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[Nagendra Singh Chauhan](https://orcid.org/0000-0003-2579-6642), [Takao Mori](https://orcid.org/0000-0003-2682-1846)
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[Alloying induced superionic β-phase formation in Mg3Sb2 based Zintl compounds](https://mdr.nims.go.jp/datasets/67ce0c17-d590-4012-80f6-3b33fc43010d)
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Alloying induced superionic β-phase formation in Mg3Sb2 based Zintl compoundsJournal of Materials Chemistry AMaterials for energy and sustainabilityrsc.li/materials-aVolume 12Number 4721 December 2024Pages 32485–33316ISSN 2050-7488PAPERNagendra Singh Chauhan and Takao MoriAlloying induced superionic β-phase formation in Mg3Sb2 based Zintl compoundsJournal ofMaterials Chemistry APAPEROpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal | View IssueAlloying inducedaInternational Center for Materials NanInstitute for Materials Science (NIMS), NE-mail: mori.takao@nims.go.jp; Fax: +81-29bGraduate School of Pure and Applied SciencTsukuba, Ibaraki 305-8573, Japan† Electronic supplementary informationpatterns of Mg3(Sb1−x−yBixGey)2; tempeSEM micrographs; off-centering alonghttps://doi.org/10.1039/d4ta06173jCite this: J. Mater. Chem. A, 2024, 12,32703Received 30th August 2024Accepted 5th November 2024DOI: 10.1039/d4ta06173jrsc.li/materials-aThis journal is © The Royal Society osuperionic b-phase formation inMg3Sb2 based Zintl compounds†Nagendra Singh Chauhan a and Takao Mori *abThe off-centering phenomenon manifests as locally distorted configurations with broken symmetry ina crystal structure due to the displacement of constituent atoms from their ideal coordination centerswithin the lattice. In contrast to the anticipated formation of anionic solid solutions ofMg3(Sb1−x−yBixGey)2, herein we report b-Mg3(Sb, Bi)2 based superionic phase formation (space group –Ia�3, 206) with off-centering of the dominant trigonal a-Mg3(Sb, Bi)2 phase and segregation of nanophaseMg3Ge upon equiatomic (Bi, Ge) alloying. The discordant nature of Ge is unveiled within the layered a-Mg3(Sb, Bi)2 structure and is assessed employing (3 + 1) dimensional superspace to reveal an off-centering (dz) along the z direction for the constituent atoms in the range of ±0–0.02 Å. The (Bi, Ge)alloying results in favourable tuning of the desired p-type conduction for attaining higher power factorsby band engineering and synergistic reduction of lattice thermal conductivity. The stable superionicpolymorph co-existing in an anionic solid solution of Mg3(Sb, Bi)2 provides a renewed basis forunderstanding the crystal structure and its transformation in CaAl2Si2-type Zintl compounds.1. IntroductionZintl compounds constitute a fascinating class of thermoelec-tric (TE) materials that exhibit viable prospects in the applica-tions of low-grade heat recovery and solid-state cooling.1–5 Oneexample is the exceptional n-type performance exhibited byMg3(Sb, Bi)2 based anionic solid solutions in recent years withTE energy conversion efficiency (h) reaching ∼10% in the mid-temperature (500–700 K) range.6–12 These compounds belong tothe 1-2-2 Zintl family and are traditionally described as layeredmaterials with a CaAl2Si2-type structure.13–15 Being extensivelyreported, the trigonal a-Mg3Sb2 (P�3m1, 164) phase constitutesan anionic framework of covalent [Mg2Sb2]2− slabs, whereinMg2+ cations are arranged as an ionically bonded interlayer. TheSb atoms in its anionic sites exhibit a trigonal pyramidal coor-dination, while Mg atoms located at interlayer Mg(I) and intra-layer Mg(II) positions occupy octahedral and tetrahedral sites,respectively, displaying distinctive bonding congurations.The evaluated Mg–Sb phase diagram (Fig. 1) based onthermal analysis suggests the occurrence of a phase trans-formation from a-Mg3Sb2 to b-Mg3Sb2 (Ia�3, 206) at highoarchitectonics (WPI-MANA), Nationalamiki 1-1, Tsukuba, 305-0047, Japan.-851-6280; Tel: +81-29-860-4323es, University of Tsukuba, 1-1-1 Tennodai,(ESI) available: Rietveld rened XRDrature dependent thermal properties;x, y, and z directions. See DOI:f Chemistry 2024temperature (∼1200 K), prior to melting congruently at ∼1500K.16–18 However, studies undertaken on theMg–Sb binary systemlargely show the realisation of only a-Mg3Sb2 experimentally,thus suggesting its retention even at high temperature.14Presumably the high-temperature b-phase exists only above∼1200 K for Mg3Sb2, which poses a synthesis challenge due tothe high reactivity and volatility of Mg atoms at elevatedtemperatures. Owing to the inherent difficulty in synthesizinghigh-quality b-phase Mg3Sb2 crystals, their structuralFig. 