionis another possibility of broadening of (220) peak. Moreover,the lattice parameter was obtained by Rietveld analysis for allsamples. Fig. 1(e) and (f) exhibits the lattice parameters as afunction of the concentration x and y for the Mn-doped andAl-rich samples, respectively. The increase in the lattice para-meter with Mn doping is basically in line with Vegard’s law.Also, we observed a slight deviation from Vegard’s law for highMn concentration, i.e., x = 0.6.68 Furthermore, we foundthat the lattice parameter for Al-rich Fe2V0.9Mn0.1Al1+y isincreasing with increasing Al doping. The increase of thelattice parameter with y in Fe2V0.9Mn0.1Al1+y does not followVegard’s law. Moreover, the lattice parameter increasesrapidly for y = 0.5, which deviates from previously reportedresults for the Al-rich Fe2VAlx.69 Nonetheless, the overalltendency of an increasing lattice parameter is consistent withthat reported on Al-rich Fe2VAlx, although the absolute valuesare smaller compared to those reported previously. The lineartrend observed in the lattice parameter indicates that Mnatoms successfully substitute the V site; the solubility limitof Al in Al-rich Fe2V0.9Mn0.1Al1+y is y = 0.5. In comparison, thesolubility limit of Al in pure Fe2VAlx was found to be x = 2 inref. 64. For Al-rich Fe2V0.9Mn0.1Al1+y, the off-stoichiometrymanifests itself through the presence of Al antisites on theFe and V sites.69 Overall, the crystallographic site occupanciescan considerably affect the physical properties of Heuslercompounds.70Fig. 1 (a) XRD patterns for Fe2V1�xMnxAl sintered samples, with the numbers in the XRD pattern indicating Miller indices. (b) Zoomed view of (220) peakat around 44.51 for Fe2V1�xMnxAl. (c) XRD patterns for Fe2V0.9Mn0.1Al1+y sintered samples. (d) Zoomed view of (220) peak at around 44.51 forFe2V0.9Mn0.1Al1+y. (e) The lattice parameter was obtained by a refinement using the Rietveld method with RIETAN-FP64 of Fe2V1�xMnxAl as a function ofconcentration x of Mn, and the solid line represents the linear Vegard’s law. (f) Lattice parameter obtained by a refinement of Fe2V0.9Mn0.1Al1+y as afunction of y.Journal of Materials Chemistry C Paper8864 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 2024Fig. 2 depicts the temperature-dependent thermoelectricproperties of Fe2V1�xMnxAl. Firstly, the electrical resistivity atT = 300 K shows a slight increase as x increases up to x = 0.1,then gradually decreases with further increasing the Mnconcentration in Fe2V1�xMnxAl [Fig. 2(a)]. We note that theSeebeck coefficient for stoichiometric Fe2VAl (x = 0) is slightlynegative in this study, whereas usually sizeable positive valueshave been reported for samples synthesized via arc or inductionmelting.45,71 This discrepancy can likely be explained by off-stoichiometry or antisite disorder induced during the synthesisvia ball milling and has been found by other groups as well.72,73However, the Seebeck coefficient becomes p-type upon substi-tuting Mn at the V site in Fe2V1�xMnxAl, even though n-typedoping would be expected from a simple estimation of theeffective valence electron concentration per atom, indicatingnon-rigid band doping behavior. The absolute value of theSeebeck coefficient reaches S = 64 mV K�1 at 300 K forFe2V1�xMnxAl with x = 0.2 [Fig. 2(b)]. For higher Mn-dopedsamples, the Seebeck coefficient decreases down to S =16 mV K�1 at 300 K for Fe2V1�xMnxAl (x = 0.6). For non-dopedFe2VAl, the Seebeck coefficient is almost temperature-independent,and the absolute value is near to zero. Again, these resultsdeviate significantly from those obtained in melted samplesand suggest the presence of defects such as antisites in thepresent sample. The highest value of the power factor PF =0.8 mW K�2 m�1 [Fig. 2(c)] is obtained for Fe2V0.8Mn0.2Al andpeaks near room temperature, like all the other samples fromFe2V1�xMnxAl.The total thermal conductivity of Fe2V1�xMnxAl [Fig. 2(d)], k,is nearly temperature-independent, and the values decreasesubstantially upon Mn doping in Fe2V1�xMnxAl. The totalthermal conductivity at room temperature for stoichiometricFe2VAl is 16 W m�1 K�1, which is significantly lower than formelted samples (B25–30 W m�1 K�1),74,75 again indicating anincreased number of defects such as antisite disorder, latticestrain, and grain boundaries that likely arise from the synthesisvia ball milling. For Fe2V1�xMnxAl (x = 0.6), the thermalconductivity further decreases down to 9 W m�1 K�1.In general, the total thermal conductivity k for ordinary metalsand semimetals is the sum of electronic and phonon terms,which can be specified as k = kelec+ kph, where kph is the latticethermal conductivity and kelec is the electronic contribution,although a bipolar share should be expected when minoritycarriers actively contribute to transport. Since previous studieson Fe2VAl-based systems have neglected the bipolar contribu-tion, we align our investigation with those works andonly distinguish between lattice and electron contributions,where kel is related to the electrical resistivity r as given bythe Wiedemann–Franz law kel = L0T/r, where L0 = 2.45 �10�8 WO K�2 is the Lorenz number, and r is the electricalresistivity. The lattice thermal conductivity (kph) can beobtained by subtracting kel from the total thermal conductivity(k). Fig. 3(a) shows kph as a function of temperature. At roomtemperature, kph drops from 14 to 7.5 W m�1 K�1 due to the Mnsubstitution, which is a significant reduction compared to pre-viously reported values for doped Fe2VAl systems,12,30,74,76–78 con-sidering that there are no significant differences in atomic massand size between Mn and V atoms. Meanwhile, it is obvious thatkelec is smaller than kph for all the studied compounds, whichsignifies that the total thermal conductivity is dominated by latticephonons rather than charge carriers. This implies that a substan-tial further enhancement can be achieved by further suppressingFig. 