Synthesis of layered gold tellurides AuSbTe and Au$_2$Te$_3$ and their semiconducting and metallic behavior
Emma A. Pappas Rong Zhang Cheng Peng Robert T. Busch Jian-Min Zuo Thomas P. Devereaux Daniel P. Shoemaker





Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA
Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA



Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA
Department of Applied Physics, Stanford University, Stanford, CA, 94305, USA


Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA




Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA


Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA
Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA


dpshoema@illinois.edu
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA
Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA






Previous studies on natural samples of pampaloite (AuSbTe) revealed the crystal structure of a potentially cleavable and/or exfoliable material, while studies on natural and synthetic montbrayite (Sb-containing Au_2Te_3) claimed various chemical compositions for this low symmetry compound. Few investigations of synthetic samples have been reported for both materials, leaving much of their chemical, thermal and electronic characteristics unknown. Here, we investigate the stability, electronic properties and synthesis of the gold antimony tellurides AuSbTe and (montbrayite). Differential thermal analysis and in situ powder x-ray diffraction revealed that AuSbTe is incongruently melting, while is congruently melting. Calculations of the band structures and four-point resistivity measurements showed that AuSbTe is a semiconductor and a metal. Various synthesis attempts confirmed the limited stable chemical composition of , identified successful methods to synthesize both compounds, and highlighted the challenges associated with single crystal synthesis of AuSbTe. 



Synthesis of layered gold tellurides AuSbTe and Au_2Te_3 and their semiconducting and metallic behavior
    Daniel P. Shoemaker
    April 29, 2024
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§ INTRODUCTION
 

Heavy elements such as gold and tellurium have the potential to give rise to compounds with large spin-orbit coupling and narrow band gaps, but the synthesis, band structures and properties of many gold tellurides are still unknown. The interesting characteristics and applications of certain materials in this family and of similar compounds justify further exploration of the gold tellurides. For example, calaverite AuTe_2 is a rare case of a natural mineral with an incommensurate crystal structure and is superconducting at high pressures,<cit.> while small band gap semiconductors Bi_2Te_3 and Sb_2Te_3 are exfoliable topological insulators and useful thermoelectric materials.<cit.> By investigating similar compounds in the Au–Sb–Te phase space, we are identifying materials with potentially interesting properties and applications. Here, we focus our efforts on AuSbTe and . 

During their respective studies of the Au–Sb–Te phase diagram, Blachnik and Gather (1976),<cit.> and Nakamura and Ikeda (2002)<cit.> had synthesized AuSbTe years before the 2018 discovery of a natural specimen.<cit.> Blachnik and Gather reported that AuSbTe decomposes into Sb_2Te_3 and a liquid at 424and presented a diffraction pattern<cit.>, but the crystal structure was only solved as part of a more recent study following the discovery of a natural sample. Pampaloite (AuSbTe), first found in the Pampalo gold mine in Finland, was described by Vymazalová et al. (2018).<cit.> The compound crystalizes in the C2/c space group. Its layered structure and chemistry makes AuSbTe a potential cleavable and/or exfoliable semiconductor, but these properties remain to be explored as very few investigations of natural and synthetic samples of pampaloite exist. Recent studies reported reflectance measurements, electron microprobe analyses, electron backscatter diffraction and Raman spectroscopy.<cit.> Here, we present synthesis, band structure calculations, and electrical resistivity measurements of AuSbTe. We also further investigate its stability through in situ powder x-ray diffraction (PXRD) and differential thermal analysis (DTA). 

The rare mineral montbrayite was first described in 1946 by Peacock and Thompson,<cit.> as they studied a specimen from the Robb-Montbray mine, Québec, Canada. Their chemical analysis revealed that it contained Au, Te and trace amounts of Ag, Bi, Pb and Sb. The mineral was given the approximate chemical formula Au_2Te_3, but fusion experiments at this stoichiometric composition only gave calaverite (AuTe_2) and gold.<cit.> In 1972, Bachechi confirmed the instability of Au_2Te_3 and reanalyzed a specimen from the Robb-Montbray mine, revealing a composition of (Au_1.89Ag_0.03Pb_0.04Sb_0.07)_2.03(Te_2.88Bi_0.12)_3.00.<cit.> She also showed that the compound could be stabilized by partial substitution of Au and Te for Bi, Pd and Sb, or Sb alone. Two stable synthetic compounds were found, Au_1.89(Sb,Pb)_0.11Te_2.88Bi_0.12 and Au_1.89Sb_0.23Te_2.88, with the former having a melting temperature of 410. 