1 The Mg–Sb phase diagram near Mg3Sb2 indicating the phasetransformation of a-Mg3Sb2 (P�3m1, 164) 4 b-Mg3Sb2 (Ia�3, 206).16–18J. Mater. Chem. A, 2024, 12, 32703–32711 | 32703http://crossmark.crossref.org/dialog/?doi=10.1039/d4ta06173j&domain=pdf&date_stamp=2024-11-29http://orcid.org/0000-0003-2579-6642http://orcid.org/0000-0003-2682-1846https://doi.org/10.1039/d4ta06173jhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA012047Fig. 2 (a) XRD diffraction of the synthesized Mg3(Sb1−x−yBixGey)2polycrystals. Magnified view indicating (b) the major (112) and (202)peaks of b-Mg3Sb2 phase, and (c) the major 101 and 011 peaks of a-Mg3Sb2 phase. (d) Le Bail analysis of the XRD pattern of the pulverizedsintered sample with a nominal composition of Mg3(Sb0.95Bi0.025-Ge0.025)2 revealing indexed phases, as indicated by the observed Braggpeaks.Journal of Materials Chemistry A PaperOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinedescription has thus remained unclear so far. Nonetheless, thehigh-temperature b-phase for the structural analogue Mg3Bi2 asevaluated by Barnes et al.19 is a disordered cubic structure(Im�3m, 229), which demonstrates superionic properties.The structural exibility and rich chemistry enabled by theZintl–Klemm concept have allowed tuning of the coordinationenvironment of Mg and Sb atoms (or their substitutes) byincorporating different dopants at the anionic and cationicsites.14,20 In this work, we report the implication of co-substituting equiatomic (Bi, Ge) in Mg3(Sb1−x−yBixGey)2 basedanionic solid solutions, to achieve higher power factors by bandengineering for p-type conduction and synergistic reduction oflattice thermal conductivity. As a discordant dopant, Ge atomsinduce local distortions and broken symmetry within thelattice, effectively enhancing phonon scattering. This mecha-nism signicantly reduces lattice thermal conductivity,contributing to an improved thermoelectric gure of merit (zT)of 0.3 (±0.1) at 673 K. Polycrystalline Mg3(Sb1−x−yBixGey)2materials with (0 < x < 0.1) were prepared by a high-energymilling process, followed by spark milling sintering. Uponequiatomic (Bi, Ge) alloying in the anionic framework[Mg2Sb2]2− and high temperature (∼1023 K) sintering, theformation of a high-temperature b-phase of Mg3(Sb, Bi)2 wasobserved, with off-centering of constituent atoms within thedominant and co-existing a-Mg3Sb2 phase.2 Results and discussion2.1 Structural parameters, phase compositions andsuperionic phase formationAnionic solid solutions of Mg3(Sb1−x−yBixGey)2 were synthe-sized using a widely explored route of ball milling followed byspark plasma sintering as described in the Experimental details.The powder diffraction proles of Mg3(Sb1−x−yBixGey)2 poly-crystals shown in Fig. 2(a) indicate major peaks correspondingto trigonal a-Mg3Sb2. Remarkably, for all the (Bi, Ge) alloyedcompositions, additional peaks (at 2q ∼23.3, 27.0) corre-sponding to the cubic b-Mg3Sb2 (space group – Ia�3, 206), besidespeaks from the trigonal Mg3Ge (space group – P�3, 147)21 phaseswere also indexed. The major peaks corresponding to the b-phase of Mg3(Sb, Bi)2 are indicated separately in Fig. 2(b) in themagnied 2q range, while the structural information is pre-sented in Table 1 along with residual factors (R-factors). Withincreasing Bi, Ge content (x, y), the PXRD peaks shi to higherangles (Fig. 1(c)) with respect to the synthesized Mg3Sb1.8Ge0.2,suggesting the contraction of the a-Mg3Sb2 lattice upon (Bi, Ge)co-substitution. Also, the rened lattice parameter of the a-Mg3Sb2 phases as shown in Table 1 indicates a reduced latticeparameter c for alloyed compositions, which decreasesmarginally for higher alloying content i.e. x ∼ 0.1. This suggeststhe shi in the overlapping major (101) and (011) peaks to be anoutcome of secondary phase formation, which alters the Sb/Biratio in the synthesized compounds. For comparison, the unitcell parameters of indexed phases in Mg3(Sb1−x−yBixGey)2nanocomposites are presented in detail in Table 1.Fig. 2(d) shows the observed, calculated, and differenceproles of the