简介概要

High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure

来源期刊：Rare Metals2018年第4期

论文作者：Atsuko Kosuga Yosuke Fujii Akito Horie

文章页码：360 - 368

摘要：In this study, we introduced the temperaturedependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe₂ and copper gallium telluride CuGaTe₂ with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hotpressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction（SXRD） measurements clarifies that impurity phases such as Te and Ag₂ Te in the AgGaTe₂ matrix and Te and CuTe in the CuGaTe₂ matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K.The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe₂ and CuGaTe₂ were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe₂ and CuGaTe₂. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe₂ and CuGaTe₂, which should aid their development for thermoelectric applications.

稀有金属(英文版) 2018,37(04),360-368

High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure

Atsuko Kosuga Yosuke Fujii Akito Horie

Department of Physical Science, Graduate School of Science,Osaka Prefecture University

JST, PRESTO

Department of Electronics,Mathematics and Physics,Graduate School of Engineering,Osaka Prefecture University

收稿日期：1 December 2017

基金：financially supported by a Grantin-Aid for Young Scientists (A) (No. 15H05548) of Japan;JST PRESTO of Japan (No. JPMJPR17R4);the Program to Support Research Activities of Female Researchers in Osaka Prefecture University in Japan;

High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure

Atsuko Kosuga Yosuke Fujii Akito Horie

Department of Physical Science, Graduate School of Science,Osaka Prefecture University

JST, PRESTO

Department of Electronics,Mathematics and Physics,Graduate School of Engineering,Osaka Prefecture University

Abstract：

In this study, we introduced the temperaturedependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe₂ and copper gallium telluride CuGaTe₂ with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hotpressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction(SXRD) measurements clarifies that impurity phases such as Te and Ag₂ Te in the AgGaTe₂ matrix and Te and CuTe in the CuGaTe₂ matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K.The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe₂ and CuGaTe₂ were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe₂ and CuGaTe₂. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe₂ and CuGaTe₂, which should aid their development for thermoelectric applications.

Keyword：

Chalcopyrite; Silver gallium telluride; Copper gallium telluride; Thermoelectric; Crystal structure;

Author： Atsuko Kosuga e-mail:a-kosuga@p.s.osakafu-u.ac.jp;

Received： 1 December 2017

1 Introduction

The ternary compounds with chalcopyrite-type structure denoted as [A=Cu or Ag (Group Ⅰ elements); B=Al,Ga or In (Group Ⅲ elements);and C=S,Se or Te (Group Ⅵ elements)] are currently of technological interest because they show promise for use in photovoltaics and optoelectronics [ 1] .These compounds crystallize in a tetragonal lattice with space group I42d and possess four formula units per cell (Fig.1).AⅠ,BⅢ,and CⅥ atoms occupy 4a (0,0,0),4b (0,0,1/2) and 8d(x,1/4,1/8) Wyckoff positions,respectively [ 2] .This structure is regarded as the stacking of two binary DⅡ-EⅥ zinc-blended structures along the c-axis,in which one cation of a Group Ⅱ element is alternately replaced by two different cations from Groups Ⅰ and Ⅲ elements.Each AⅠ and BⅢ atom is tetrahedrally coordinated to four CⅥ atoms,while each CⅥ atom is tetrahedrally coordinated to two AⅠ and BⅢ atoms (Fig.1).The tetrahedral coordination implies that the bonding is primarily covalent with mainly sp3 hybrid bonds,although there is some ionic character because the cations are different.This feature of thechalcopyrite lattice can be evaluated by two factors: tetragonal distortion,defined by η=c/2a,which is a measure of the tetragonal compression (η<1) and tension (η>1) along the c-axis,and the anion displacement parameter (u),which is the x-coordinate of the CⅥ anion. The difference between AⅠ-CⅥ and BⅢ-CⅥ bond lengths shifts the u value away from the ideal cubic value of 0.25 [ 3] .