The composition of the Au_2Te_3-like phase is not consistent across previous studies. In 1976, Blachnik and Gather synthesized a compound with approximate composition AuSb_0.077Te_2 (33.0 mol% Au, 2.5 mol% Sb, 64.5 mol% Te) that decomposes into AuTe_2 and liquid at 460.<cit.> In their investigation of the Au–Sb–Te system, they also reported that the compound forms at 444by reaction of this melt and AuTe_2.<cit.> When normalized to five atoms, Au_1.65Sb_0.125Te_3.225 has a different composition than the previously synthesized montbrayite. In 2002, Nakamura and Ikeda's investigation of the Au–Sb–Te phase diagram revealed another composition for montbrayite, namely , and estimated its solid solution field to be limited to within one atomic percent of this chemical formula.<cit.> 

More recently, Bindi et al. (2018) performed a chemical and structural study on a natural specimen of montbrayite, also from the Robb-Montbray mine.<cit.> They addressed some of the concerns associated to the structure and found the centrosymmetric space group P1 to be a better fit than the previously reported non-centrosymmetric structure P1<cit.>. Their analysis revealed a composition of Au_21.60Sb_0.68Te_37.41Bi_1.31, where Sb substitutes for Au in a disordered fashion and Bi substitutes for Te in an ordered fashion. However, this formula does not agree well with the available chemical data of other natural samples. The large chemical variations of natural montbrayite for different samples and localities was one of the reasons why the formula for montbrayite was revised and approved by the Commission on New Minerals as (Au,Ag,Sb,Bi,Pb)_23(Te,Sb,Bi,Pb)_38.<cit.> 

Many studies have been performed on natural specimens of montbrayite,<cit.> but like pampaloite, the band structure and electrical transport of the compound are unknown. Optical properties and spectral reflectance data for mineral identification purposes<cit.>;  Mössbauer spectroscopy chemical state characterization for gold recovery purposes<cit.>; and microscopy techniques, reflection and Raman spectra analyses for the study of chemical substitution and decomposition mechanisms<cit.> have been reported. More investigations of natural samples are reviewed by Bindi et al.<cit.>, which is of limited interest for this paper, as we focus on the synthetic analog. Only a few synthetic samples have been investigated,<cit.> with most focusing on the chemical composition and stability of montbrayite. Those few studies report stable chemical compositions in the Au–Sb–Te phase diagram that are conflicting: Au_1.89Sb_0.23Te_2.88, Au_1.65Sb_0.125Te_3.225 and . To address these inconsistencies and reveal more of its characteristics, we synthesize montbrayite compounds with a range of chemical compositions, investigate its properties using in situ powder x-ray diffraction (PXRD) and differential thermal analysis (DTA), and present new band structure calculations and electrical resistivity data. 





§ METHODS


A polycrystalline sample of AuSbTe was synthesized by mixing gold (99.99% trace metal basis, <45 μm), antimony (99.5%), and tellurium (99.8% trace metal basis, -200 mesh) powders in stoichiometric ratios and sealing them in an evacuated silica tube of 10 mm inner diameter. The ampoule was placed in a muffle furnace, heated to 600at 5/min, annealed for 1 h, cooled to 350over 2 h, and annealed for 180 h before the furnace was turned off and cooled naturally. The sample was ground, pressed into a pellet, sealed in an evacuated silica tube, heated to 350at 5/min, annealed for approximately 185 h, then water quenched. These steps were repeated thrice with annealing times of 82 h, 190 h, and 168 h. Re-grinding and re-annealing proved to be essential and increased the phase purity of the product with each step. The incompletely reacted Sb_2Te_3, AuSb_2 and Au phases decreased with each annealing cycle as increasing amounts of AuSbTe formed. 

The polycrystalline sample of used for in situ powder x-ray diffraction (PXRD) and differential thermal analysis (DTA) measurements was synthesized by mixing powders of gold (99.99%, -100 mesh), antimony (99.5%), and tellurium (99.8% trace metal basis, -200 mesh) in an atomic ratio of 1.9:0.46:2.64. The reactants were sealed in an evacuated silica tube of 10 mm inner diameter. The ampoule was placed in a muffle furnace, heated to 600at 5/min, and annealed for 1 h before water quenching. The resulting ingot was ground and pressed into a pellet, which separated into a few large pieces as it was sealed in an evacuated silica tube. The sample was heated to 350at 5/min and annealed for two weeks before water quenching. The resulting product was re-ground, pelletized, and re-annealed at 350for 10 days, which proved to be an optional step as the sample was essentially unchanged. Pellets of lacked physical integrity, so the polycrystalline sample used for four point resistivity measurements was instead heated to 600at 5/min, annealed for 1 h, cooled to 350over 2 h, and annealed for 180 h before the furnace was turned off and cooled naturally. This heating profile produced a solid ingot with grains too small (≲ 200 nm) to perform high quality scanning transmission electron microscopy (STEM) measurements. The STEM sample was heated to 600at 5/min, annealed for 30 min, cooled to 450over 2 h, cooled to 350over 50 h, then cooled to room temperature over 2 h. This heating profile provided a sample that appeared to have multiple phases (dark spots and gold veins on a silver ingot), but the main phase had larger grains (≲ 1 μm) that were more appropriate for STEM imaging. 