2 Thermoelectric properties of Fe2V1�xMnxAl (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) (a)–(d) temperature dependence of electrical resistivity r, Seebeckcoefficient S, power factor PF, and total thermal conductivity k, respectively.Paper Journal of Materials Chemistry CThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8865heat conduction through phonons, e.g. by tuning the microstruc-ture or via co-substitution as employed in the second part of thisstudy. The dimensionless figure of merit, ZT, was determined andreaches its highest value ZT = 0.025, for Fe2V1�xMnxAl (x = 0.4)at about T = 350 K [Fig. 3(b)].In order to further improve the TE properties of Mn-substituted Fe2V1�xMnxAl, additional co-substitution/off-stoichiometry with Al at the Fe and V sites has been performed.This type of off-stoichiometry has been previously shown to notonly significantly increase the p-type Seebeck coefficientof Fe2VAl-based thermoelectrics but also greatly reduces thelattice thermal conductivity via substitutions on differentsublattices.69 Fig. 5 demonstrates the temperature-dependentthermoelectric properties of Al-rich Fe2V0.9Mn0.1Al1+y. Indeed,within this study we have observed that r decreases withincreasing Al concentration (see Fig. 5(a)). Furthermore, thetemperature-dependent behavior of r(T) changes fromsemiconductor-like, dr/dT o 0, towards metallic-like,dr/dT 4 0. It is interesting to note, however, that r reachesits lowest values for Fe2V0.9Mn0.1Al1.3 (y = 0.3) and increasesagain dramatically as y further rises up to y = 0.5, concomitantwith an unexpected decrease of the carrier concentration. Thisindicates peculiar changes in the electronic structure andclearly shows that such Al off-stoichiometry cannot be regardedas a conventional doping mechanism.Regarding the Seebeck coefficient, the additional off-stoi-chiometry clearly has a beneficial effect and not only increasesthe absolute values of the Seebeck coefficient but also modifiesthe overall temperature-dependent behavior such that S stayslarge over a broader temperature interval (see Fig. 5(b)). Uponincreasing the amount of Al in Fe2V0.9Mn0.1Al1+y, the maximumof |S| shifts from below room temperature up to almost 500 Kfor Fe2V0.9Mn0.1Al1.5 (y = 0.5). Moreover, the peak value S =82 mV K�1 for Fe2V0.9Mn0.1Al1.5 (y = 0.5) is about 20% larger thanthe maximum value of the Fe2V1�xMnxAl series, S = 61 mV K�1for x = 0.2 (shown in Fig. 3(b)). Hence, as expected, the Al-excessin terms of antisites on Fe and V sublattices has brought anenhancement in the Seebeck coefficient, consistent with find-ings in ref. 69. The value S = 82 mV K�1 is comparable to thelargest ones in the p-type Fe2VAl-based systems12,19,44–49,69,74and establishes such type of Al-off stoichiometry andconcomitant co-substitution, e.g., with V/Mn, as a desirableroute to improve the ZT of p-type full-Heusler systems. Thetemperature dependence of the Seebeck coefficient shows amaximum at around T = 350 K for y = 0.1 and 0.2, and aroundT = 450 K for x = 0.3 and 0.5. The maximum for y = 0 is not seenin Fig. 4(b), but is expected below T = 300 K. Thus, themaximum temperature systematically increases with increasingy. This reflects the increase in the hole carrier concentration byextra Al doping and also hints at a possible expansion of theband gap.Compared to the single substitution series Fe2V1�xMnxAl,where the power factor peaks at 0.8 mW K�2 m�1, PF issignificantly improved up to 2.0 mW K�2 m�1 for Fe2V0.9-Mn0.1Al1.1. However, the power factor decreases with increasingAl concentration in Fe2V0.9Mn0.1Al1+y, hovering around PF =1.6–1.8 mW K�2 m�1 at 450–500 K for y = 0.3 and 0.5. Thethermal conductivity of Fe2V0.9Mn0.1Al1+y shows only weaktemperature dependence for the measured temperature range300–550 K (see Fig. 5(d)). The thermal conductivity atroom temperature for stoichiometric Fe2V0.9Mn0.1Al, is12.1 W m�1 K�1, while it decreases monotonically as a functionof y down to 9 W m�1 K�1 for Fe2V0.9Mn0.1Al1+y (y = 0.5). Weobserved an almost 40% drop in thermal conductivity forFe2V0.9Mn0.1Al1+y (y = 0.5) compared to the stoichiometric Fe2VAlcompound, despite the lack of any heavy element substitution inthe investigated samples. The significant decreases in thermalconductivity with increasing Al in Fe2V0.9Mn0.1Al1+y is consistentwith the findings in ref. 69 and is assumed to be stemming fromthe sizeable atomic disorder and point defects brought about by thelarge number of Al/Fe and Al/V antisites.The temperature dependence of kph is shown in Fig. 5(a),which was again obtained by applying the Wiedemann–Franzlaw to estimate the electronic contribution from the experi-mental electrical resistivity. Overall, the thermal conductivity isphonon-driven, while the kelec contribution increases for Al-richsamples. As discussed in the previous section, the electricalresistivity was further decreased, and the Seebeck coefficientenhanced for the Al-rich Fe2V0.9Mn0.1Al1+y samples, Conse-quently, the power factor has improved, and thermal conductivitydecreased for Al-rich Fe2V0.9Mn0.1Al1+y samples. Moderately, the ZTvalues have been improved for Fe2V0.9Mn0.1Al1+y (y = 0.5), with theFig. 