Fig.1 a Crystal structure of AⅠBⅢCⅥwith a chalcopyrite structure with four formula units per cell and b CⅤ-centered tetrahedron in which each CⅥatom being tetrahedrally coordinated to two AⅠand BⅢatoms

The degree of tetragonality of compounds with chalcopyrite-type structure depends on the kinds of elements and temperature,which thereby has a considerable effect on their physical properties.In particular,the thermoelectric (TE) transport properties of these compounds has attracted considerable attention recently because they have favorable electrical properties to result in high TE performance,as shown in Fig.2 [ 4, 5, 6, 7, 8, 9] These properties are attributed to a good combination of the covalent and ionic bonding in the crystal structure,and are mostly controllable by changing the tetragonal parameters.Therefore,improving such properties through tuning the crystal structure of chalcopyrite-type compounds has been widely examined.For example,the band structure of this system was reported to be optimized through control ofη,which is a guiding parameter to realize the highly degenerate electronic band structures that yield good electrical properties [ 10] .However,the temperature-dependent crystallographic parameters,includingηof compounds with chalcopyritetype structure have not been extensively studied.Furthermore,their temperature-dependent formation phases have not been studied sufficiently [ 11, 12, 13, 14] .Although many attempts have been made to improve the TE properties of chalcopyrite-type compounds [ 15, 16, 17] ,they were founded on the assumption that these materials are stable over the operating temperature range,which has not been confirmed.For practical applications,TE materials should have long operating lifetimes and be maintenance-free;therefore,it is essential to consider the thermal stability of such materials to assess their reliability.

Fig.2 Temperature dependence of dimensionless figure of merit(ZT) for representative compounds with chalcopyrite-type structure

In this study,we focused on polycrystalline AgGaTe₂and CuGaTe₂ consolidated by hot pressing,which is a typical method to prepare sintered TE materials,and their high-temperature formation phases and crystallographic parameters were examined.Hot-pressed samples are intentionally used with the purpose of evaluating the hightemperature behavior of samples intended for TE applications.This study enables understanding of the high-temperature behavior of hot-pressed AgGaTe₂ and CuGaTe₂and provides clues to improve the high-temperature TE properties and stability of these materials for use in real TE applications.

2 Experimental

The starting materials of Ag (99.99%,shot,Rare Metallic Co.,Ltd.,Japan),Ga₂Te₃ (99.999%,chunk,Furuuchi Chemical Co.,Ltd.,Japan) and Te (99.999%,shot,Rare Metallic Co.,Ltd.) were mixed to form AgGaTe₂ and Cu(99.99%,shot,Rare Metallic Co.,Ltd.),Ga₂Te₃ (99.999%,chunk,Furuuchi Chemical Co.,Ltd.) and Te (99.999%,shot,Rare Metallic Co.,Ltd.) were mixed to form CuGaTe₂ according to the corresponding stoichiometric compositions and placed in silica vacuum tubes.The mixture was melted at 1273 K for AgGaTe₂ and 1173 K for CuGaTe₂ and kept overnight in the furnace before being quenched in water.The obtained ingot was annealed at 873 K for 72 h for AgGaTe₂ and 773 K for 72 h for CuGaTe₂ to obtain a homogeneous ingot sample.The ingot was then crushed into a fine powder.The fine powder was sintered into a disc-like shape with a hot press using conditions reported previously [ 4, 5] .

To characterize the crystal structural parameters and identify impurity phases of the hot-pressed samples,synchrotron X-ray diffraction (SXRD) was performed at BL02B2 beamline of the SPring-8 synchrotron radiation facility,Japan.At beamline BL02B2,a large DebyeScherer camera and imaging plate were used for the detector.The wavelength used in this study was0.0419712 nm refined with a CeO₂ standard.Each hotpressed sample was crushed into a fine powder and then sealed in a silica capillary under vacuum.It was measured diffraction profiles of hot-pressed AgGaTe₂ and CuGaTe₂with temperature increasing from 300 to 800 K in increments of 50 K to observe changes in crystal structural parameters and detect impurity phases with temperature.Rietveld analyses were carried out for the observed SXRD profiles using the RIETAN-FP program [ 18] .The average crystal structure of the main phases of AgGaTe₂ and CuGaTe₂ was refined using the tetragonal space group as a starting model.Minority phases that appeared with temperature were identified using data analysis software integrated with X-ray powder diffraction software PDXL (Rigaku,Japan).A modified split pseudo-Voigt function with Lorentz and Gauss functions for the full width at half maximum (FWHM) were applied for the profile function.The fit indicators R_wp and R_B were used to assess the quality of the refined structural model.