powder diffraction proles of the representative32704 | J. Mater. Chem. A, 2024, 12, 32703–32711synthesized Mg3(Sb0.95Bi0.025Ge0.025)2 samples measured at 295K. The short vertical lines below the patterns indicate the peakpositions of possible Bragg reections for a and b-phases ofMg3(Sb, Bi)2, implying their coexistence due to the locally dis-torted congurations with broken symmetry. The Mg–Sb phasediagram suggests that the b-phase melts above∼1500 K and hasa cubic structure, whose details beside the space group (i.e. Ia�3,206) are unknown.16–18 The rapid transition to the superionicstate, oen occurring at elevated temperatures, presents chal-lenges in determining intermediate states or precise structuraldetails during the transition. This complexity is further aggra-vated by dynamic disorder, thermal expansion, and increasedatomic vibrations, which obscure the characterization of thesephases. In the superionic phase, a subset of ions, typicallycations, becomes highly mobile, creating a “liquid-like” moltensublattice within the solid framework. Such ion redistributioncan dramatically alter the scattering intensity of certain reec-tions. Moreover, the superionic state is typically characterizedby the strongly anharmonic vibrations of the mobile ions,leading to unusual changes in peak intensities and positions.Such changes are oen difficult to interpret using conventionalcrystallographic models, complicating the structural analysis.Thus, superspace formalism is well-suited for describing thecomplex structures in the synthesized Mg3(Sb1−x−yBixGey)2samples using the superspace group (P�3m1(00g)0s0), whichThis journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jTable 1 Constituting phases in the synthesized Mg3(Sb1−x−yBixGey)2 nanocomposites. Space group, lattice parameters, unit cell volume (Å2), andrefinement parameters (weighted profile (Rwp); profile (Rp); and expected (Rexp) R-factor). Standard deviations are given in parenthesesComposition Space group Lattice parameters VolumeR-factorsaGOF Rp Rwpx = 0; y = 0.1 P�3m1 a = b = 4.5797(3), c = 7.2758(6), a = b =90°, g = 120°132.16(1) 2.95 6.06 8.55Ia�3 a = b = c = 9.2634(7), a = b = g = 90° 794.90(1)P�1 a = b = 12.557(1), c = 4.252(1), a = b =90°, g = 120°580.76(8)x = y = 0.025 P�3m1 a = b = 4.5840(2), c = 7.2531(4), a = b =90°, g = 120°131.99(1) 1.84 4.13 5.72Ia�3 a = b = c = 9.3502(4), a = b = g = 90° 817.45(5)P�1 a = b = 12.629(2), c = 4.166(1), a = b =90°, g = 120°575.60(6)x = y = 0.05 P�3m1 a = b = 4.5785(2), c = 7.2494(4), a = b =90°, g = 120°131.60(1) 2.38 5.06 7.25Ia�3 a = b = c = 9.3504(4), a = b = g = 90° 817.52(4)P�1 a = b = 12.627(2), c = 4.162(1), a = b =90°, g = 120°574.68(5)x = y = 0.1 P�3m1 a = b = 4.5798(2), c = 7.2542(4), a = b =90°, g = 120°131.77(1) 2.20 4.65 6.45Ia�3 a = b = c = 9.3317(3), a = b = g = 90° 812.63(1)P�1 a = b = 12.613(2), c = 4.268(1), a = b =90°, g = 120°588.09(4)a Rwp ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPwiðyobsi � ycalci Þ2Pwiðyobsi Þ2vuut ;Rp ¼P��yobsi � ycalci��Pyobsi;GOF ¼ RwpRexpPaper Journal of Materials Chemistry AOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineeffectively restores the translational symmetry that is lost in thesuperionic transition due to its incommensurate nature.22–24The superionic transitioning of a lower-symmetry (P�3m1,164) phase to a higher-symmetry (Ia�3, 206) phase representsa notable increase in crystal symmetry. This phase transition isconsistent with the Mg–Sb phase diagram reported in previousstudies, further supporting the observed structuralevolution.16–18 Renements of the powder X-ray diffractionevaluate the cubic b-Mg3Sb2 unit cell (Formula unit = 8) witha lattice parameter i.e. a = b = cz 9.350 Å, having a volume performula unit of z817.5 Å3 as also presented in Table 1. A moreisotropic arrangement of atoms in the superionic b-Mg3Sb2phase likely facilitates easier ion movement in multiple direc-tions, compared to the trigonal structure, which has distinctatomic positions and potentially layered or anisotropic features.It is also noteworthy that the synthesized polycrystals exhibit anirreversible and co-existing b-phase, in contrast to the pressure-induced displacive and reversible (trigonal 4 monoclinic)phase transition at above 7.8 and 4.0 GPa, respectively, forMg3Sb2 and Mg3Bi2, as reported previously.25 The constitutingsuperionic phases