A Chemical, High-Temperature Way to Ag1.9Te via Quasi-Topotactic

Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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A Chemical, High-Temperature Way to Ag1.9Te via Quasi-Topotactic Reaction of Stuetzite-type Ag1.54Te: Structural and Thermoelectric Properties Franziska Baumer and Tom Nilges* Synthesis and Characterization of Innovative Materials Department of Chemistry, Technical University of Munich, Lichtenbergstrasse 4, 85748 Garching b. München, Germany S Supporting Information *

ABSTRACT: Semiconducting silver tellurides gained reasonable interest in the past years due to its thermoelectric, magneto-caloric, and nonlinear optic properties. Nanostructuring has been frequently used to address quantumconfinement effects of minerals and synthetic compounds in the Ag−Te system. Here, we report on the structural, thermal, and thermoelectric properties of stuetzite-like Ag1.54Te (or Ag4.63Te3) and Ag1.9Te. By a quasi-topotactic reaction upon tellurium evaporation Ag1.54Te can be transferred to Ag1.9Te after heat treatment. Crystal structures, thermal and thermoelectric properties of stuetzitelike Ag1.54Te (or Ag4.63Te3) and Ag1.9Te were determined by ex situ and in situ experiments. This method represents an elegant chemical way to Ag1.9Te, which was so far only accessible electrochemically via electrochemical removal of silver from the mineral hessite (Ag2Te). The mixed conductors show reasonable high total electric conductivities, very low thermal conductivities, and large Seebeck coefficients, which result in a significant high thermoelectric figure of 0.57 at 680 K.

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INTRODUCTION Covalent interactions in the tellurium substructures in silver(I) chalcogenide halides of Peierls-type led to intriguing thermoelectric and electric properties.1 The prominent substructure responsible for this kind of unexpected property is a linear arrangement of tellurium featuring different grades of covalent bonding interactions of Peierls-type in Ag10Te4Br3, dependent on temperature and the mobility of the surrounding silver ions. As a consequence, a strong modulation of the density of states at the Fermi level occurs, which is the origin of significant changes in the electronic structure and the Seebeck coefficient of such compounds. A p-n-p switch of the semiconducting properties is a direct consequence of this modulation. It has been shown that this feature can be generalized and can also be observed for other types of interactions that are able to modulate the density of states close to the Fermi level.2−4 Inspired by this finding we screened closely related binary and ternary phase diagrams containing tellurium for structural features capable to perform such Peierls-like interactions. We identified stuetzite, a naturally occurring and also synthetically accessible mineral within the Ag−Te system, as a potential candidate according to its structural features. The crystal structure of stuetzite or Ag4.5Te3 is still controversially discussed, because a certain phase broadening occurs that gives rise to an ongoing modulation of the structural features of this compound. Therefore, it is not surprising that a plethora of structure models and compositions has been suggested in the past 70 years (see Figure 1 and Table S1, Supporting Information).5−15 The range of compositions last from Ag5.25Te3 to Ag3.56Te3, and the materials might crystallize in a hexagonal metric described with several possible space groups. © XXXX American Chemical Society

Figure 1. Ag−Te phase diagram reprinted with permission from ref 19. Temperatures (in K) were added for clarity. Copyright 1991 Springer.

Breaking this down to a more descriptive formula sum and scaling this to the tellurium content the variation of the silver is more clearly documented resulting in Ag1.75Te to Ag1.19Te. The most recent and also most reliable structure determination was done in 1996 resulting in Ag4.53Te3 (or Ag1.51Te) for synthetic stuetzite.8 In the following we benchmark our experimental results on the findings in this report. Ag4.5Te3 is composed by a Received: August 8, 2017