3 (a) Temperature dependence of lattice thermoelectric conductivity for Fe2V1�xMnxAl. (b) The thermoelectric performance, ZT values vs.temperature for Fe2V1�xMnxAl.Journal of Materials Chemistry C Paper8866 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 2024highest ZT value 0.1 achieved for Fe2V0.9Mn0.1Al1+y (y = 0.5) ataround T = 500 K [Fig. 5(b)].Furthermore, we have investigated the valence electronconcentration (VEC) dependence of the Seebeck coefficient (S)at T = 300 K for Fe2V1�xMnxAl and Fe2V0.9Mn0.1Al1+y. Fe2VAl has24 valence electrons per formula unit, thereby VEC = 6.40 Asaliovalent elements are doped in the Fe2VAl system, the VECdeviates from 6, which affects largely the Seebeck coefficient.For Mn-doped Fe2V1�xMnxAl, the VEC scenario is not in agree-ment with the behavior observed within the majority of single-element doping studies reported in the literature so far; rather,we have observed a p-type Seebeck coefficient for the VEC 4 6.0(see Fig. 7(a)). The universal curve based on the VEC is onlyvalid for the rigid band case.40 However, substitutions on the Fe-or V-sites with certain transition metals, can cause significantchanges in the electronic band structure; thereby, the simple VECrule is no longer valid. For instance, off-stoichiometric Heusleralloys, Fe2�xV1+x�yTiyAl (p-type Seebeck coefficient), have beeninvestigated using soft X-ray photoelectron spectroscopy.79 Theelectronic structure near the Fermi level is found to be drasticallyaltered by off-stoichiometry, while doping with Ti has a moremoderate effect in a rigid-band-like manner.79 In the case of off-stoichiometric Fe2�xV1+x Al-based Heusler alloys, Soda et al.proposed that the excess V or Fe (antisite) defects may induceimpurity states near the valence band edge, resulting in a potentialenhancement of the Seebeck coefficient.80 To elucidate the anom-alous p-type Seebeck coefficient in nominally n-doped Fe2V1�x-MnxAl, we have performed extensive density functional theorycalculations of the alloy-averaged densities of states of substitutedFe2V1�xMnxAl, which are summarized in the subsequent section.Our theoretical investigations reveal that the Fermi level remainspinned near the valence band edge inside the pseudogap, owing tothe creation of additional electronic states below EF, which bal-ances the chemical potential.Fig. 5 (a) Temperature dependence of lattice thermoelectric conductivity for Fe2V0.9Mn0.1Al1+y. (b) The thermoelectric performance, ZT values vs.temperature for Fe2V0.9Mn0.1Al1+y.Fig. 4 Thermoelectric properties of Fe2V0.9Mn0.1Al1+y (y = 0, 0.1, 0.2, 0.3 and 0.5) (a)–(d) temperature dependence of electrical resistivity r, Seebeckcoefficient S, power factor PF, and total thermal conductivity k, respectively.Paper Journal of Materials Chemistry CThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8867To further shed light on the anomalous doping dependenceof Fe2V1�xMnxAl and Fe2V0.9Mn0.1Al1+y, we have measured theHall resistivity to estimate the carrier concentration.The magnetic field-dependent Hall resistivity for Fe2V1�xMnxAland Fe2V0.9Mn0.1Al1+y at T = 300 K is shown in the ESI† [seeFig. S7]. A linear magnetic field dependence rxy was observed forall samples. The Hall coefficient RH was then determined fromthe slope of rxy versus the applied magnetic field B. The carrierconcentration (p) was estimated using the simple equationp = 1/eRH. All the samples (except for x = 0 and y = 0) are p-type,and the hole carrier concentration appears to increase monotoni-cally with doping, except for y = 0.5.The Seebeck coefficient as a function of carrier concen-tration at room temperature, known as the Ioffe–Pisarenkoplot, is shown in Fig. 7(b). The results here deviate from theIoffe–Pisarenko curve, especially in the heavily doped region.Although a general trend with a maximum is similar, a cleardifference is that the peak is much wider than the normal IPcurve, with a shift of maximum to higher carrier concentra-tions. This is consistent with an increase of the density of stateseffective mass. Indeed, DFT calculations reveal that ratherlocalized electronic states are formed at both sides of the Fermilevel and that the slope of the energy-dependent density ofstates is enhanced, which is in good agreement with experi-mental findings. Similar features were reported by Parzer et al.for the Al-rich Fe2VAl system, where they suggested that a shiftin a maximum of S