Portions of the samples were crushed and analyzed using PXRD on a Bruker D8 diffractometer with a Mo x-ray source in transmission geometry. The crystal structures were refined using the Rietveld method with GSAS-II<cit.>. For the in situ PXRD measurements, the samples were diluted with ground quartz before being sealed in evacuated quartz capillaries. 

Chemical composition of the samples was also verified using scanning electron microscopy (SEM) energy dispersive x-ray spectroscopy (EDS) analyses, the results and detailed description of which can be found in the Supplementary Material.<cit.>

Differential thermal analysis (DTA) measurements were performed on a Shimadzu Differential Thermal Analyzer DTA-50. Crushed portions of the samples were sealed in evacuated tubes and Al_2O_3 powder was used as reference. 

Resistivity measurements were carried out using the 4-point contact probe method in a Quantum Design Physical Property Measurement System DynaCool. Using silver epoxy, four gold wire contacts were made on a rod of (1.56 × 1.56 × 3.92 mm^3). The rod was then mounted using Kapton tape. Silver epoxy and silver paint were used to make the four contacts on the AuSbTe rod (0.98 × 0.6 × 4 mm^3). The detailed description and results of the Van Der Pauw measurements on AuSbTe are in the Supplementary Material.<cit.> 

The scanning transmission electron microscopy (STEM) sample was prepared by grinding the crystals into a fine powder in an agate mortar and pestle then dispersing onto an ultra-thin lacy carbon grid. The final samples were well dispersed particles several hundred nanometers in size. The STEM images were obtained using a FEI Themis Z (S)TEM operated at 300 kV. Atomic-resolution high-angle annular dark-field (HAADF) imaging was performed in STEM mode with a convergence angle of 18 mrad, a camera length of 115 mm, and a screen current of ∼29 pA with a 10 μs dwell time. EDS maps were obtained using a FEI Talos (S)TEM equipped with four windowless silicon drift detectors (SuperXTM) operated at 200 kV. EDS maps were collected with a 20.5 mrad convergence angle and a current of ∼85 pA. Data processing was performed in the on-board Velox software to calculate atomic percentages including only Au, Te, and Sb during atomic calculations.

The calculations of band structures using first principles method based on the density functional theory (DFT) were conducted with the VASP code<cit.>, where VASP 5.4 PBE<cit.> projector-augmented wave (PAW) pseudopotentials<cit.> were used to carry out the calculations. The total valence electrons explicitly treated in our calculations of AuSbTe consist of Au s1d10, Te s2p4, and Sb s2p3, which are described by a plane-wave basis set with a cut-off energy of 550 eV. We performed the self-consistent calculations on the primitive cell aligned with the space group C2/c for bulk. The experimental lattice constants in primitive cell are a = 11.947(3) Å, b = 4.4812(10) Å, c = 12.335(3) Å, α = 90^∘, β = 105.83(2)^∘, and γ = 90^∘. The total valence electrons explicitly treated in the DFT calculations of Au_2Te_3 consist of Au s1d10 and Te s2p4, and with a cut-off energy of 500 eV. We performed the self-consistent calculations on the primitive cell aligned with the space group P1 for bulk. The experimental lattice constants in primitive cell are a = 10.8045(7) Å, b = 12.1470(7) Å, c = 13.4480(7) Å, α = 108.091(5)^∘, β = 104.362(5)^∘, and γ = 97.471(5)^∘. Bands for both materials are plotted along the k-paths recommended by the SeeK-path tool <cit.>.





§ RESULTS AND DISCUSSION




 §.§ Structure of AuSbTe




AuSbTe crystallizes in the monoclinic space group C2/c<cit.> and is very similar to both the PdSe_2 verbeekite-type structure (C2/c) and the AuTe_2-type structure (C2/m).<cit.> Fig. <ref>(a) displays the atomic arrangement of AuSbTe in such a way to highlight its resemblance to verbeekite PdSe_2. The Au atoms (gold) are bonded to two Sb (pink) and two Te (green) atoms, with an additional bond between the Sb and Te atoms. The Au–Sb and Au–Te bonds range from 2.693(1) to 2.771(1) Å, while the Sb–Te bond is 2.829(2) Å.<cit.> In verbeekite, the Au–Te and Au–Sb bonds are replaced by four Pd–Se bonds of 2.4844(14) and 2.4966(11) Å, and the Sb–Te bond is replaced by a Se–Se bond of 2.442(2) Å<cit.>. Fig. <ref>(b)-(d) does not display the Sb–Te bond in AuSbTe and instead displays additional Au–Sb and Au–Te bonds of 3.071(1) and 3.257(1) Å.<cit.> With these longer bonds, the antimony and tellurium atoms form [AuSb_3Te_3] octahedra around the gold atoms, giving rise to flat single layers similar to those formed by the [AuTe_6] octahedra in the AuTe_2-type structure. In the latter, the Au–Te bonds (2.67 and 2.98 Å) are all shorter than the Te–Te bonds (≥3.20 Å)<cit.>, resulting in disconnected sheets of [AuTe_6] octahedra. This is in contrast to the actual structure of AuTe_2 (calaverite) where an incommensurate modulation creates Te–Te bonds shorter than some of the Au–Te bonds (2.88 Å).<cit.>
When seen from above, a single layer of AuSbTe (see Fig. <ref>(d)) resembles that of transition metal dichalcogenides with distorted octahedral structure (TMDCs-1T). AuTe_2 also shares this resemblance with TMDCs-1T, but unlike the semiconducting AuSbTe (see the next section), it is a metal that becomes superconducting at high pressures or upon platinum doping.<cit.> 