The electrical resistivity (ρ) and Seebeck coefficient(S) of the hot-pressed AgGaTe₂ and CuGaTe₂ samples were measured by the standard four-probe method using a commercially available apparatus (ZEM-3;ULVAC Technologies) at 300-800 K in helium atmosphere.To assess the high-temperature stability of the hot-pressed samples,ten cycles of temperature-dependent measurements of p and S were repeated.

3 Results and discussion

3.1 AgGaTe2

To evaluate the temperature-dependent phase formation of hot-pressed AgGaTe₂,the impurity phases were identified.The chalcopyrite-type structure of AgGaTe₂ is the main phase at all temperatures studied (300-800 K),as shown in Fig.3.Depending on the impurity phase(s) present,the profiles are classified into four regions:RegionⅠ(300-350 K),where only the chalcopyrite-type structure of AgGaTe₂ exists;RegionⅡ(400-650 K),where a small amount of Te appear as a secondary phase;RegionⅢ(at700 K),where the Te disappears and again only the chalcopyrite-type structure of AgGaTe₂ exist;and RegionⅣ(750-800 K),where small amounts of Ag₂Te and unidentified materials appear as impurity phases.

By taking a closer look at each region,all of the peaks are fully indexed as those of AgGaTe₂ with a chalcopyritetype structure [ 19] in RegionⅠ,similar to the results for previously reported samples prepared by the same method [ 7] .In RegionⅡ,the peaks from the secondary phase of Te [ 20] appear at 400 K and increase in intensity up to 600 K,then their intensity decreases slightly at 650 K,corresponding to the phase contents in Fig.3.The estimated maximum phase content of Te is～4.2 wt%at 600 K.SXRD patterns and results of Rietveld refinement at 350 K as a typical example of RegionⅠand 600 K to represent RegionⅡare shown in Fig.4a,b.There are two possible reasons why Te appears with temperature increasing in RegionⅡ.First,small amounts of Te existing at the grain boundaries in the hot-pressed AgGaTe₂ would aggregate with temperature increasing.In this case,crystallite Te would be thinly dispersed through the sample or dispersed Te would be amorphous,so that crystalline Te structure is not able to be detected in SXRD pattern measured at300 K.In an attempt to detect precipitated Te,energydispersive X-ray analysis was performed at room temperature on the surface of hot-pressed AgGaTe₂ at various observation scales,but no precipitated Te is observed.Therefore,it could be ruled out this possibility.The second possible reason why Te appears with temperature increasing in RegionⅡis that Te was segregated from the AgGaTe₂ matrix with temperature rising.The occupancy of Te of AgGaTe₂ was refined at 600 K,resulting in a decrease of occupancy from 1.00 to 0.84 with better reliability factor (R-factor).This indicates that Te could segregate from the AgGaTe₂ matrix with temperature increasing in RegionⅡ.

As for Region Ⅲ,the peaks from Te disappear and only the chalcopyrite-type structure of AgGaTe2 is present.This result implies that Te sublimated or melted as the temperature rises.The thermogravimetry-differential thermal analysis (TG-DTA) of hot-pressed AgGaTe2 reveals that there is no weight loss from the sample in this temperature range;therefore,the possibility of sublimation of Te could be disregarded.The possibility of Te melting could also be excluded because Te has a melting temperature higher than 700 K.Instead,the eutectic reaction between Te and AgGaTe2 occurs at 673 K [ 21] ,meaning that Te would melt to enter the liquid phase at this ratio of Te to AgGaTe2.Thereby,the peaks from Te disappear and only those of the chalcopyrite-type AgGaTe2 structure are observed in Region Ⅲ.In Region Ⅳ,peaks from impurity phases of Ag2Te [ 22] and an unidentified material aredetected.Some of the unidentified peaks could be indexed to those of AgO [ 23] ,indicating that AgO is formed by a reaction between Ag in Ag2Te or AgGaTe2 and small amounts of oxygen in the glass capillary tube.detected.Some of the unidentified peaks could be indexed to those of AgO [ 23] ,indicating that AgO is formed by a reaction between Ag in Ag2Te or AgGaTe₂ and small amounts of oxygen in the glass capillary tube.