vs. carrier concentration occurs due toresonant states located close to the Fermi level.69 Therefore,Mn- and extra Al-doping can be a very fast and effective way toincrease the Seebeck coefficient through disorder-related reso-nant levels in the Heusler compounds. In the present case, theMn- and Al-doping provoked too much hole carriers, therebythe enhancement in the Seebeck coefficient was just moderate.It should also be stressed that thermally quenched samples ofFe2VAl showed only n-type behavior and an effective strategyfor enhancing the p-type Heusler compound has not beenreported. The present findings, Mn- and/or excess Al-dopinginduced disorder may cast a new way for achieving p-type highpower factor.The Hall mobility was derived using mH = rxy/(rxxB) [seeTable S2, ESI†]. mH decreases by increasing the Mn dopingconcentration in Fe2V1�xMnxAl or increasing Al in Fe2V0.9-Mn0.1Al1+y due to the increasing charge carrier density and/orincreasing static disorder in the crystalline unit cell.Fig. 7 shows the Seebeck effective mass (m�s ) as a function ofhole carrier concentration p estimated at T = 300 K forFe2V1�xMnxAl and Fe2V0.9Mn0.1Al1+y. m�s was estimated usingeqn (S1) (ESI†), where the m�s values are directly proportional tothe Seebeck coefficient and the carrier concentration (p).81The formula used was taken from ref. 81 where it is suggestedthat there are two classes of effective masses; one describesan inertial effective mass that is relevant to a specific electron,and the other one depends more on the density of the electronstates (DOS), and is hence called a DOS mass, m�DOS. Further,m�DOS can be directly calculated from the Seebeck coefficientemploying eqn (S2) (ESI†); thus, m�DOS can be consideredas Seebeck effective mass (m�s ). We found that with increa-sing carrier concentration, the Seebeck effective mass m�sincreases linearly in the case of Fe2V1�xMnxAl, while for theFig. 6 (a) The Seebeck coefficient as a function of valence electron concentration (VEC) at 300 K. The dotted lines are doped off-stoichiometric Fe2VAldata taken from ref. 78. (b) The Seebeck coefficient as a function of hole carrier concentration p estimated at T = 300 K for Fe2V1�xMnxAl andFe2V0.9Mn0.1Al1+y with Ioffe–Pisarenko cure.Fig. 7 The Seebeck effective mass (m�s ) as a function of hole carrierconcentration p estimated at T = 300 K for Fe2V1�xMnxAl and Fe2V0.9-Mn0.1Al1+y. The dotted line represents a linear trend in m�s with increasingMn doping in Fe2V1�xMnxAl.Journal of Materials Chemistry C Paper8868 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 2024Fe2V0.9Mn0.1Al1+y, it slightly deviates from linearity. Accordingto literature, the tendencies of the Seebeck effective mass m�s atroom temperature and above upon doping are straight-forwardly explained by changes in the band structure, e.g.,band convergence or changes in the dominant scatteringmechanism. In fact, it has been mentioned for Al-rich Fe2VAlxsystems that a local distortion of the electronic density of statesarises for very large Al concentrations.69 Again, the increase ofm�s is fully consistent with first-principles calculations, reveal-ing a modification of the alloy-averaged densities of states,where both conduction and valence band edges become moresteep. Thus, the density of states effective mass increases due tothe rather localized impurity states by Mn, which spawn at bothsides of the Fermi energy.As can be seen in Fig. 6(a), p-type full Heusler materials havea smaller absolute value of the Seebeck coefficient compared ton-type materials. Additionally, it’s challenging to obtain p-typematerials due to a more restricted number of doping elements.Mn doping Al-rich Fe2VAl could lead to improved thermo-electric properties in p-type materials near room temperature.The achieved power factor around 2.2 mW K�2 m�1 at T =350 K, involving a Mn-doped Al-rich Fe2VAl strategy, is largerthan Ta-doped or Al-rich Fe2VAl systems, while it is lower thanfor off-stoichiometric and Ti-doped samples, reaching up toB4 mW K�2 m�1.12,52 In the present cases, the highest ZT valueof 0.05 was achieved for Fe2V0.9Mn0.1Al1+y (y = 0.1, 0.2) nearroom temperature. The value of power factor for Mn-dopedAl-rich Fe2V0.9Mn0.1Al1+y (y = 0.1) around 2.2 mW K�2 m�1 is arelatively high value for p-type Heusler compound near roomtemperature. The explored compounds could be useful forapplications in thermoelectric power generation.The magnetic properties of selected samples were alsoinvestigated. Fig. 8(a) depicts the temperature-dependent mag-netization M under an applied external magnetic field B = 1 Tfor the Fe2V1�xMnxAl (x = 0, 0.1, 0.2, 0.4) samples in thetemperature range 2–300 K. We fitted the inverse of themagnetic susceptibility H/M vs. temperature measured at B =1 T for x = 0, 0.1, 0.2 and 0.4 in the paramagnetic region above200 K employing the Curie–Weiss function, H/M = (T � y)/C,where C and y are the Curie constant and Weiss temperature,respectively [see Fig. 8(b)]. From least squares fit, the obtainedvalues of parameters C, and y are summarized in Table 1.Because of the decreasing Curie constants upon an increasingMn content, the overall susceptibility