AuSbTe can be seen as replacing one Te^2- ion from the AuTe_2-type structure by a Sb^3- ion. Upon substitution with Sb^3-, the material can be charge balanced by forming Sb–Te dimers, effectively Au^3+(Sb–Te)^3-.
This Sb–Te bond is shorter (2.829(2) Å<cit.>) than the interlayer tellurium bond length in C2/m AuTe_2 (approximately 3.2 Å<cit.>) and creates a resemblance to the structure of verbeekite. Interestingly, the verbeekite structure only appears after PdSe_2 undergoes a phase transition at high temperature and pressure<cit.> or when selenium is partially substituted with tellurium<cit.>, and it has been predicted that, with more pressure, it could transform into the C2/m structure<cit.>. One could imagine that AuSbTe might also take the more layered C2/m structure at high pressure, which remains to be verified.  



 §.§ Stability of AuSbTe by synthesis, diffraction and calorimetry


The particular C2/c layered structure of AuSbTe indicates its potential for being a cleavable and/or exfoliable material, which could be verified through the synthesis of large single crystals. Unfortunately, the presence of other stable competing phases in the Au–Sb–Te ternary phase diagram<cit.> and its reported decomposition at 430(see the 67.1 at.% Au, 32.9 at.% Sb – Sb_2Te_3 binary phase diagram<cit.>) makes single crystal syntheses of pampaloite difficult to achieve. Indeed, various synthesis attempts were made, with few giving the desired phase at all. At stochiometric composition, cooling the liquid from 600to room temperature at a rate of 1/min mostly gave Sb_2Te_3 with some AuSb_2 and Au. Melting this product at 700, followed by rapid cooling to 400or 500, a one day hold, and then water quenching gave similar results. Grinding, pelleting, and annealing the reacted compounds at 350for many days increased the AuSbTe content and reduced the impurities. Re-grinding and re-annealing the pellets multiple times proved essential to improve the homogeneity of the samples. 

Based on some of the available Au–Sb–Te phase diagrams<cit.>, non-stoichiometric reactions were performed in an attempt to find more promising routes towards single crystal flux syntheses of AuSbTe. Gold, antimony, and tellurium powders were mixed in a 26.84:37.16:36 ratio and sealed into an evacuated silica tube. The tube was heated to 700to melt the reactants, then was slow cooled to 450over 18 h, to 400over 192 h, and then to room temperature over 12 h. Such synthesis was expected to give AuSbTe with some impurities, but once again resulted in mostly Sb_2Te_3 and no AuSbTe. (See the 67.1 at.% Au, 32.9 at.% Sb – Sb_2Te_3 binary phase diagram.<cit.>) Multiple cycles of grinding, pelleting, and re-annealing the compound at 400increased the AuSbTe content, with the sample still having impurities due to its non-stoichiometry. This supports the fact that annealing cycles at 400are also appropriate for the synthesis of polycrystalline AuSbTe<cit.>. Another synthesis with mixed powders in a 50:36:14 atomic ratio was expected to give a majority of AuSbTe. (See the 50 at.% Au, 50 at.% Sb – 50 at.% Au, 50 at.% Te binary phase diagram.<cit.>) The mixture was annealed at 500for 1 h, followed by a rapid cooling to 400, where it was held for 3.5 days before water quenching. The resulting product did not contain any AuSbTe, but instead was composed of Au, AuSb_2, and Sb_2Te_3. The results from these two syntheses seem to contradict the AuSbTe-rich products predicted by the 67.1 at.% Au, 32.9 at.% Sb – Sb_2Te_3 and the 50 at.% Au, 50 at.% Sb – 50 at.% Au, 50 at.% Te binary phase diagrams.<cit.> A recurrent finding was that Sb_2Te_3 is kinetically favored, making single crystal syntheses of AuSbTe difficult. No promising path for the slow cooling or the flux growth of single crystals of AuSbTe has been confirmed yet. Chemical vapor transport was attempted, the details are available in the Supplementary Material.<cit.> 

Among all of the synthesis techniques explored, the one resulting in the highest quality polycrystalline sample of AuSbTe is reported in the Methods section. It consists of grinding, pelleting, and annealing multiple times an ingot with stoichiometric composition. The success of this method is attributed to the repeated mixing and annealing, which increased the phase purity of the compound. Annealing was done at 350for a few days at a time. As can be seen in the PXRD analysis (see Fig. <ref>), the sample has minimal Au and Sb_2Te_3 impurities. SEM-EDS analyses of the resistivity samples (see Supplementary Material<cit.>) displayed the same minor impurities. Various portions of the as-synthesized pellet of AuSbTe were used for all measurements, so the PXRD and SEM-EDS results are representative of the chemical composition of all studied samples. 