exhibit a bigger, and highly disorderedsuperionic cell, indicating their complex anionic structuralmotifs, while demonstrating the structural transition prevalentat elevated temperatures in similar Zintl compounds.2.2 Domain separation, nanoprecipitation andtransgranular fractureFor a comprehensive understanding of the coexisting phases,polished and fractured samples were investigated usingThis journal is © The Royal Society of Chemistry 2024Scanning Electron Microscopy (SEM) in BSE (back scatteredelectron) and SE (secondary electron) modes. The BSE image ofthe representative polished Mg3(Sb1−x−yBixGey)2 sample, shownin Fig. 3(a), reveals nanoprecipitation and compositional vari-ations within the microstructure. The presence of secondaryMg3Ge phases or impurities, identiable by their darkercontrast, suggests potential structural changes associated withthe superionic transition. The back scattered electrons beingsensitive to the atomic number and crystallographic orientationare scattered differently, resulting in contrast in the BSEmicrographs. The EDS (energy dispersive X-ray) composition fornanoprecipitates corresponds to a Mg3Ge based phase, whilea higher Bi content is measured for the brighter domains withinthe matrix. The EDS spectrum for the mapped region, shown inFig. 3(b), identies the elemental constituents, with the averageEDS composition of the primary phase or matrix (tabulated inthe inset) closely aligning with the nominal composition. Theaverage EDS composition of the dark appearing nano-precipitates is measured to be Mg3GeBi0.4Sb0.6, which corre-sponds to the trigonal Mg3Ge phase (as designated previouslyand hereaer),21 as indexed in Fig. 2. Notably, compositionaluctuations within the domains, along with overlapping char-acteristic X-ray peaks of Mg (Ka peak is at 1.254 keV) and Ge (La∼1.188 keV and Lb peaks ∼1.218 keV) in the EDS spectrum,make accurate identication and quantication of the exactcompositions challenging. The elemental mapping shown inFig. 3(c) indicates compositional variations more clearly, whichare also identiable on the grayscale contrast. Compositionalcontrast within grains points to localised variations inJ. Mater. Chem. A, 2024, 12, 32703–32711 | 32705http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jFig. 3 (a) BSE micrographs of the polished sintered surface, showing dark nanoprecipitates; (b) EDS spectrum accompanied by tabulated EDScompositions; (c) elemental mapping; and (d) line scan of a zoomed-in region, highlighting microstructural heterogeneity in a representativeMg3(Sb1−x−yBixGey)2 sample with x = y ∼ 0.05.Journal of Materials Chemistry A PaperOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinestoichiometry or the distribution of mobile ions, attributed tocoexisting a and b-phases. Notably, the superionic transition isoen accompanied by defect clusters or extensive Frenkeldisorder,26 which could also explain the observed contrastvariations in BSE imaging and observation of high densityFrenkel defects, as reported previously.11,27 The line scan inFig. 3(d) further conrms that the nanoprecipitates arecomposed of Mg- and Ge-rich regions, contrasting with thebrighter, disordered primary phase.The SE-SEM micrograph shown in Fig. 4(a) reveals a typicalmorphology of the fractured surface indicating a transgranularmode of fracture (i.e. crack propagation through the grains).The cleavage facets and aligned lamellae patterns were observedlargely on the fracture surfaces, which are oen associated withplastic deformation within the grains. The average grain size isevaluated to be larger than ∼1 mm, suggesting their contribu-tion towards lowering of electrical resistivity due to reducedgrain boundary scattering.28 At higher magnication, theMg3Genanoinclusion segregation within the microstructure becomesdistinctly visible. This can be attributed to local compositionaluctuations induced by discordant Ge atoms. The dark-appearing nanoprecipitates, observed embedded within thegrains, further indicate the role of Ge in promoting phaseseparation at the nanoscale. We anticipate Mg3Ge nucleationand segregation from the super-saturated Mg3(Sb, Bi)2 solidsolution to be an outcome of limited solid solubility of Ge. TheEDX line scan (Fig. 4(c)) for the zoomed-in region conrms thepresence of Mg and Ge excess in the same regions corre-sponding to dark appearing