decreases. The value of ycan be seen as a measure of the inter-site magnetic interactionin the case of a molecular-field approximation. Thus, thesamples with x = 0 and 0.1 are weakly ferromagnetic, with aWeiss temperature y = 35 K and y = 50 K, respectively, whichcan most likely be attributed to intrinsic antisite defects invol-ving the Fe sublattices.A further increase of x, however, causes a crossover of y tonegative values (x = 0.4: yB�16 K), indicating that the growingMn content triggers eventually antiferromagnetic interactions.This is very much in line with our first-principles calculationsof the exchange interactions between Mn atoms shown in theESI† (Table S7).The derivative of magnetization dM/dT vs. T shows a peak ataround 16 K for undoped Fe2VAl and 30 K for x = 0.1, whichsuggests a Curie temperature TC B 16 K for x = 0 and TC B 30 Kfor x = 0.1 [see ESI,† Fig. S6(a)]. The isothermal magnetizationat T = 2 K decreases rapidly with increasing Mn concentrationin Fe2V1�xMnxAl for x Z 0.2. This decrease in magnetization isFig. 8 (a) The temperature dependence of magnetization M for Fe2V1�xMnxAl (x = 0, 0.1, 0.2, 0.4), (b) The inverse of magnetic susceptibility H/M vs. T forFe2V1�xMnxAl (x = 0, 0.1, 0.2, 0.4). The dotted line shows the fitting to the Curie–Weiss function H/M = (T � y)/C with C = NPeff2/3kB, where Peff, N, and kBare the effective magnetic moment, number of magnetic ions and the Boltzmann constant, respectively. (c) and (d) The magnetization M vs. B curves forFe2V1�xMnxAl (x = 0, 0.1, 0.2, 0.4) at 2.0 K and 300 K.Paper Journal of Materials Chemistry CThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8869expected due to the antiferromagnetic correlation between Mnatoms. The higher Mn concentration, for example, the x Z 0.2samples, probably shows a ferrimagnetic nature, which isanalogous to the ferrimagnetism reported in the Fe2MnAlcompound.82 However, above the Curie point, the inversemagnetic susceptibility is no longer following the Néel hyper-bola. This deviation is due to the limitations of magnetizationmeasurements in the paramagnetic state of the alloys withoutperfect atomic ordering. The effective magnetic moments eval-uated from the respective Curie constants C are meff = 3.44mB peratoms for x = 0, meff = 2.87mB per atoms for x = 0.1, meff = 2.69mBper atoms for x = 0.2, and meff = 1.85mB per atoms for x = 0.4. Themeff values for x r 0.2 are close to those calculated forFe2V0.9Cr0.1Al and other Fe2VAl-based full-Heusler compoundsetc.31,53,83Beyond that, it is interesting to point out that the Seebeckcoefficient of the Mn-doped Fe2V1�xMnxAl exhibits its max-imum value of about 60 mV K�1 at 300 K for a wide range of Mnconcentrations, x = 0.1 to 0.4, as seen in Fig. 2(b). This plateau-like behavior cannot be explained by a normal doping depen-dence (see Fig. 6(a)). In addition, the magnetic interaction isalso found to be reduced by Mn-doping, as is seen in Fig. 8. Theundoped Fe2VAl shows large effective magnetic moment meff =3.44mB per atoms, which suggests off-stoichiometry or Fe anti-site defects is present in the sample. The values of effectivemagnetic moments decrease with increasing Mn doping levelsin Fe2V1�xMnxAl. We can assume that at low Mn doping level,isolated magnetic ions might act as local perturbations, buttheir influence on the overall magnetic interaction might beminimal. As the Mn doping level increases, the competinginteractions could become more pronounced, potentially lead-ing to a more significant reduction in magnetic interaction.Consequently, the Mn-doped samples need to assume otherchanges in the electronic structure emerging from the substitu-tion of Mn atoms at the V sublattice that cause an enhancementof the Seebeck coefficient.The magnetization (M) as a function of applied magneticfield (B) for Fe2V1�xMnxAl (x = 0.1, 0.2, 0.4) is shown in Fig. 8(c)and (d) at 2.0 K and 300 K, respectively. The magnetizationcurves up to B = 5 T reveal a gradual change, fromferromagnetic-like features (x = 0.1) towards a paramagneticbehaviour for x = 0.4. Most importantly, the magnetizationdecreases with increasing Mn concentration. It might be pos-sible that introducing Mn magnetic ions at the V site in Fe2VAlcould create several competing magnetic interactions. Depend-ing on the specific magnetic ion and its interactions with Feand Al, these competing interactions might partially cancel outthe ferromagnetic order between Fe atoms, leading to a netreduction in the overall magnetic interaction strength. Also, wehave studied magnetic properties for Al-rich Fe2V0.9Mn0.1Al1+ycompounds [see ESI,† Fig. S5(a)–(d)].Density functional theory calculationsTo further elucidate the anomalous behavior of Mn substitu-tion in Fe2V1�xMnxAl, both with respect to the thermoelectrictransport and magnetic properties, we performed extensivedensity functional theory (DFT) calculations on substitutedFe2V1�xMnxAl Heusler compounds across different concentra-tions x. Our calculations indicate that Mn atoms, when sub-stituted at the V site, exhibit a large local magnetic moment ofaround 2.43mB, while they stay non-magnetic, when substitutedat the Fe sites.To simulate the true paramagnetic