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DOI: 10.1021/acs.inorgchem.7b01991 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

easily separated from the sample, and we obtained phase-pure stuetzite Ag4.63Te3 on a gram scale. Single-Crystal X-ray Measurements. Single-crystal intensity data of the same crystal were collected at room temperature and at 660 K on a STOE STADIVARI with a microfocus X-ray source Genix 3DX of Xenocs (Mo Kα1 radiation, λ = 0.710 73 Å) and a hybrid pixel detector Pilatus3R 110 K of Dectris. High temperature was set with the system Heatstream of Stoe. To avoid oxidation upon heating, the single crystal was put in a quartz glass capillary and fixed at the bottom with a smaller one (Scheme 1). The capillaries were sealed under vacuum after evacuating and refilled with argon several times. The starting atomic parameters were determined with the Superflip algorithm, which is implemented in the Jana2006 program package.32 Because of the small number of reflections for the high-temperature phase only a numerical absorption correction was applied for roomtemperature data. The quality of the high-temperature data is in general limited due to the quasi-topotactic reaction taken place and the in situ data collection performed right after on the diffractometer. Therefore, an internal R value of 0.54 was observed, which is a direct consequence of the formation process. As a consequence, the refinement of the Flack parameter was not possible, but a manual test resulted in better overall R values for the given enantiomer. We therefore set the Flack parameter manually to 0.01. Tellurium displacement parameters were refined anisotropically (full matrix, least squared on F2), whereas silver displacement parameters are described by using a nonharmonic approach with third-order terms for both refinements.33 Further details of the structure refinements are available in the Supporting Information. X-ray Powder Diffraction. Powder X-ray diffraction (XRD) measurements of polycrystalline Ag4.63Te3 were executed on a STOE STADI P diffractometer (Cu Kα1 radiation, λ = 1.540 51 Å, Ge monochromator) with a flat-bed sample holder. α-Si (a = 5.43096 Å) was used as internal standard. Lattice parameters were indexed with the program package WinXPOW. Phase purity was substantiated by brief phase analyses. Scanning Electron Microscope and Energy-Dispersive X-ray Spectroscopy. Several single crystals of Ag4.63Te3 were subject to scanning electron microscopy (SEM) and semiquantitative energydispersive X-ray spectroscopy (EDS) analysis using a JEOL JCM-6000 analyzer with a JED 2200 detector applying an acceleration voltage of 15 kV. No impurities heavier than sodium were detected. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) measurements of polycrystalline samples of Ag4.63Te3 were performed in aluminum crucibles under argon atmosphere with a NETZSCH DSC 200 F3Maja using heating and cooling rates of 10 K min−1. Measurements of Electrical Conductivity and Seebeck Coefficient. Grinded samples of Ag4.63Te3 were cold-pressed to a pellet with 15 mm diameter. Electrical conductivity and Seebeck coefficient were measured simultaneously with a NETZSCH SBA 458 Nemesis. For the determination of the Seebeck coefficient we used at least a temperature difference of A direct-current (DC) of 50 mA was applied for measuring the electrical conductivity. During data collection the apparatus is flushed continuously with argon (flow rate 60 mL min−1). Thermal Conductivity Measurements. The thermal conductivity was investigated in a temperature range of 300−675 K using a NETZSCH LFA 467 Hyper Flash under argon atmosphere. Phasepure Ag4.63Te3 was pressed to a pellet (d = 10 mm, h = 0.9 mm) reaching a density of 6.34 g cm−3 (83% of the theoretical density). For a better emission and absorption capability the front and back sides were coated with graphite. The values for the thermal diffusivity a are the mean value of five independent measurements (see Figure S4, supplement). The heat capacity Cp was determined by the standard test method ASTM-E 1461−2011 using stainless steel 310 as reference (see Figure S5, Supporting Information). The thermal conductivity λ was calculated using the formula λ = ρ·Cp·a, whereas the density ρ was considered as being constant.