nanoprecipitations embedded in32706 | J. Mater. Chem. A, 2024, 12, 32703–32711the grains. Similar to the polished surface of the bulk samples,the elemental mapping (Fig. 4(d)) of the fractured surface alsoconrms the presence of Mg and Ge excess in dark appearingembedded nanoprecipitates. The EDS mapping revealingspatial distribution indicates that all the constituting elements,i.e.Mg, Sb, Bi and Ge, are well distributed uniformly throughoutthe sample.Interestingly, the incorporation of Ge in n-type Mg3.2-Sb1.49−2xBi0.5Te0.01+xGex has led to the formation of Bi/Ge-richJanus nanoprecipitates,29 which may possess similarity to theobserved secondary phase formation. As understood previ-ously,29 despite negligible mutual solid-state solubilities of Geand Bi, during sintering (at a sintering temperature of z923 K)they become fully miscible through a co-melting process.During crystallization, the a-Mg3(Sb, Bi)2 and Bi–Ge liquidphases compete for formation, which we anticipate has inducedMg3Ge phase formation and a partial displacive structuraltransition from the trigonal (P�3m1) phase to the cubic (Ia�3)phase during sintering. As the nominal composition of thesynthesized Mg3(Sb1−x−yBixGey)2 nanocomposites lies wellwithin the solid solubility range (i.e. x ∼ 0.4) of Bi in Mg3-Sb2−xBix compounds,30 the superionic phase formation andobservation of Mg3Ge are indicative of the discordant nature ofGe atoms, which exhibit limited solubility with Bi and undergolocal co-melting of Bi and Ge during sintering.29 These ndingssuggest that the synthesized nanocomposites possess a complexmicrostructure that necessitates high-resolution techniques tofully characterize and understand the origins of these compo-sitional variations. The coexistence of these phases hasThis journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jFig. 4 (a) Low magnification and (b) high magnification images of the fractured specimen, showing transgranular fracture and the presence ofnanoprecipitates. (c) High-resolution line scan of the zoomed-in region, indicating embedded Mg and Ge-rich nanoprecipitates. (d) Elementalmapping of the region indicated in (b).Paper Journal of Materials Chemistry AOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinea signicant impact on the microstructure, deformationbehaviour, and transport properties of the sintered material, asdiscussed in the subsequent sections.2.3 Distorted coordination polyhedra, off centering anddiscordant Ge atomsSuperionic phases are characterized by the presence of highlymobile ions (usually cations) moving within a stable crystalframework formed by another set of ions (most likely theanionic framework). Their formation and stability of superionicphases are highly dependent on pressure and temperatureconditions. The body-centered cubic structure (Ia�3) of b-Mg3(Sb,Bi)2 is a distorted variant of the trigonal a-phase (P�3m1) withhigh symmetry, complex coordination geometries and distortedcoordination polyhedra, as shown in Fig. 5(a). TheMg atoms aretetrahedrally and octahedrally coordinated by Sb/Bi atoms ina trigonal a-phase and adapt a more complex coordinationgeometry having rapid diffusion of Mg ions at cationic crystal-lographic sites in the superionic phase. Similarly, trigonalpyramidal coordination of Sb/Bi atoms in the trigonal a-phaseoccupy a complex and presumably octahedral coordinationwithin the larger superionic b-phase structure. We expect aninherent conict to exist between the tetrahedral coordinationpreference of Ge atoms and the trigonal pyramidal coordinationof its hosting Sb-site, which could lead to signicant localdeviations, such as off-centering, in the structure from the idealstate. Such local distortion resembles cluster behaviour and lossof nearest-neighbours, wherein the constituent Mg(I), Mg(II) andThis journal is © The Royal Society of Chemistry 2024Sb atoms adopt a local coordination environment whileproviding charge balancing for the formation of Mg vacancydefects.31As conventional 3-dimensional structural models overlooklocal and random distortions within the crystal structure, weemploy a (3 + 1) dimensional superspace group (P�3m1(00g)0s0)to characterize inherent modulation in the synthesizedsamples.22–24 Modulations can arise from displacive modula-tions (atomic positions modulated) or occupational modula-tions (site occupancies modulated). The superspace