state with thermallydisordered magnetic moments at high temperatures, calcula-tions were performed within the framework of the disorderedTable 1 Obtained results from Curie–Weiss function H/M = (T � y)/C forthe Fe2V1�xMnxAl samplesSamples C (K emu mol�1) meff (mB per atoms) y (K)x = 0 5.88 3.44 35x = 0.1 4.05 2.87 50x = 0.2 3.59 2.69 �2.5x = 0.4 2.46 1.85 �14.6Fig. 9 (a) Spin-polarized density of states of a single Mn atom in Fe2V0.9Mn0.1Al, showing a sizeable splitting between majority and minoritycontributions, yielding a large magnetic moment m B 2.43mB for Mn atoms substituted at the V site. (b) Partial atom-projected density of states ofFe2V0.9Mn0.1Al.Journal of Materials Chemistry C Paper8870 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 2024local moment (DLM) approximation.67 Moreover, the substitu-tional alloy disorder due to Mn atoms randomly replacing Vatoms in the unit cell, is fully accounted for in the calculationsof the electronic structure by making use of the CoherentPotential Approximation (CPA).63 Fig. 9(a) shows the atom-projected and spin-polarized density of states of a single Mnatom in Fe2V0.9Mn0.1Al. A sizeable splitting between majorityand minority spin channels is obvious, resulting in a largemagnetic moment of around 2.43mB. Interestingly, both min-ority- and majority-spin contributions exhibit a gap at the Fermienergy. Moreover, rather sharp and localized features in theelectronic density of states occur at both sides of the Fermi levelfor the majority-spin bands. These impurity states contribute tothe total DOS of Fe2V1�xMnxAl at the conduction and valenceband edge as shown in Fig. 10(b), effectively narrowing thepseudogap of pristine Fe2VAl with increasing Mn substitution.Fig. 9(b) displays the atom-projected partial DOS ofFe2V0.9Mn0.1Al in the paramagnetic state with the respectivecontributions from all atoms. We point out that, despite thelarge electron doping concentration of 0.2 e per formula unitupon substituting 10% Mn at the V site, the Fermi level remainsvirtually unaffected and stays within the pseudogap. Thisbecomes even more evident in Fig. 10, where the total DOS ofFe2V1�xMnxAl is shown for various Mn concentrations in theparamagnetic state, matching our experimentally investigatedsamples. Even for Mn substitutions as high as x = 0.5, corres-ponding to an entire additional electron, the Fermi levelremains pinned within the pseudogap and does not get shiftedinto the conduction band. The anomalous doping behavior canbe traced back to Mn forming magnetic impurity states belowthe Fermi energy (see Fig. 10(a)), which can accommodate theadditional electrons doped by Mn in Fe2V1�xMnxAl. Thus,although the valence electron concentration increases, the totalnumber of states below EF increases as well, rendering anunchanged position of EF. This readily explains the unconven-tional VEC dependence of the Seebeck coefficient in Fig. 7(a),where S retains positive values even for VEC c 6.We also studied the effect of substituting Mn at the Fe site,i.e., considering that Mn may occupy the Fe site with Fe atomssimultaneously occupying the vacant V positions as antisitedefects (Fe1.9Mn0.1V0.9Fe0.1Al), which cannot be ruled out by ourstandard X-ray diffraction experiments due to the very similaratomic scattering factors of Mn, Fe and V. However, theelectronic density of states of Fe1.9Mn0.1V0.9Fe0.1Al (see ESI,† Fig.S8) indicates that this seems very unlikely, since a metallic groundstate with a very large DOS at EF emerges, which contradicts theexperimental Seebeck coefficient and its VEC dependence.ConclusionOverall, p-type Fe2VAl compounds are less developed comparedto n-type ones. Here, we performed a systematic experimentaland theoretical study of the thermoelectric properties ofMn-doped Fe2V1�xMnxAl Heusler compounds across a widerange of Mn concentrations x = 0 – 0.6. We found an un-expected hole-dominated Seebeck coefficient in nominallyn-doped Fe2V1�xMnxAl. Using DFT calculations, the origin forthis unconventional doping behavior was found to be rooted indistinct modifications of the electronic density of states uponMn substitution. More specifically, Mn atoms at the V site formmagnetic impurity states with a large local magnetic moment(m B 2.43mB) and antiferromagnetic exchange interactions. Theadditional electronic states contributed below EF exactly bal-ance the chemical potential, such that EF remains pinned at thevalence band edge, within the pseudogap, even for very largeMn concentrations up to x = 0.6. Moreover, rather localized Mnstates spawn at both sides of the pseudogap, yielding anincrease of the DOS effective mass, albeit a narrowing of thepseudogap is triggered as well. Consequently, relatively largep-type Seebeck coefficients and an anomalous doping/VECdependence emerges in Fe2V1�xMnxAl.The thermoelectric properties could be further improvedby co-substituting Al antisites at the Fe and V sublattices inFig. 