telluride substructure characterized by significant covalent tellurium interactions resulting a distorted Kagomé-net and localized as well as disordered Te dumbbells. The observed disorder in parts of the Te2 dumbbell substructure gives rise to a Peierls-like behavior as observed in aforementioned Ag10Te4Br3. Stuetzite is a mixed ion/electron conductor with a significant silver ion mobility resulting in polymorphism. While the room-temperature structure has been analyzed many times, polymorphism, electronic, and thermoelectric properties are only barely analyzed and documented. Because of the fact that the p-n-p switch was addressed by the polymorphism and structural changes during the phase transition in Ag10Te4Br3 we intended to check whether the same is true for stuetzite. The variety of compounds within the Ag/Te ratio of 1:1 to 2:1 is rich. Three minerals are known, empressite AgTe,5,12,16 the aforementioned stuetzite, and hessite Ag2Te.17−19 Polymorphism is a common finding for those compounds because of the high silver ion mobility and its tendency to localize on distinct positions upon cooling. Starting from hessite with no covalent bonding of tellurium in the anion substructure a continuous increase of covalently bonded Te can be expected going toward AgTe. Consequently, because of the formation of covalent bonds the silver content decreases up to the point where only covalently bonded Te is found in AgTe. Stuetzite is located between the two extremes, and it therefore consists of isolated Te2− ions, covalently bonded [Te2]2− dumbbells, and more complex 3.6.3.6 Kagomé nets also containing significant amounts of covalently bonded tellurium. Beside hessite and stuetzite another compound with the composition Ag1.9Te is known, which can be synthesized electrochemically by removing silver from hessite.20−22 This compound is only stable at elevated temperatures between 393 and 733 K according to the given Ag−Te phase diagram (see Figure 1).19 To the best of our knowledge no structural information has been reported so far for Ag1.9Te. All binary silver tellurides are of interest due to their electrical properties and have been intensively investigated with respect to their mixed ion and electron conductivity. Ag2Te has generated reasonable interest in materials science due to its promising thermoelectric, magneto resistivity, and nonlinear optical properties.23−27 Also significant efforts are focused on nanostructuring of Ag2Te to address its quantum-confined properties.28 Polycrystalline bulk α-Ag2Te shows a thermoelectric figure of merit ZT of 0.27 at 370 K, which can be improved to a ZT of 0.55 at 400 K being a reasonable value for applications.29,30 In contrast to this, only limited information is known for stuetzite or Ag1.9Te concerning these properties. In a study where a mixture of 40−45 wt % tellurium was present (stuetzite is ∼38 wt %) the authors found promising Seebeck values of 290 to 360 μV K−1 and electrical conductivities of 2 to 20 S cm−1.31 The lack of both structural and thermoelectric information triggered us to analyze stuetzite and Ag1.9Te in more detail to close this gap.

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EXPERIMENTAL SECTION

Preparation of Stuetzite Ag4.63Te3 (or Ag1.54Te). Ag (Heareus, pieces, 99.9%) and Te (Chempur, pieces, 99.999%) were sealed in evacuated quartz glass ampules in stoichiometric ratios between Ag/Te 4.53:3 and 4.71:3, heated to 1200 K in 10 h, and held at this temperature for 24−48 h. After the ampule was quenched in an ice bath the melt was homogenized and annealed for two months at 473 K. Independent of the synthesis parameters we always observed small amounts of tellurium crystals (Figure S1). However, they could be B


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Inorganic Chemistry

Figure 2. (a−c) Crystal-structure representation of the anion substructure of Ag4.63Te3. (d) Representative view on the full structures of α-Ag4.53Te3 according to Krebs,8 the title compound α-Ag4.63Te3 and β-Ag10Te4Br3.34

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RESULTS AND DISCUSSION The Ag−Te phase diagram has been intensively examined in the past 70 years and is shown in Figure 1. Stuetzite is stated as Ag5Te3 in this diagram featuring a small homogeneity range and a decomposition temperature of 693 K. According to this diagram stuetzite decomposes peritectically into Ag1.9Te (thermal stability range ∼393 to 733 K) and Te at this temperature. The homogeneity range has been reliably determined for natural and synthetic stuetzite. According to the generally accepted and used formula Ag5−xTe3 the homogeneity range for natural stuetzite is 0 < x < 0.5, while for synthetic stuetzite it was corrected to −0.01 < x < 0.236.5,17 Depending on the composition, the phase transition temperature from roomtemperature α-Ag5−xTe3 to high-temperature β-Ag5−xTe3 was given as 570 K for tellurium-rich and as 540 K for silver-rich stuetzite. On the basis of the structure details known for αAg5−xTe3 (see Figure 2), the close structural relation to Ag10Te4Br3 concerning the polyanion substructures, and the reported polymorphism, we intended to check whether stuetzite also showed a modulation in its thermoelectric/ electric properties and if we are able to determine the structure of the high-temperature β-phase. After careful analysis of the final product after the hightemperature synthesis we found significant amounts of tellurium as a side phase beside the expected stuetzite independent to the fact that either a starting composition of Ag/Te = 4.53:3 (the composition reported by Krebs et al. in 1996) or a slight excess of silver (4.71:3) was used. A light