approachcan model both types by introducing modulation functions foratomic coordinates and occupancies to capture the complexstructural variations. Fig. 5(b) reveals the evaluated off-centering in angstroms of Mg(I), Mg(II) and Sb atoms alongthe z axis with varying alloying content, evaluated upon Rietveldrenement of the dominant a-Mg3Sb2 phase from their ideallattice positions, i.e., 1a (0, 0, 0), 2d (1/3, 2/3, 0.664), and 2d (1/3,2/3, 0.225), respectively, against the fourth superspace coordi-nate t. All the displacements (in angstroms) are periodic in theinterval 0# t# 1.0, considering up to the fourth order of cosineand sine components of the Fourier terms, with an isotropicdisplacement parameters of each atoms. The off-centeringphenomenon in a crystal structure involves the displacementof atoms from their ideal coordination centers, leading tolocally distorted congurations and broken symmetry, whilemaintaining the overall crystallographic symmetry of thematerial. Interestingly, the displacement occurs only along the zdirection, which is likely triggered by the presence of Ge atomsJ. Mater. Chem. A, 2024, 12, 32703–32711 | 32707http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jFig. 5 (a) Schematic depicting trigonal and superionic phase transitions. The tetrahedral and octahedral polyhedra are highlighted in the trigonalphase. (b) Evaluated off-centering for Mg(I), Mg(II) and Sb atoms for varying compositions. (c) Corresponding positional modulations of the atomiccoordinates of Mg(I) ∼ (0, 0, 0), Mg(II) ∼ (1/3, 2/3, 0.634), and Sb ∼ (1/3, 2/3, 0.229) atoms plotted as a function of the fourth superspacecoordinate (t).Journal of Materials Chemistry A PaperOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinein layered anionic solid solutions of Mg3(Sb1−x−yBixGey)2. Thiscounterintuitive site preference of Ge atoms arises from factorssuch as ionicity, covalency, and the exibility of the hoststructure to accommodate the dopant atoms in energeticallyfavourable sites. The off-centering of the constituting atoms isonly allowed along the z direction in all the alloyed composi-tions, indicative of a low ideal shear strength of ∼1.95 GPafound for Mg3Sb2.32The overlapping periodicity of displacive modulations in thealloyed compositions, which corresponds to the atomic coor-dinates in the a-Mg3(Sb, Bi)2 phase, suggests off-centering as aninherent characteristic of the synthesized anionic solid solu-tions, which are independent of alloyed content as revealed inFig. 5(b). The evaluated positional modulations of the atomiccoordinates from their ideal, symmetric lattice position areshown in Fig. 5(c–e). Substituting Sb with Ge inMg3(Sb1−x−yBixGey)2 will lead to an uncommon and destabiliz-ing coordination environment for Sb. Intuitively, Ge might tendto shi its lattice position from the anionic site to lower itsenergy, thereby disrupting local symmetry and enabling orbitalhybridization that would otherwise be forbidden. Thus, even forthe isostructural compounds, the local coordination and bonddistances may vary, and detailed structural analysis is thereforerequired to draw any conclusions. The high symmetry superi-onic phase of Mg3(Sb, Bi)2 and Mg3Ge rich nanoprecipitates32708 | J. Mater. Chem. A, 2024, 12, 32703–32711accommodates alterations in the anionic structural motifswithin their large unit cells, wherein the valence electron countper anionic atom determines the type of motif formed. Throughelectron transfer and the resulting electronic congurations,the trigonal a-Mg3(Sb, Bi)2 phase facilitates Bi-dopant solubility,as indicated by its relatively expanded unit cell. This highlightsthe versatility of Zintl phases in coexisting with diverse anionicsubstructures that can be explained well through the Zintl–Klemm concept.2.4 Synergistic k reduction and enhanced power factorRecent investigations have unveiled off-centering behaviour ina diverse array of TE materials including GeTe compounds,PbSe, and I–V–VI2 chalcogenides (such as AgSbTe2 andAgSbSe2), establishing it as the origin of their remarkably lowthermal conductivity (k).33–36 Consequently, off-centering hasemerged as an effective strategy to modulate thermal transportproperties and enhance TE performance by exploiting theanharmonic lattice dynamics induced by the off-centre atomicdisplacements. Alloying and doping have become prevalentapproaches