10 (a) Total density of states (DOS) of Fe2V1�xMnxAl calculated in the disordered local moment (DLM) theory. (b) DOS around the Fermi energy,featuring contributions from rather localized Mn impurity states at the valence and conduction band edges, as well as a pinning of the Fermi level withinthe pseudogap. To indicate the energy range, relevant for thermoelectric transport, the derivative of the Fermi–Dirac distribution at 300 K is shownin grey.Paper Journal of Materials Chemistry CThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 8861–8872 | 8871Al-rich Fe2V0.9Mn0.1Al1+y compounds. Fe2V0.9Mn0.1Al1.1 exhibitsthe largest Seebeck coefficient (S = 75 mV K�1) and PF =2.2 mW K�2 m�1 at T = 350 K. Also, the thermal conductivitydecreased to 9 W m�1 K�1 for Fe2V0.9Mn0.1Al1.5 (y = 0.5).Consequently, the largest ZT value, B0.1, was found forFe2V0.9Mn0.1Al1.5 (y = 0.5), at T = 500 K. Our study motivatesfurther investigations and optimization studies of Fe2VAl-basedHeusler compounds, co-substituted with Mn at the V site, torealize high-performance p-type thermoelectrics, which arerelevant for technological applications.Conflicts of interestThere are no conflicts to declare.AcknowledgementsThis work was mainly supported by JST Mirai Program GrantJPMJMI19A1. N. T. has been supported by JSPS KAKENHI,22H01761.References1 L. E. Bell, Science, 2008, 321, 1457.2 J. Heand and T. M. Tritt, Science, 2017, 357, eaak9997.3 N. Nandihalli, C.-J. Liu and T. Mori, Nano Energy, 2020,78, 105186.4 I. Petsagkourakis, K. Tybrandt, X. Crispin, I. Ohkubo,N. Satoh and T. Mori, Sci. Technol. Adv. Mater., 2018, 19, 836.5 T. Zhu, Y. Liu, C. Fu, J. P. Heremans, J. G. Snyder andX. Zhao, Adv. Mater., 2017, 29, 1605884.6 Z. Liu, N. Sato, W. Gao, K. Yubuta, N. Kawamoto,M. Mitome, K. Kurashima, Y. Owada, K. Nagase, C.-H.Lee, J. Yi, K. Tsuchiya and T. Mori, Joule, 2021, 5, 1196.7 T. Mori, Small, 2017, 13, 1702013.8 J. Mao, Z. Liu, J. Zhou, H. Zhu, Q. Zhang, G. Chen andZ. Ren, Adv. Phys., 2018, 67, 69–147.9 T. Hendricks, T. Caillat and T. Mori, Energies, 2022, 15,7307.10 Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki andU. Mizutani, Phys. Rev. Lett., 1997, 79, 1909.11 H. Kato, M. Kato, Y. Nishino, U. Mizutani and S. Asano,J. Jpn. Inst. Met., 2001, 65, 652.12 Y. Nishino and Y. Tamada, J. Appl. Phys., 2014, 115, 123707.13 G. Y. Guo, G. A. Botton and Y. Nishino, J. Phys.: Condens.Matter, 1998, 10, L119.14 D. J. Singh and I. I. Mazin, Phys. Rev. B: Condens. MatterMater. Phys., 1998, 57, 14352.15 R. Weht and W. E. Pickett, Phys. Rev. B: Condens. MatterMater. Phys., 1998, 58, 6855.16 A. Laila, M. Nanko and M. Takeda, Mater. Trans., 2020,61, 2216.17 M. Weinert and R. E. Watson, Phys. Rev. B: Condens. MatterMater. Phys., 1998, 58, 9732.18 H. Okamura, J. Kawahara, T. Nanba, S. Kimura, K. Soda,U. Mizutani, Y. Nishino, M. Kato, I. Shimoyama, H. Miura,K. Fukui, K. Nakagawa, H. Nakagawa and T. Kinoshita, Phys.Rev. Lett., 2000, 84, 3674.19 F. Garmroudi, A. Riss, M. Parzer, N. Reumann, H. Muller,E. Bauer, S. Khmelevskyi, R. Podloucky, T. Mori, K. Tobita,Y. Katsura and K. Kimura, Phys. Rev. B, 2021, 103, 085202.20 F. Garmroudi, M. Parzer, A. Riss, A. V. Ruban,S. Khmelevskyi, M. Reticcioli, M. Knopf, H. Michor,A. Pustogow, T. Mori and E. Bauer, Nat. Commun., 2022,13, 3599.21 F. Garmroudi, M. Parzer, A. Riss, N. Reumann,B. Hinterleitner, K. Tobita, Y. Katsura, K. Kimura, T. Moriand E. Bauer, Acta Mater., 2021, 212, 116867.22 F. Garmroudi, M. Parzer, A. Riss, S. Beyer, S. Khmelevskyi,T. Mori, M. Reticcioli and E. Bauer, Materials Today, Physics,2022, 27, 100742.23 Z. Liu, W. Gao, H. Oshima, K. Nagase, C.-H. Lee and T. Mori,Nat. Commun., 2022, 13, 1120.24 K. Imasato, S. D. Kang and G. J. Snyder, Energy Environ. Sci.,2019, 12, 965–971.25 L. Wang, N. Sato, Y. Peng, R. Chetty, N. Kawamoto,D. H. Nguyen and T. Mori, Advanced Energy, Materials,2023, 13, 2301667.26 Y. C. Lan, B. Poudel, Y. Ma, D. Z. Wang, M. S. Dresselhaus,G. Chen and Z. F. Ren, Nano Lett., 2009, 9, 1419.27 Y.-Z. Li, Q. Zhang, K. Liu, Y.-J. Lin, N. Lin, Y. Yu, F. Liu,X.-B. Zhao, B.-H. Ge, O. Cojocaru-Mirédin, C.-G. Fu andT.-J. Zhu, Materials Today, NANO, 2023, 22, 100340.28 J. Pei, B. Cai, H.-L. Zhuang and J.-F. Li, Natl. Sci. Rev., 2020,7, 1856–1858.29 Y. Feng, J. Y. Rhee, T. A. Wiener, D. W. Lynch, B. E.Hubbard, A. J. Sievers, D. L. Schlagel, T. A. Lograsso andL. L. Miller, Phys. Rev. B: Condens. Matter Mater. Phys., 2001,63, 165109.30 K. Soda, H. Murayama, K. Shimba, S. Yagi, J. Yuhara,T. Takeuchi, U. Mizutani, H. Sumi, M. Kato, H. Kato,Y. Nishino, A. Sekiyama, S. Suga, T. Matsushita andY. Saito, Phys. Rev. B: Condens. Matter Mater. Phys., 2005,71, 245112.31 M. Kato, Y. Nishino, U. Mizutani and S. Asano, J. Phys.:Condens. Matter, 2000, 12, 1769–1779.32 T. Naka, K. Sato, M. Taguchi, T. Nakane, F. Ishikawa,Y. Yamada, Y. Takaesu, T. Nakama and A. Matsushita, Phys.Rev. B: Condens. Matter Mater. Phys., 2012, 85, 085130.33 S. Jemima, A. Mani, A. Bharathi, N. Ravindran andY. Hariharan, J. Alloys Compd., 2001, 326, 183–187.34 E. P. Amaladass, A. T. Satya, S. Sharma, K. Vinod,V. Srinivas, C. S. Sundar and A. Bharathi, J. Alloys Compd.,2015, 648, e34–e38.35 O. Nashima, T. Kanomata, Y. Yamaguchi, S. Abe, T. Harada,T. Suzuki, H. Nishihara, K. Koyama, T. Shishido,K. Watanabe and T. Kaneko, J. Alloys Compd., 2004, 383,298–301.36 M. Mikami, Y. Kinemuchi, K. Ozaki, Y. Terazawa andT. Takeuchi, J. Appl. Phys., 2012, 