Fig.3 SXRD data for heating process over temperature range of 300-800 K in angle range of 2.5°-20.0°(filled triangles,open triangles and question marks denoting Te,Ag₂Te,and unidentified phases,respectively;numbersⅠ-Ⅳreferring to temperature regions used to classify profiles

Fig.4 SXRD patterns and results of Rietveld refinement of hotpressed AgGaTe2 at a 350 K and b 600 K (dotted and solid lines representing observed data and fitted patterns,respectively;upper and lower vertical marks indicating Bragg peaks of AgGaTe2 with chalcopyrite structure and Te,respectively;blue curve at bottom being difference between observed and calculated patterns;GOF, goodness of fit;Rs and Rwp,reliability factors)

The temperature dependence of tetragonal lattice constants (a and c) and tetragonal lattice volume (V) of AgGaTe₂ are shown in Fig.5.It is found that a increases and c decreases with temperature increasing,leading to monotonic expansion of V.The calculated thermal expansion coefficients ofα_a=18.9×10^-6 K^-1,α_c=-2.48×10^-6 K^-1,andα_v=33.8×10^-6 K^-1 are slightly different from previously reported values ofα_a=13.5×10^-6 K^-1,α_c=-8.8×10^-6 K^-1 [ 11] andα_a=16.0×10^-6 K^-1,α_c=-6.52×10^-6 K^-1 andα_v=25.48×10^-6 K^-1 [ 24] .Figure 6a reveals thatηdecreases with temperature rising,which is caused by both the decrease of c-axis and increase of a-axis with temperature elevating.The anion displacement|u-0.25|increases with temperature.These results indicate that the structure of AgGaTe₂ becomes more distorted with temperature increasing in terms of the tetragonal lattice and Te-centered tetrahedra.This is probably because the more ionic Ag-Te bonds have higher thermal expansion coefficients than the covalent Ga-Te bonds,and thus the anisotropy increases with temperature [ 1] .The isotropic displacement factor (U) for Ag,Ga and Te is illustrated in Fig.6b.For all atoms,U increases with temperature,indicating their enhanced thermal vibration.It is notable that U of Ag is much larger than those of Ga and Te within the temperature range examined.This is probably because Ag atoms deviates considerably from their equilibrium position or Ag occupies interstitial sites in the lattice.

At high temperature (around 800 K),the hot-pressed AgGaTe₂ exhibits high TE performance;therefore,values of transport properties S andρat 800 K were plotted as a function of the number of measurement cycles (Fig.7).The values of S andρwere normalized to the corresponding value obtained from the first measurement.Both S andρrise with temperature and increases by 7%and26%,respectively,after ten measurement cycles.It is presumed that these increases could originate from decrease in carrier concentration due to mainly loss of Te from the AgGaTe₂ matrix phase caused by repeated temperature-dependent measurement cycles from 300 to800 K.The relative amounts of precipitations of the impurity phases of Ag₂Te and Te to the matrix phase are subtle,therefore,metallic properties of such phases would not contribute to S andρof the sample.

Fig.5 Temperature dependence of a tetragonal lattice constants (a and c) and b tetragonal lattice volume (V) for hot-pressed AgGaTe₂ (dotted red lines showing fitting lines used to estimate linear thermal expansion coefficients of a and b and volume thermal expansion coefficient of V)

Fig.6 Temperature dependence of a tetragonal distortion(η)and anion displacement|u-0.25|and b isotropic displacement factor (U) for Ag,Ga and Te

Fig.7 Normalized values of Seebeck coefficient (S) and electrical resistivity (ρ) of AgGaTe₂ at 800 K based on value obtained in the first measurement against number of measurement cycles