microscopic picture of the resulting product is given in the Supporting Information in Figure S1. We performed a thermal analysis of the stuetzite main phase after careful removal of tellurium. An X-ray powder phase analysis of the remaining sample proved the full removal of tellurium (see Figure S2). Two endothermic effects are present, namely, a broad and textured one from 545 to 555 K and a sharp one at 625 K (see Figure S3). According to the previously mentioned studies and the temperature of the first endothermic effect, we supposed to be in a silver-rich regime for our title compound. The second thermal effect is consistent with the peritectic decomposition of Ag5−xTe3 into Ag1.9Te and Te according to the phase diagram. It was also possible to separate suitable stuetzite crystals for a temperature-dependent single-crystal structure analysis. In a first study we tried to redetermine the crystal structure of αAg5−xTe3. In Figure 2 we denoted the structures of α-Ag4.53Te3 according to Krebs and our own structure determination on αAg4.63Te3.8 As one might realize, our refined composition is slightly different and more silver-rich than the one reported by Krebs. This feature results in a slightly different tellurium distribution within the distorted Te2 dumbbell region as highlighted in Figure 2. Nevertheless, the general structure features of the polyanion substructure like the honeycomb net of isolated Te2− ions, the distorted Kagomé net of partially covalent-bonded Te and the ordered Te2 dumbbells remain the same. Of course, the silver distribution differs slightly due to the different compositions and the high ion mobility. Selected crystallographic data are given in Tables 1 and 2. Semiquantitative EDS analyses of several single crystals including the C


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Inorganic Chemistry

performed at 660 K finally resulted in the complete transformation of single-crystalline β-Ag4.63Te3 (equivalent to Ag1.54Te) to single-crystalline Ag1.9Te by a quasi-topotactic reaction according to Scheme 1.

Table 1. Selected Crystallographic Data of α-Ag4.63Te3 at 293 K refined composition temperature/K space group a/Å c/Å V/Å3 Z molar mass/g mol−1 calculated density/g cm−3 2θ-range/deg transmission ratio (min/max) F(000) total number of reflection independent reflections; Rint reflections with I ≥ 3σ(I) data/parameter goodness of fit on F2 R1/wR2 [I ≥ 3σ(I)] R1/wR2 (all data) largest diff peak and hole/e Å−3

Ag4.63Te3 293 P6̅2m 13.4680(5) 8.4701(2) 1330.53(7) 6 1022.3 7.65 1.8−42.2 0.2698/0.4990 2597 30 650 1434; 0.149 1145 1434/144 1.85 0.0401; 0.0857 0.0522; 0.0884 1.96; −1.86

Scheme 1. Quasi-Topotactic Reaction of a Synthetic Stuetzite to a Ag1.9Te Single Crystal within a Capillary

Surprisingly, the refined composition of Ag5.3(6)Te3 (or Ag1.8(2)Te) is very close to that of Ag1.9Te, as stated in the phase diagram and derived from electrochemical titrations done in the past. Because of the uncertainty of the refined X-ray composition this small deviation is acceptable. A reliable semiquantitative EDS measurement through the capillary was not possible. We decided to call this compound Ag1.9Te instead of Ag1.8(2)Te in the following to make it easier for the readers to compare our results with reports of Ag1.9Te in the literature. We call this transformation only quasi-topotactic, because the transformation of single-crystalline α-Ag4.63Te3 results in singlecrystalline Ag1.9Te, but both substructures, the cation, and most parts of the anion substructure are significantly rearranged during this process. The only anion substructure that remains intact is the Te2 dumbbell in both compounds. Results of the single-crystal structure determination at 660 K are summarized in Tables 3 and 4. It is also the first report of the Ag1.9Te crystal structure, to the best of our knowledge. So far, Ag1.9Te was postulated after electrochemical removal of silver ions from Ag2Te, and its existence is still controversially discussed. Our quasi-topotactical approach represents a chemical way to synthesize Ag1.9Te, and it proves the existence of this compound. Ag1.9Te can be regarded as a polychalcogenide, where isolated Te2− form a 36 net embedded in a silver ion framework (Figure 3d). Te−Te distances are 4.69 Å, well-larger than the