Contributing to the difficulties associated with single crystal synthesis of AuSbTe is the fact that it is incongruently melting, which we have verified through in situ PXRD and DTA measurements. Fig. <ref> shows PXRD scans collected at temperatures up to 600. The room temperature scan performed before heating is shown in black at the bottom of the figure. Below it, in dotted lines, are displayed the contributions of the AuSbTe (93.9(4) wt.%), Sb_2Te_3 (4.3(2) wt.%) and Au (1.8(3) wt.%) phases to the powder pattern. At 480, the signal of the AuSbTe phase diminishes while the signal of the Sb_2Te_3 phase becomes much stronger. Above 500, all signal is lost indicating that the sample is in the liquid phase. As the sample cools, Sb_2Te_3 precipitates. Below 400, Au and a phase that matches well to the high pressure diffraction pattern of AuTe_2<cit.> start to precipitate. Below 350, AuSb_2 and AuSbTe start to appear. The final room temperature scan is shown in black at the top of Fig. <ref>. Above it, in dotted lines, are displayed the contributions of the Sb_2Te_3 (47.9(3) wt.%), Au (28.2(3) wt.%), AuSb_2 (10.8(4) wt.%), high pressure AuTe_2 (10.5(2) wt.%) and AuSbTe (2.6(3) wt.%) phases to the powder pattern. A possible explanation for the presence of the AuTe_2 high pressure diffraction pattern at ambient conditions is that antimony substitutions are stabilizing the structure. No further investigation of this phase was performed. For simplicity, this phase will be referred to as HP-AuTe_2. Comparison of the PXRD patterns before and after the insertion of the furnace indicates that the broad peaks around 5.8^∘, 7.9^∘, 12.3^∘ and 16.7^∘ are due to scattering from the furnace. Consequently, these peaks still appear in the high temperature scans when the sample signal is absent due to its melting. 

When compared to the available literature, the phase transitions seen in the in situ PXRD data differ from the ones predicted by the 67.1 at.% Au, 32.9 at.% Sb – Sb_2Te_3 binary phase diagram<cit.>. Even if one ignores the temperature differences, which can be explained by insulation effects, the phases observed upon cooling are different. The binary phase diagram predicts that from the liquid, Sb_2Te_3 precipitates, then AuSbTe, then Au and more AuSbTe, followed by the solidification of Au, AuSb_2 and any remaining AuSbTe. The in situ PXRD data suggests that from the liquid, Sb_2Te_3 precipitates, then Au and HP-AuTe_2, then AuSb_2 and a small amount of AuSbTe. An important difference concerns the resulting product; the in situ PXRD shows that AuSbTe does not reappear at the temperature where it decomposed (near 480), and that other compounds like HP-AuTe_2, which is not predicted by the binary phase diagram, are kinetically favorable. The phase diagram then overestimates the amount of AuSbTe reformed, which is minimal upon cooling of the in situ PXRD sample.



Three consecutive cycles of DTA (defined as exothermic up), shown in Fig. <ref>, also confirm that AuSbTe is incongruently melting. The first DTA cycle, drawn in red, displays a strong trough around 434upon heating (bottom portion of the curve), which would correspond to the decomposition of the sample into Sb_2Te_3 and liquid. The next trough near 459would then indicate the transition towards the fully liquid phase. Upon cooling (top portion of the curve), the features around 500would be signs that Sb_2Te_3 precipitates. Based on the results of the in situ PXRD, the peaks near 391and 351could correspond to the precipitation of Au and HP-AuTe_2, then AuSb_2 and a small amount of AuSbTe, which completes the re-solidification of the sample. Indeed, the fast heating and cooling rates of the DTA cycles should lead to reactions more similar to the step and hold temperature scans of the in situ PXRD rather than the equilibrium reactions predicted by phase diagrams. The next DTA cycles, shown in purple, begin with different materials (Sb_2Te_3, Au, AuSb_2, AuSbTe and maybe HP-AuTe_2) compared to the first cycle, which had mostly AuSbTe. As such, the heating (bottom) portions of the curves display different features, like the trough around 361, which could correspond to the melting of AuSb_2 and some AuSbTe. The trough near 425is possibly Sb_2Te_3 melting or AuSbTe decomposing. The next heating features close to 455could then be associated with the melting of Sb_2Te_3, HP-AuTe_2 and/or the decomposition of AuSbTe. The broad feature close to 500could be the slow dissolution of gold. The cooling phases of all three cycles are then almost identical since they all start with the same liquid. 