3.2 CuGaTe2

Figure 8 shows the temperature-dependent SXRD profiles for the hot-pressed CuGaTe₂ sample from 300 to 800 K.The profiles obtained at all the temperatures contain the chalcopyrite-type structure of CuGaTe₂ as the main phase.The profiles are classified into three regions according to the phase(s) present:RegionⅠ(300-450 K),where only the chalcopyrite-type structure of CuGaTe₂ exists;RegionⅡ(500-650 K),where small quantities of impurity phases corresponding to Te and CuTe appear;and RegionⅢ(700-800 K),where these impurity phases disappear and only the chalcopyrite-type structure of CuGaTe₂ is observed.The detailed changes in each region are as follows.In RegionⅠ,all of the peaks are well fitted to those of CuGaTe₂ with a chalcopyrite-type structure [ 25] ,confirming that the same phase is obtained as that of previously reported samples [ 5] .The result of Rietveld refinement at 300 K as a typical example in RegionⅠis presented in Fig.9a.In RegionⅡ,peaks from the impurity phases Te and CuTe appear at 500 K and increase in intensity up to 600 K,then decrease in intensity and almost disappear at 650 K,as shown by the phase content in Fig.8.The estimated phase contents of Te and CuTe are～1 wt%at 500 K and reach maximum values of 2.41 wt%and 1.69 wt%,respectively,at 600 K.The result ofRietveld refinement at 600 K as a typical example in RegionⅡis presented in Fig.9b.Similar to the case for AgGaTe₂,Te and CuTe are considered to segregate from the CuGaTe₂ matrix with temperature increasing.In RegionⅢ,the peaks from Te and CuTe disappear and only the chalcopyrite-type structure of CuGaTe₂ is detected.The eutectic reaction between Te and CuTe occurs at 613 K,causing Te to melt and enter the liquid phase [ 26] .Thereby,the peaks of Te disappear and only those of the chalcopyrite-type CuGaTe₂ structure are observed in RegionⅢ.

Fig.8 SXRD data for heating process over temperature range of 300-800 K in angle range of 2.5°-15.0°(filled and open triangles denoting Te and CuTe,respectively;NumbersⅠ-Ⅲreferring to temperature regions used to classify profiles)

Fig.9 SXRD patterns and results of Rietveld refinement of hot-pressed CuGaTe₂ at a 300 K and b 600 K (dotted and solid linesrepresenting observed data and fitted patterns,respectively;upper, middle and lower vertical marks indicating Bragg peaks of CuGaTe2 with chalcopyrite structure,Te and CuTe,respectively;blue curve at bottom being difference between observed and calculated patterns)

Figure 10 shows the temperature dependence of a,c,and V of hot-pressed CuGaTe₂.Both a and c increase with temperature,leading to linear expansion of V.The calculated thermal expansion coefficients areα_a=18.9×10^-6 K^-1,α_c=8.15×10^-6 K^-1,andα_v=34.3×10^-6 K^-1 are slightly different from previously reported values ofα_a=11.7×10^-6 K^-1 andα_c=6.6×10^-6 K^-1 [ 27] .It is found thatα_c of CuGaTe₂is a positive value,whereas that of AgGaTe₂ is negative.This result is accordance with the empirical relation proposed by Neumann [ 28] ,who stated that the thermal expansion coefficient ratioα_a/α_c is a continuous function of the axial ratio c/a,and increases with c/a decreasing.Also,there exists a critical c/a value for chalcopyrite materials,which is approximately 1.90,below whichα_c becomes negative.In the current study,c/a of AgGaTe₂ is 1.89,which is below the critical value,while that of CuGaTe₂ is1.98,above the critical value.

Figure 11a shows thatηis almost independent of temperature,which is caused by the similar expansion along the a-axis (α_a) and the c-axis (α_c).Compared with the results for AgGaTe₂ which exhibits a positiveα_a and negativeα_c,the thermal expansion anisotropy of CuGaTe₂is apparently smaller than that of AgGaTe₂.The anion displacement|u-0.25|of CuGaTe₂ decreases with temperature increasing,meaning that the difference between Cu-Te and Ga-Te bond length in the CuGaTe₂ structure becomes small.This result implies that some defects at the Cu/Ga cation sites are likely to occur along with slipping out of Cu from the CuGaTe₂ matrix with elevated temperature.Considering the similarity of CuGaTe₂ with CuGaSe₂,in which formation energy of defects including Cu_Ga and GaCu is estimated to be low [ 29] ,it is not surprising to arise such defects in Cu-deficient CuGaTe₂,possibly contributing to decrease in|u-0.251.

Fig.10 Temperature dependence of a tetragonal lattice constants (a and c) and b tetragonal lattice volume (V) for hot-pressed CuGaTe₂ (dotted red lines showing fitting lines used to estimate linear thermal expansion coefficients of a and b and volume thermal expansion coefficient of V)

Fig.11 Temperature dependence of a tetragonal distortion (η) and anion displacement|u-0.25|and b isotropic displacement factor (U) for Cu,Ga and Te

The U values for Cu,Ga and Te in CuGaTe₂ are shown in Fig.11b.These values increase with temperature,indicating enhanced thermal vibration of each atom.It is notable that U of Cu is considerably larger than those of Ga and Te within the temperature range examined.This is probably because Cu atoms deviates substantially from their equilibrium positions or Cu occupies the interstitial sites in the lattice,as is the case for AgGaTe₂.