one of the X-ray measurement reveal an atomic Ag/Te ratio of 61(1):39(1) being in good agreement with our refined composition (Ag/Te 60.7:39.3). The deviation to the published composition of Krebs (Ag/Te 60.1:39.9) is within the error of the EDS experiment. After the successful determination of the room-temperature α-Ag4.63Te3 polymorph we intended to determine the hightemperature β-Ag4.63Te3 structure. We heated this crystal to a temperature of 600 K to allow the phase transformation safely within the determined temperature window according to our thermal analyses. Unfortunately, the crystal showed a tendency to decompose, which we determined in three independent experiments of different crystals. We observed Bragg reflections for the high-temperature phase, but it was not possible to index and refine the structure properly. A slight but also continuous evaporation of tellurium was found to occur within the time frame of the diffraction experiment of ∼1 d. We increased the temperature further to accelerate the evaporation of tellurium and to check whether the decomposition to Ag1.9Te might occur at temperatures higher than 625 K. Our experiment

Table 2. Coordinates and Isotropic Displacement Parameters (Å2) of α-Ag4.63Te3 at 293 K atom

sof

site

x

y

z

Ueq

Te1 Te2 Te3 Te4 Te5 Te6 Te7 Ag1 Ag2 Ag3 Ag4 Ag5 Ag6 Ag7 Ag8

1 1 1 1 1 0.92(1) 0.04(1) 0.17(2) 0.69(2) 0.19(2) 1 0.62(2) 0.65(4) 0.94(1) 0.06(2)

6k 4h 3g 3f 3f 2e 2e 12l 12l 12l 6k 6j 6i 6i 3f

0.12196(11) 1/3 0.56822(14) 0.31367(17) 0.66922(16) 0 0 0.070(5) 0.2105(9) 0.2709(18) 0.19497(18) 0.1416(9) 0.432(2) 0.8052(3) 0.098(4)

0.29848(11) 2/3 0 0 0 0 0 0.520(5) 0.4201(5) 0.4395(14) 0.53833(17) 0.5650(10) 0 0 0

1/2 0.2254(2) 1/2 0 0 0.1623(3) 0.288(4) 0.171(4) 0.1919(15) 0.2624(13) 1/2 0 0.267(3) 0.2507(5) 0

0.0235(5) 0.0322(4) 0.0341(7) 0.0351(8) 0.0277(7) 0.0238(7) 0.014(13) 0.16(3) 0.054(2) 0.125(9) 0.0408(8) 0.0478(19) 0.076(4) 0.0476(11) 0.12(3)

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Inorganic Chemistry Table 3. Selected Crystallographic Data of Ag5.3(6)Te3 (or Ag1.8(2)Te) at 660 K refined composition temperature/K space group a/Å c/Å V/Å3 Z molar mass/g mol−1 calculated density/g cm−3 θ-range/deg F(000) total number of reflection independent reflections; Rint reflections with I ≥ 3σ(I) data/parameter goodness of fit on F2 R1/wR2 [I ≥ 3σ(I)] R1/wR2 (all data) largest diff peak and hole/e Å−3

Ag5.3(6)Te3 (or Ag1.8(2)Te) 660 P6̅m2 4.693(3) 11.486(7) 219.1(2) 1 954.0 7.25 1.8−29.3 406 5603 278; 0.542 54 278/54 1.01 0.0333; 0.0777 0.0913; 0.2437 0.72; −0.74

Figure 3. Crystal structure sections of Ag1.9Te determined at 660 K. (a, b) Two sets of subunits. View along [110] in (a, b). (c) Te2 dumbbells coordinated by silver ions. (d) 36 Te net. Displacement parameters are given at 50% probability in (a).