The presence of different peaks in the second and third DTA cycles confirms the formation of additional phases and the incongruent melting of AuSbTe. It is possible that the first DTA cycle and the in situ PXRD data show the same phase transitions, with the temperature differences being attributed to the different insulation, but it is also possible that the continuous cooling of the DTA experiment gave different products than the step-and-hold cool of the in situ PXRD scans. Nevertheless, the DTA and in situ PXRD measurements both confirm that AuSbTe is incongruently melting and that the kinetics of its formation are especially slow compared to the competing phases. This is in agreement with the fact that repeated grinding and annealing was necessary to obtain a synthetic sample of pampaloite.  





 §.§ Electronic properties of AuSbTe




We expect the heavy elements Au and Te to give rise to compounds with large spin-orbit coupling and narrow band gaps. The layered structure of AuSbTe could make it a cleavable and/or exfoliable semiconductor. The semiconducting behavior of AuSbTe was confirmed using our calculations, which revealed an indirect band gap of 0.11 eV near V_2. The closest-approach direct-gap of the material also occurs near V_2 and the direct gap at Γ is 0.52 eV, see Fig. <ref>. 

The reported DFT calculations for PdSe_1.25Te_0.75 verbeekite, which crystallizes in the same space group as AuSbTe (C2/c), also show that it is an indirect band gap semiconductor.<cit.> The proximity of the layers in AuSbTe and PdSe_2-xTe_x verbeekite (0.3 ≤  x ≤ 0.8)<cit.> seems to create interlayer interactions strong enough to localize the electrons and open a gap in the band structure. 
Indeed, the unfilled Se, Te and Sb p-orbitals in the conduction bands and the similar shape of their contribution to the density of states are hallmarks of covalent Se–Se dimers in PdSe_2-xTe_x verbeekite and covalent Sb–Te dimers in AuSbTe. 
This is in contrast with the structurally analogous compound AuTe_2 in which an incommensurate modulation of the C2/m structure gives rise to various Te–Te interlayer bond distances. These variations in the electronic interactions could then leave charge carriers available for conduction, in agreement with the observed metallic behavior of AuTe_2.<cit.> Upon chemical substitutions, one can obtain PdSe_2-xTe_x (for 1.0 ≤ x ≤ 1.2 and x=2)<cit.> and Au_0.65Pt_0.35Te_2 in the P3m1 structure, resulting in larger interlayer distances and metallic low-temperature superconductors.<cit.> 



To confirm the semiconducting behavior of AuSbTe, we performed four-point resistivity measurements. Fig. <ref>(a) shows the temperature dependent resistivity with various applied magnetic fields. The overlap of the data points with and without magnetic fields indicate negligible magnetoresistance, which is confirmed by the low temperature field sweep found in the Supplementary Material.<cit.> The global increase of the resistivity as the temperature decreases indicates a dominant semiconducting behavior associated with the main AuSbTe phase. At low temperature, a small decrease in the resistivity suggests that some metallic impurities may contribute to the signal. The presence of small amounts of Au and Sb_2Te_3 in the resistivity sample was confirmed by SEM-EDS analysis (see the Supplementary Material<cit.>). These impurities are known to be metallic<cit.> and explain this small decrease in resistivity at low temperature and the non-linear shape of the Arrhenius plot seen in Fig. <ref>(b). No other phases were found in the sample, meaning that the main AuSbTe phase must be the source of the large semiconducting behavior seen in the data. The shape of the temperature-dependent resistivity reflects this mixing between the dominant semiconducting AuSbTe and the metallic impurities. As such, the Arrhenius plot displays an almost linear shape at high temperatures. Linear fits between 50 and 100 K and between 200 and 300 K yield activation energies of 4.3(2) meV and 29.5(6) meV, respectively. The high temperature value of 29.5(6) meV is a better estimate of the activation energy of AuSbTe. Indeed, upon cooling, the resistivity of AuSbTe increases, while the resistivity of the metallic impurities decreases, allowing more transport through the impurities, which lowers the activation energy. Temperature dependent Van Der Pauw measurements (see Supplementary Material<cit.>) displayed the same shape as in Fig. <ref>, confirming that the observed behavior is directly caused by the sample and not by some experimental artifact. 