for enhancing the electrical transport in Mg3Sb2compounds as they exhibit inherently low kL. In the synthesizedMg3(Sb1−x−yBixGey)2 nanocomposites, the discordant Ge atomsinduce displacive modulation in the dominant a-Mg3Sb2This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jFig. 6 Temperature dependence of the (a) total thermal conductivity, (b) power factor, (c) Seebeck coefficient, (d) electrical conductivity, (e)weighted mobility, and (f) thermoelectric figure of merit (zT) of nominal Mg3(Sb1−x−yBixGey)2 polycrystals.Paper Journal of Materials Chemistry AOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinephases, demonstrating off-centering behaviour for constituentatoms.High temperature and pressure sintering leads to superionicphase formation, off-centering of constituting atoms in thetrigonal a-Mg3Sb2, and Mg3Ge phase nanoprecipitation. Thelattice thermal conductivity (kL) being intimately related to themicrostructure is signicantly lowered (∼30%) in comparisonto the Ge doped Mg3Sb2 as shown in Fig. 6(a). Nanoprecipitatesand the superionic phase owing to their large and complex unitcell are anticipated to effectively scatter the phonons, contrib-uting to kL reduction. The off-centering feature of the trigonal a-Mg3Sb2, wherein all the constituting atoms undergo displacivemodulation, can be considered as a primary cause for kLreduction as it disrupts the overall periodic arrangement ofatoms. The off-centering ranges from ∼0 to 0.02 Å at 300 K andoccurs periodically in the structure, wherein locally off-centeredatoms are expected to hinder the smooth propagation ofphonons.Pure and intrinsic p-type Mg3Sb2 exhibits poor electricalperformance quantied as the power factor (S2s), primarily dueto its high electrical resistivity. However, the synthesizedMg3(Sb1−x−yBixGey)2 (0 < x,y < 0.1) nanocomposites displaya signicant improvement in temperature dependent electricalThis journal is © The Royal Society of Chemistry 2024conductivity s(T), as shown in Fig. 6(b). For comparison, the TEproperties of the synthesized Ge doped Mg3Sb2 are also shownalongside, wherein at 300 K the s(T) of Mg3Sb1.8Ge0.2 is∼1.74×103 S m−1, and it improved signicantly to ∼1.45 × 104 S m−1.All samples indicate degenerate semiconducting behaviour,wherein s decreases with increasing temperature. Thetemperature-dependent S(T) shown in Fig. 6(c) displays p-typeconduction, which gradually increases with rising temperatureand decreases with increasing alloying content showcasing aninverse correlation with s(T). A higher power factor as displayedin Fig. 6(d) was attained for all the alloyed compositions withthe maximum value approaching 3 × 10−4 W m−1 K−1 fora higher alloy content (x > 0.05). The enhancement can beunderstood as an outcome of improved weighted mobility (mw)37derived S(T) and s(T) measurements, as shown in Fig. 6(e). Itprovides a weighted average contribution of all energy levels tothe overall mobility of the carrier in the crystal and providesgood results at room temperature and above, and for mobilitiesas low as 10−3 cm2 V−1 s−1. All the alloyed compositions exhibithigher mw and correspond well with the evaluated power factorenhancement. The temperature-dependent zT presented inFig. 6(f) indicates comparatively higher values for alloyed off-centered compositions with a maximum zT ∼ 0.3(±0.05) atJ. Mater. Chem. A, 2024, 12, 32703–32711 | 32709http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4ta06173jJournal of Materials Chemistry A PaperOpen Access Article. Published on 18 November 2024. Downloaded on 12/9/2024 2:02:20 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online673 K. Optimizing the Fermi level by cation site doping or bandbending is anticipated to further enhance the zT of thesynthesized alloyed compositions, which exhibits an inherentlylowered kL. Thus, the high-symmetry superionic phase inMg3(Sb, Bi)2-based compositions enhances electrical transportproperties, providing a renewed basis for exploring structure–property relationships. This insight paves the way for designingMg3(Sb, Bi)2 based Zintl phases as advanced functional mate-rials with improved performance.3. ConclusionComplex crystal structures andmixed ionic-covalent bonding inMg3(Sb, Bi)2 based Zintl compounds have remained critical forunderstanding their unique electronic structure and transportproperties. In this