sum of the van der Waals radii of ∼4.4 Å at this temperature. The isolated Te2− ions are surrounded by silver ions as shown in Figure 3b, top and bottom. A certain fraction of covalently bonded Te2 dumbbells is preserved (as one might expect from the composition of Ag1.9Te, which is significantly different than Ag2Te, where only isolated Te2− are present) separating the aforementioned silver-coordinated 36 nets. The Te2 dumbbells are denoted in Figure 3a,c. The full crystal structure can be described by two alternating substructures stacked along the caxis, a silver-coordinated 36 Te and a layer of parallel-oriented and silver-coordinated Te2 dumbbell arrangement. All Te sites are fully occupied. The Te2 dumbbells featuring a Te−Te bond length of 2.71(2) Å are also embedded in a silver ion environment. As compared with the element (Te−Te bond lengths of 2.83 Å at room temperature) this distance seems to be short, but other examples for dumbbell-like tellurium substructures are reported, where this distance is realized. Examples are UTe3 with Te−Te = 2.75 Å or ZrTe3 with 2.79 Å just to name some.35,36 The Ag−Te bond lengths are fully distributed in an expected range of 2.5 to 2.9 Å. Silver ions are completely delocalized in this compound on five different site featuring occupancy factors between 0.22 and 0.73 substantiating the large mobility and diffusion within the anion substructure. The short Te−Te distance might be the reason for the decomposition of Ag1.9Te at temperatures below 393 K, where the silver mobility significantly goes down, and an effective coordination of the Te2 dumbbell by highly mobile silver ions at various sites is no longer possible. In good accordance with

literature we were also not able to stabilize Ag1.9Te at room temperature. We can also rule out another possible structure model for Ag1.9Te, which has been published for Cu2−xTe with 0 < x < 0.26.37−43 In contrast to our suggested model tellurium tends to be more localized in this postulated anion substructure and the d10 ions supposed to segregate between the anions (Figure S6). As a result, tellurium is expected to be four (!) coordinated by other tellurium, and a certain under-occupancy of tellurium sites are necessary to describe the anion part of this model. Further details of the structure data are given in the Supporting Information. Because of structure chemical considerations including an even shorter Te−Te distance of 2.68(2) Å for the Te2 dumbbells we can safely skip this structure model. Because of the promising thermoelectric properties of Ag2Te with a reported ZT of up to 0.55 at slightly elevated temperatures we performed a full thermoelectric characterization of Ag4.63Te3 and Ag1.9Te as well.30 We started our thermoelectric investigation for Ag4.63Te3 at 300 K and moved to 680 K directly into the existence region of Ag1.9Te. At room temperature, a Seebeck coefficient of 375 μV K−1 was detected (see Figure 4). The Seebeck coefficient decreases in the temperature range from 300 to 680 K to 166 μV K−1, which is consistent with an increase of the total electrical conductivity from 2 to 122 S cm−1. Especially in the region of the α- to β-Ag4.63Te3 phase transition we used a narrow measurement grid to follow the evolvement of the Seebeck

Table 4. Coordinates and Isotropic Displacement Parameters (Å2) of Ag5.3(6)Te3 (or Ag1.9Te) at 660 K atom

sof

site

x

y

z

Ueq

Te1 Te2 Ag1 Ag2 Ag3 Ag4 Ag5

1 1 0.22(3) 0.45(6) 0.56(9) 0.59(6) 0.73(4)

2i 1a 3k 2i 2h 2h 2g

2/3 0 0.278(9) 2/3 1/3 1/3 0

1/3 0 0.722(9) 1/3 2/3 2/3 0

0.3822(13) 0 1/2 0.168(4) 0.095(2) 0.280(5) 0.245(4)

0.122(4) 0.20(2) 0.22(3) 0.43(3) 0.27(2) 0.12(2) 0.18(2)

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Inorganic Chemistry

mobility increase and Peierls distortion as in the previous case. After the quasi-topotactic reaction to Ag1.9Te, the Seebeck coefficient drops more significantly from 200 to 170 μV K−1. During both drops we observe an increase of the electrical conductivity, which is in line with this finding. To fully characterize the thermoelectric properties, we measured the thermal diffusivity of the title compounds and calculated the thermal conductivity hereof. The thermal diffusivities are denoted in the Supporting Information. At room temperature we found a very small thermal conductivity of 0.21 W m−1 K−1 (thermal diffusivity 1.1 × 10−7 m2 s−1), being among the smallest observed for thermoelectric materials. The thermal diffusivity is equal to the one observed for Ag20Te10BrI and only slightly lower than that for Ag10Te4Br3 (1.9 × 10−7 m2 s−1).44 It is even as small as the thermal diffusivity found in isolating polymers like polyvinyl chloride (1.0 × 10−7 m2 s−1) or polyethylene (2.2 × 10−7 m2 s−1).45,46 The small anomalies in the thermal diffusivity might be related to the start of the Te evaporation (at 450 K) and the two phase transitions at 545 and 625 K, but these statements need to be further substantiated. Putting all experiments together the ZT value starts at 0.04 at 300 K and increases to a reasonable number of 0.57 for Ag1.9Te at 680 K (see Figure 5). Such a value at 680 K is comparable to commonly used or known thermoelectric materials like polycrystalline PbTe, the skutterudite-type Ba0.30Ni0.05Co3.95Sb12, or the clathrate Ba8Ga16Ge30.47−49 Nanostructuring like in the case of Ag2Te might be an option to further improve this figure of merit.30 As a major drawback it must be stated at this point that Ag4.63Te3 significantly loses tellurium upon heating to the point where Ag1.9Te is formed, and Ag1.9Te itself is only stable above 393 K. Therefore, neither Ag4.63Te3 nor Ag1.9Te are suitable materials for thermoelectric applications.