 §.§ Structure of 






Montbrayite crystallizes in the low symmetry structure P1. Fig. <ref> shows that the tellurium ions (green) arrange themselves into distorted octahedra (gold polygon) around the gold ions (the partial occupation of antimony is omitted in the figure for clarity). The Au–Te bonds that form the AuTe_6 polyhedra are all between 2.66 and 3.33 Å.<cit.> These polyhedra form double layers similar to the Bi_2Se_3 structure, except for periodic displacements that give them a staircase appearance. The montbrayite layers are connected by gold ions that only make contacts with five tellurium ions. The sixth bond is with another gold ion and has a length of 3.154(2) Å.<cit.> Between the layers, the shortest Te–Te distance is 2.83 Å.<cit.> 

The PXRD patterns for the samples used in this study were fitted to the reported structure for Au_1.81Sb_0.11Bi_0.12Te_3.04.<cit.> Despite the slightly different chemical composition, we find that this P1 structure better fits our data compared to the previously reported P1 structure for Au_2Te_3.<cit.> A few small angle peaks in the diffraction pattern of the non-centrosymmetric P1 space group were absent from our patterns. Earlier refinements of PXRD data for also reported some deviations between the sample's diffraction pattern and the P1 structure.<cit.> As in this previous work, we conclude that the initial indexing of the montbrayite phase was not yet properly solved. The more recent indexing with the centrosymmetric P1 space group does not have any additional peaks or large deviations from our powder patterns, addressed the structural concerns associated with the P1 structure, and provided more precise information regarding Sb and Bi substitution in a natural specimen of montbrayite.<cit.> 

Fig. <ref> is representative of the PXRD pattern refinement results for all the samples, except for slightly different impurity contents. The sample used for in situ PXRD and DTA (seen in Fig. <ref>) has 1.2(1) wt.% of gold, the one used for resistivity measurements has 6.0(1) wt.% of gold (see the Supplementary Material<cit.>), and the sample used for STEM imaging has 6.5(5) wt.% of AuTe_2 and 1.6(3) wt.% of Sb_2Te_3 (see the Supplementary Material<cit.>). No other phases were observed in the PXRD patterns. SEM-EDS confirmed the presence of a minimal gold impurity in the sample used for in situ PXRD and DTA and revealed small Te-rich regions in all samples. The specimens appear otherwise homogeneous. The SEM-EDS results are available in the Supplementary Material.<cit.> 

Scanning transmission electron microscopy (STEM) was used to confirm the chemical composition and structure of our compound. Fig. <ref> shows STEM images and EDS of the crystal. EDS profiles of Te, Au, and Sb are displayed for the shown particle over the marked region. Taken as atomic percent, Te is found to maintain 52.24 ± 1.62%, Au maintains 36.64 ± 1.59%, and Sb maintains 11.12 ± 1.24%. This is in reasonably good agreement with the formula, which corresponds to 52.8 at.% Te, 38 at.% Au and 9.2 at.% Sb. Edges of a 500 nm particle were used for high resolution imaging with atom spacing matching the [543] and [341] orientations. Atom spacing for the [543] orientation is 2.88 Å matching the expected 2.89 Å projected distance for the XRD derived model<cit.> as shown with the atomic model overlay. The [341] has atomic spacings of 1.14 Å by 2.494 Å compared to the expected projected distances of 1.14 Å by 2.441 Å, indicating approximate agreement with the expected atomic structure.







 §.§ Stability of by synthesis, diffraction and calorimetry


Differentiating from other gold telluride phases can be subtle. In particular, the powder x-ray diffraction pattern of is very similar to the diffraction pattern of AuTe_2. A diffraction peak near 4.42 Å d-spacing (∼ 9.19^∘ in Fig. <ref>), and other low angle peaks, present in but not in AuTe_2, and a diffraction peak near 5.02 Å d-spacing (which would be at ∼ 8.07^∘ in Fig. <ref>) present in AuTe_2 but not in , can help to distinguish the phases. These differences helped to confirm the results of synthesis attempts with lower antimony contents. Indeed, the PXRD patterns of samples with compositions Au_2Te_3 and Au_1.89Sb_0.11Te_3 lacked any indication of the montbrayite phase, while the composition Au_1.89Sb_0.23Te_2.88 gave a small peak near 9^∘ associated with montbrayite. Unlike , the three antimony-deficient compounds had significant AuTe_2 and Au impurities, suggesting that these compositions do not result in stable montbrayite. This is in agreement with Nakamura and Ikeda's conclusion that, within the Au–Sb–Te phase space, the stable composition of montbrayite is with a very narrow region of stability.<cit.> As previously reported,<cit.> our own results confirm that the stoichiometric formula Au_2Te_3 is unstable to AuTe_2 and Au. 