study, we observed off-centering of consti-tuting atoms in the dominant trigonal phase of a-Mg3Sb2,superionic b-phase formation and nano-segregation of theMg3Ge phase upon equiatomic (Bi, Ge) alloying inMg3(Sb1−x−yBixGey)2. The superionic phase formation signi-cantly impacts the coordination environments of the cations(Mg) and anions (Sb/Bi), resulting in more complex structuralmotifs. The concentration of nanoprecipitates is determined bythe alloying content, whereas the extent of off-centering andatomic disorder is independent of the alloying content. The off-centering enables the realization of a synergistic reduction inlattice thermal conductivity and power factor enhancement,resulting in zT enhancement. The strategic co-alloying of Geand Bi and the formation of nano-precipitates offer promisingavenues for optimizing the p-type TE performance of Mg3Sb2-based Zintl nanocomposites.4 Experimental details4.1 Material synthesis and processingHigh-purity elemental constituents, i.e., magnesium turnings(Mg, 99.8%), antimony powder (Sb, 99.9999%), bismuth powder(Bi, 99.999%), and germanium powder (Ge, 99.999%), werestoichiometrically weighed in accordance with nominalcompositions Mg3(Sb1−x−yBixGey)2 wherein (0 < x < 0.2).Subsequently, they were loaded into a stainless-steel ball mill ina glovebox (MBRAUN UNILAB PLUS ECO.) under an argonatmosphere with the oxygen level (<0.1 ppm) for a high-energymilling (SPEX 8000D) process. The milling was carried out for2 hours and the milled powder was collected and loaded intoa high-density graphite die (Ø 10 mm) inside the glove box forsintering. To obtain dense samples, isothermal sinteringthrough spark plasma sintering (SPS, SPS-1080 System, SPSSYNTEX INC) was carried out at 1023 K with a pressure of∼60 MPa for 5 min. The volumetric densities of polycrystallinebulk samples were determined to be >97% of the theoreticaldensity, using the Archimedes principle.4.2 Structural characterizationX-ray diffraction (XRD) was performed at RT in the 2q range of20–60° using monochromatized CuKa radiation (l = 1.54 Å) on32710 | J. Mater. Chem. A, 2024, 12, 32703–32711a MiniFlex 600/600-C (Rigaku) equipped with a high-speed 1Ddetector D/teX Ultra2. The microstructure and compositionwere characterized using a eld emission scanning electronmicroscope (FESEM, Hitachi S 4800) equipped with an energydispersive spectrometer (EDS, Horiba EMAX Evolution X-Max).4.3 Thermoelectric characterizationTemperature-dependent thermoelectric transport measure-ments were conducted on bulk sintered samples withina temperature range of 300–673 K. For thermal diffusivity (D)and specic heat (CP), circular disc specimens of approximately10 mm diameter and 2 mm thickness were analysed using theash diffusivity method (LFA467 HT HyperFlash, Netzsch,Germany) in an argon environment. Electronic thermalconductivity (ke) was assessed in accordance with the Wiede-mann–Franz law ke = L × s × T, where L is the Seebeck-dependent Lorenz number.38 The L values estimated using therelationship L ¼�1:5þ exp�� jSj116��� 10�8 were found to be inthe range of (1.6–1.9) × 10−8 W U K−2. Simultaneously,temperature dependent measurements of electrical conduc-tivity and Seebeck coefficients were performed using the four-probe DC method (ZEM-3, Ulvac-Riko, Japan) in a heliumatmosphere on samples cut into rectangular bars with dimen-sions of approximately 9 mm × 2 mm × 2 mm. The accuracy intransport measurements is ±10% for thermal conductivity,±7% for electrical conductivity, and ±7% for Seebeckcoefficients.Data availabilityThe data that support the ndings of this study are availableupon request from the authors.Conflicts of interestThere is no conict of interest to declare.AcknowledgementsThis research was supported by the JST Mirai Program, Japan(grant number – JPMJMI19A1).References1 S. M. Kauzlarich, Chem. Mater., 2023, 35, 7355–7362.2 J. Shuai, J. Mao, S. Song, Q. Zhang, G. Chen and Z. Ren,Mater. Today Phys., 2017, 1, 74–95.3 J. Wang, J. Mark, K. E. Woo, J. Voyles and K. Kovnir, Chem.Mater., 2019, 31, 8286–8300.4 J. Mao, H. Zhu, Z. Ding, Z. Liu, G. A. Gamage, G. 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Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld... Alloying induced superionic tnqh_x03B2-phase formation in Mg3Sb2 based Zintl compoundsElectronic supplementary information (ESI) available: Rietveld...