111, 093710.Journal of Materials Chemistry C Paper8872 | J. Mater. Chem. C, 2024, 12, 8861–8872 This journal is © The Royal Society of Chemistry 202437 R. Jha, N. Tsujii, C. Bourgès, W. Gao, E. Bauer and T. Mori,Z. Anorg. Allg. Chem., 2022, 648, e202200058.38 W. Lu, W. Zhang and L. Chen, J. Alloys Compd., 2009, 484,812–815.39 F. Kobayashi, N. Ide and Y. Nishino, J. Jpn. Inst. Met., 2007,71, 208.40 Y. Nishino, IOP Conf. Ser.: Mater. Sci. Eng., 2011, 18, 142001.41 T. Sugiura and Y. Nishino, J. Jpn. Inst. Met., 2009, 73, 846.42 Y. Sandaiji, N. Ide, Y. Nishino, T. Owada, S. Harada andK. Soda, J. Jpn. Soc. Powder Powder Metall., 2010, 57, 207.43 I. Knapp, B. Budinska, D. Milosavljevic, P. Heinrich,S. Khmelevskyi, R. Moser, R. Podloucky, P. Prenningerand E. Bauer, Phys. Rev. B, 2017, 96, 045204.44 K. Iwase, Y. Nishino, H. Miyazaki and K. Soda, J. Jpn. Inst.Met., 2008, 72, 464.45 Y. Nishino, H. Kato, M. Kato and U. Mizutani, Phys. Rev. B:Condens. Matter Mater. Phys., 2001, 63, 233303.46 Y. Hanada, R. O. Suzuki and K. Ono, J. Alloys Compd., 2001,329, 63.47 C. S. Lue and Y.-K. Kuo, Phys. Rev. B: Condens. Matter Mater.Phys., 2002, 66, 085121.48 T. Nakama, Y. Takaesu, K. Yagasaki, T. Naka, A. Matsushita,K. Fukuda and Y. Yamada, J. Phys. Soc. Jpn., 2005, 74, 1378.49 Y. Nishino, S. Kamizono, H. Miyazaki and K. Kimura, AIPAdv., 2019, 9, 125003.50 B. Hinterleitner, I. Knapp, M. Poneder, Y. Shi, H. Müller,G. Eguchi, C. Eisenmenger-Sittner, M. Stöger-Pollach,Y. Kakefuda, N. Kawamoto, Q. Guo, T. Baba, T. Mori,S. Ullah, X.-Q. Chen and E. Bauer, Nature, 2019, 576, 85–90.51 H. Matsuura, Y. Nishino, U. Mizutani and S. Asano, J. Jpn.Inst. Met., 2002, 66, 767.52 H. Miyazaki, S. Tanaka, N. Ide, K. Soda and Y. Nishino,Mater. Res. Express, 2014, 1, 015901.53 N. Tsujii, A. Nishide, J. Hayakawa and T. Mori, Sci. Adv.,2019, 5, eaat5935.54 R. Ang, A. U. Khan, N. Tsujii, K. Takai, R. Nakamura andT. Mori, Angew. Chem., Int. Ed., 2015, 54, 12909.55 F. Ahmed, N. Tsujii and T. Mori, J. Mater. Chem. A, 2017,5, 7545.56 Y. Zheng, T. Lu, Md. M. H. Polash, M. Rasoulianboroujeni,N. Liu, M. E. Manley, Y. Deng, P. J. Sun, X. L. Chen,R. P. Hermann, D. Vashaee, J. P. Heremans and H. Zhao,Sci. Adv., 2019, 5, eaat9461.57 J.-B. Vaney, S. Aminorroaya Yamini, H. Takaki,K. Kobayashi, N. Kobayashi and T. Mori, Mater. Today Phys.,2019, 9, 100090.58 S. Hebert, R. Daou, A. Maignan, S. Das, A. Banerjee,C. Bourgès, N. Tsujii and T. Mori, Sci. Technol. Adv. Mater.,2021, 22, 583.59 W. L. Lee, S. Watauchi, V. L. Miller, R. J. Cava and N. P. Ong,Phys. Rev. Lett., 2004, 93, 226601.60 R. Ramos, M. H. Aguirre, A. Anadon, J. Blasco, I. Lucas,K. Uchida, P. A. Algarabel, L. Morellón, E. Saitoh andM. R. Ibarra, Phys. Rev. B: Condens. Matter Mater. Phys.,2014, 90, 054422.61 H. Matsuura, M. Ogata, T. Mori and E. Bauer, Phys. Rev. B,2021, 104, 214421.62 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,1996, 77, 3865.63 B. L. Gyorffy, Phys. Rev. B: Solid State, 1972, 5, 2382.64 F. Izumi and K. Momma, Solid State Phenom., 2007, 130, 15.65 I. A. Abrikosov and H. L. Skriver, Phys. Rev. B: Condens.Matter Mater. Phys., 1993, 47, 16532.66 A. V. Ruban and H. L. Skriver, Comput. Mater. Sci., 1999,15, 199.67 B. L. Gyorffy, A. J. Pindor, J. Staunton, G. M. Stocks andH. Winter, J. Phys. F: Met. Phys., 1985, 15, 1337.68 A. Denton and N. Ashcroft, Phys. Rev. A: At., Mol., Opt. Phys.,1991, 43, 3161.69 M. Parzer, F. Garmroudi, A. Riss, S. Khmelevskyi, T. Moriand E. Bauer, Appl. Phys. Lett., 2022, 120, 071901.70 F. Garmroudi, M. Parzer, A. Riss, A. Pustogow, T. Mori andE. Bauer, Phys. Rev. B, 2023, 107, L081108.71 B. Hinterleitner, F. Garmroudi, N. Reumann, T. Mori,E. Bauer and R. Podloucky, J. Mater. Chem. C, 2021, 9,2073–2085.72 K. Kurosaki, H. Muta, Y. Endo, A. Charoenphakdee, M. Unoand S. Yamanaka, J. Alloys Compd., 2009, 486, 507–510.73 Y. Takagiwa and Y. Iwasaki, ACS Appl. Energy Mater., 2023,6(15), 8256–8265.74 Y. Nishino, S. Deguchi and U. Mizutani, Phys. Rev. B:Condens. Matter Mater. Phys., 2006, 74, 115115.75 B. Hinterleitner, P. Fuchs, J. Rehak, F. Garmroudi,M. Parzer, M. Waas, R. Svagera, S. Steiner, M. Kishimoto,R. Moser, R. Podloucky and E. Bauer, Phys. Rev. B, 2020,102, 075117.76 Y. Nishino, H. Sumi and U. Mizutani, Phys. Rev. B: Condens.Matter Mater. Phys., 2005, 71, 094425.77 C. S. Lue, C. F. Chen and K. Kuo, Phys. Rev. B: Condens.Matter Mater. Phys., 2007, 75, 064204.78 M. Mikami and K. Kobayashi, J. Alloys Compd., 2008, 466,530–534.79 K. Soda, S. Harada, M. Kato, S. Yagi, M. Inukai, H. Miyazaki,Y. Sandaiji, Y. Tamada, S. Tanaka, T. Sugiura and Y. Nishino,J. Electron Spectrosc. Relat. Phenom., 2011, 184, 236.80 K. Soda, S. Harada, M. Kato, S. Yagi, Y. Sandaiji andY. Nishino, IOP Conf. Ser.: Mater. Sci. Eng., 2011, 18, 142004.81 G. J. Snyder, A. Pereyra and R. Gurunathan, Adv. Funct.Mater., 2022, 32, 2112772.82 N. I. Kourov, V. V. Marchenkov, A. V. Korolev, L. A.Stashkova, S. M. Emelyanova and H. W. Weber, Phys. SolidState, 2015, 57, 700–708.83 K. Sato, T. Naka, M. Taguchi, T. Nakane, F. Ishikawa,Y. Yamada, Y. Takaesu, T. Nakama, A. de Visser andA. Matsushita, Phys. Rev. B: Condens. Matter Mater. Phys.,2010, 82, 104408.Paper Journal of Materials Chemistry C