Our stability investigations of revealed that it is congruently melting. Its in situ PXRD data is displayed in Fig. <ref>. The lower intensity of the data at 480, which is particularly noticeable due to the decrease in size of the peak close to 9^∘, is indicative of melting past 470. At 490, all the major peaks are suppressed, suggesting that the sample is completely in the liquid phase. The peak intensities slowly increase as the temperature decreases, with the final room temperature data being misshaped due to the fact that the sample is slow cooled. The final scan shows that the sample is still mostly composed of with minimal decomposition into competing phases AuTe_2 (10.3(6) wt.%) and Au (3.4(2) wt.%), with a diffraction peak of the latter being indicated by a star in Fig. <ref>. Refinement of the final room temperature data is available in the Supplementary Material<cit.>. Comparison of the PXRD patterns before and after the insertion of the furnace indicates that the broad peaks around 5.8^∘, 7.8^∘, 12.3^∘ and 16.6^∘ are due to scattering from the furnace. Consequently, some of these peaks, like the ones at 12.3^∘ and 16.6^∘, still appear in the high temperature scans when the sample signal is absent due to its melting. 



The stability of is also confirmed by two consecutive cycles of DTA (defined as exothermic up) shown in Fig. <ref>. Upon heating, troughs at 446indicate that the sample is melting. Upon cooling, peaks at 410indicate that the sample solidifies. The overlap of the troughs and peaks indicate the repeatability of this heating and cooling cycle, while the absence of additional strong features suggest that no other transition is taking place. The solidification temperature of at 410is akin to the 410melting temperature of Au_1.89(Sb,Pb)_0.11Te_2.88Bi_0.12 found by Bachechi,<cit.> although the synthetic sample used in that study has a reported composition that we did not attempt to replicate. 

The in situ PXRD data and the DTA measurements show that synthetic is congruently melting. Samples of annealed above the melting point, at 450, 500and 600for two days, then water quenched, showed signs of melting and decomposition. PXRD analysis indicated the presence of 47.8(7), 44.7(6) and 27.9(5) wt.% of AuTe_2, the rest of the samples being mostly with small amounts of gold. This goes to show that quenching is sufficiently fast to prevent the compound from fully reaching its equilibrium structure. This is in agreement with previous reports on natural and synthetic compounds which found that montbrayite decomposed into AuTe_2 and gold at high temperatures.<cit.> 





 §.§ Metallicity of 




Density functional theory (DFT) calculations of the band structure and density of states of Au_2Te_3 show it is metallic (see Fig. <ref>). The large number of bands is a hallmark of the large unit cell and low P1 symmetry of Au_2Te_3.
The ideal chemical formula of montbrayite, Au_2Te_3, was used for the calculations to reduce the computational complexity. This composition could lead one to assume that the compound is made up of Au^3+ and Te^2- ions, but the similar shape of the density of states of the gold and tellurium atoms indicate that the compound is covalent, not ionic. The presence of covalent bonds is consistent with the metallic behavior, the partial substitution of gold and tellurium with antimony necessary to stabilize the compound, and the close proximity of these elements in the periodic table. 



The metallic nature of was confirmed using four-point resistivity measurements. Fig. <ref> shows a decrease of the resistivity as the temperature decreases. The residual resistivity ratio is 1.6. Data collected with applied magnetic fields of 500 Oe, 1000 Oe and 10000 Oe almost overlap the data collected without a magnetic field, indicating that no significant magnetoresistance is observed. This is confirmed by the room temperature field sweep found in the Supplementary Material.<cit.> 





§ CONCLUSIONS


We have presented a comprehensive investigation of the electronic properties of pampaloite and montbrayite. Despite previous reports, their band structure and resistivity were still unknown and information about their thermal and chemical stability was at times conflicting. We find both compounds to be compositionally well-defined and stable. The narrow band gap of AuSbTe is intriguing, in combination with its layered structure which could indicate a cleavable and/or exfoliable semiconductor. 

Four-point resistivity measurements revealed the semiconducting behavior of AuSbTe and the metallic nature of , in agreement with our DFT-calculated band diagrams and density of states. The DFT-PBE method gave a calculated indirect band gap of 0.11 eV for AuSbTe, which is likely an underestimate.

We investigated the thermal and chemical stability of AuSbTe and by reporting the effectiveness of various syntheses and by performing DTA and in situ PXRD experiments. Our results showed that AuSbTe is incongruently melting and that re-grinding and re-annealing the compound multiple times is essential to obtain a high quality polycrystalline sample. We confirmed the narrow stable chemical composition of and demonstrated that it is congruently melting with a few possible heating profiles resulting in a mostly pure polycrystalline product. 

Overall, our exploration of the properties and syntheses of gold antimony tellurides clarifies inconsistencies regarding synthetic montbrayite and emphasized the potential for AuSbTe to be a layered semiconductor. Both compounds remain to be synthesized as single crystals, but the results of our numerous syntheses will be useful in guiding future attempts. 




Materials synthesis, transport, and microstructure characterization were supported by the Center for Quantum Sensing and Quantum Materials, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0021238. The authors acknowledge the use of microscopy facilities at the Materials Research Laboratory Central Research Facilities, University of Illinois, and the use of facilities and instrumentation supported by NSF through the University of Illinois Materials Research Science and Engineering Center DMR-1720633. Computations conducted by RZ and CP were supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-AC02-76SF00515. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0027203.




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