Fig.12 Normalized values of Seebeck coefficient (S) and electrical resistivity (ρ) of CuGaTe₂ at 800 K based on value obtained for the first measurement against number of measurement cycles

Figure 12 shows the values of transport properties S andρfor CuGaTe₂ at 800 K as a function of the number of measurement cycles.Both S andρincrease with temperature,by 2%and 20%,respectively,after ten measurement cycles.These increases could partially originate from the gradual change in ratio and/or type of impurity phases and loss of Cu and Te from the CuGaTe₂ matrix phase caused by repeated temperature-dependent measurements from300 to 800 K.As is the case in AgGaTe₂,it is believed that these increases could originate from the decrease in carrier concentration due to mainly loss of Te from the CuGaTe₂matrix phase caused by repeated temperature-dependent measurement cycles from 300 to 800 K.The relative amounts of precipitations of the impurity phases of CuTe and Te to the matrix phase are subtle,therefore,metallic properties of such phases would not contribute to S andρof the sample.Furthermore,the reported anomalous decrease in thermal conductivity of CuGaTe₂ in the intermediate temperature range [ 30] would be probably correlated with this formation changes observed in the current study.

It is clarified that with elevated temperature,certain amounts of impurity phases separate from both the AgGaTe₂ and CuGaTe₂ matrixes in polycrystalline samples consolidated by hot pressing.It is currently unclear whether these phenomena occur only in hot-pressed samples.Further work is needed to clarify this point,such as high-temperature SXRD analysis of powder samples of AgGaTe₂ and CuGaTe₂ before hot pressing to compare with the results of the current study.However,the results do at least suggest that the high-temperature TE properties of hot-pressed samples reported previously could be affected by changes in temperature-dependent formation phases and crystallographic parameters [ 5, 7, 8, 9, 10, 15, 17] .Therefore,results in this study provide clues about the long-term high-temperature stability and transport properties of hot-pressed AgGaTe₂ and CuGaTe₂ as candidates for TE applications.

4 Conclusion

The temperature-dependent formation phases of hot-pressed AgGaTe₂ and CuGaTe₂ were investigated by collecting SXRD data over the temperature range of 300-800 K and analyzing their crystallographic parameters.Hot-pressed samples were intentionally used with the purpose of evaluating the high-temperature behavior of samples intended for TE applications.Separation of Te and Ag₂Te from the AgGaTe₂ matrix and Te and CuTe from the CuGaTe₂matrix was observed at some temperatures between 300and 800 K.These phenomena could cause the gradual increases of S andρobserved over ten measurement cycles from 300 to 800 K.The U values of Ag and Cu increase with temperature and are much larger than those of the other elements in AgGaTe₂ and CuGaTe₂,respectively,within the temperature range examined,which is likely attributed to either deviation of such atoms from their equilibrium positions,or the loss of Ag or Cu atoms that occupy interstitial sites.Theα_a,α_c andα_v values are slightly different from those reported previously.Analysis of c/2a and u indicate that the structure of AgGaTe₂becomes more distorted with temperature increasing in terms of both the tetragonal lattice and Te-centered tetrahedron.For CuGaTe₂,the distortion of the tetragonal lattice is almost constant with temperature.Additionally,Te moves to the equilibrium position with temperature increasing,probably caused by arising the defects of Cu/Ga cation sites.These crystallographic parameters could substantially affect the TE properties of materials;thus,the insight obtained here reveals the important parameters to evaluate the high-temperature transport properties of hotpressed AgGaTe₂ and CuGaTe₂.

Acknowledgements This work was financially supported by a Grantin-Aid for Young Scientists (A)(No.15H05548) of Japan,JST PRESTO of Japan (No.JPMJPR17R4) and the Program to Support Research Activities of Female Researchers in Osaka Prefecture University in Japan.Synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI;Proposal Nos.2014B1334,2015A1363 and 2015B1377).We thank Prof.S.Yamanaka's group at Osaka University,Japan for hot pressing AgGaTe₂ and CuGaTe₂.