Figure 4. DSC curve (green), Seebeck coefficient (black), and total electrical conductivity (blue) for Ag4.63Te3 and Ag1.9Te, determined in a temperature interval from 300 to 680 K. An enlarged section is given in the red box.

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CONCLUSION In this study the two silver polychalcogenides Ag4.63Te3 and Ag 1.9Te were subject to structural and thermoelectric investigations. Ag4.63Te3, synthesized by a solid-state melting and annealing process, is closely related to the mineral stuetzite Ag5−xTe3, and it shows a certain phase broadening according to both elements. The room-temperature structure of this polymorphic compound was redetermined, and only slight

coefficient during the phase transition. While for Ag10Te4Br3 a significant switch of the Seebeck coefficient including a change of sign took place caused by the aforementioned p-n-p switch, no such behavior was present in the case of Ag4.63Te3. Upon the α- to β-Ag4.63Te3 phase transition it is only slightly reduced from 225 to 205 μV K−1. It can therefore be concluded that the tellurium substructure in Ag4.63Te3 does not show a severe

Figure 5. Thermal conductivities and ZT values of Ag4.63Te3 and Ag1.9Te determined in a temperature interval from 300 to 680 K. The error of the ZT is in the order of 10%. F


Inorganic Chemistry

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variations in the crystal structure were found in comparison to the previously reported one. By a quasi-topotactic reaction upon tellurium release we were able to find a chemical access to Ag1.9Te, a compound that has been observed only by electrochemical titrations from Ag2Te so far. By the aid of a temperature-dependent single-crystal structure experiment we were able to follow this quasi-topotactic reaction from Ag4.63Te3 to Ag1.9Te directly on a diffractometer, and it was also possible to determine the crystal structure of the latter. According to thermal analysis Ag4.63Te3 is transferred to Ag1.9Te at 625 K. Ag1.9Te shows two different anion substructures, a 36 net of isolated Te2− ions and Te2 dumbbells, which are both coordinated by disordered and highly mobile silver ions. Thermoelectric properties were measured showing that a significant increase of the ZT value takes place up to 680 K. With continuous increase of temperature and after formation of Ag1.9Te it reaches 0.57, which is comparable to state-of-the-art thermoelectric materials at this temperature. Unfortunately, because of the continuous loss of tellurium during heating of Ag4.63Te3, this compound will not be suitable for any thermoelectric applications. While Ag1.9Te seems to be stable concerning tellurium losses its thermal stability window of 393 to 625 K will also limit its usage.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01991. Literature overview of published compositions and space groups reported for Ag5−xTe3, details of the synthesis, powder diffraction measurement, DSC measurements, additional crystallographic information like displacement parameters, and a Ag1.9Te structure model at 660 K, analogous to Cu2−xTe (PDF) Accession Codes

CCDC 1570026−1570027 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tom Nilges: 0000-0003-1415-4265 Funding

The support by the TUM Graduate School for F.B. is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The measurements of the thermal diffusivity and the specific heat capacity were performed by Dr. A. Lindemann from NETZSCH, which is gratefully acknowledged. The authors also would like to thank Dr. W. Klein for the measurement of the temperature-dependent single-crystal data. G


Article

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A Chemical, High-Temperature Way to Ag1.9Te via Quasi-Topotactic

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