Synthesis, X-ray data, and thermodynamic properties of the AgTe3 high-pressure phase in the Ag–Te system

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Highlights

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    Silver telluride, AgTe3, was synthesized in a high-pressure gas vessel.

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    The thermodynamic properties of AgTe3 have been determined for the first time.

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    The measurements were carried out in using solid-state electrochemical cell.

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    The thermodynamic properties at normal conditions were obtained.

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    The instability of AgTe3 compound under normal conditions was confirmed.

Abstract

Silver telluride, AgTe3, was synthesized in a high-pressure gas vessel at a temperature of 473 K and a hydrostatic pressure of pure Ag at 6500 bar. A Rietveld structure refinement of X-ray powder diffraction data has been performed. The structure has been refined in the trigonal space group, R3m(H), and has the following cell parameters: a = 8.653 Å, c = 5.278 Å, and V = 342.24 Å3. It was found that AgTe3 stored in a sealed container for three months partially decomposes into Ag5Te3 (stützite) and tellurium.

The thermodynamic properties of AgTe3 have been determined for the first time. The electromotive force (EMF) method for a solid-state galvanic cell with RbAg4I5 solid electrolyte.

(−) Pt | Ag | RbAg4I5 | AgTe3, Te | Pt (+).

was used to study a virtual chemical reaction of silver telluride formation from the elements;

Ag(s) + 3Te(s) = AgTe3(s)

The measurements were carried out in the temperature range of 402–474 K and pressures of 3921–6169 bar. The standard Gibbs energy of AgTe3 formation and its standard thermodynamic properties and molar volume at 298 K and 1 bar (105 Pa) pressure were derived from the EMF measurements:

ΔrG(T, p)/J mol−1 = (−21470 ± 3880) + (133.0 ± 62.5)∙T – (21.96 ± 8.82)∙Tln(T).

– (0.178 ± 0.009)∙(p – 1).

ΔfG° (298.15 K, 1 bar) = −19.12 ± 0.24 kJ/mol; ΔfH° (298.15 K, 1 bar) = −14.92 ± 1.25 kJ/mol.

S° (298.15 K, 1 bar) = 205.8 ± 3.5 J/(mol K).

V° (298.15 K, 1 bar) = 69.93 ± 0.09 cm3/mol.

Introduction

The Ag–Te binary system is of particular interest for solid-state physics and materials science, since Ag2Te is a 3D-topological insulator at atmospheric pressure. When the pressure increases, some phases undergo polymorphic transformations [1,2], and phase stability areas change significantly, mainly through the formation of new stable compounds, such as AgTe3. In order to determine the phase relations, it is necessary to know the thermodynamic properties of all the binary compounds in the system.

Voronin et al. [3] compiled original experimental and literature data on the thermodynamic properties (ΔfG°, S°, ΔfH°) of silver tellurides (α-Ag2Te, β-Ag2Te, Ag1.9Te, Ag5Te3, AgTe), analyzed the phase relations in the binary Ag–Te system and calculated the ternary phase diagram of the Ag–Te–O system. Currently, AgTe3 – the newly discovered mineral lingbaoite [4] – is the only silver telluride which thermodynamic properties are unknown.

The phase with the chemical composition AgTe3 was first synthesized from elements at high pressure (>12 kbar) and a temperature of 100–1200 °C [5]. The resulting phase is close to the phase obtained by Luo and Klement [6] by rapid tempering of a silver-telluric melt of 20.5–30.5 at% Ag composition. The effect of temperature and pressure on phase relations in the Ag–Te system containing 75 at% Te was studied by high-pressure DTA and quenching experiments at pressures up to 32 kbar [7]. These studies allowed determining the lower stability limit of AgTe3, which is 4 kbar at 355 °C. The AgTe3 compound with a primitive cubic cell (π-phase) was also found in a mixture with other silver tellurides obtained as a result of mechanical alloying in the Ag–Te system [8]. Recently AgTe3 has been found in nature inside inclusions in pyrite (FeS2) along with AgAuTe4 (sylvanite) and chalcopyrite (CuFeS2) and was called lingbaoite [4].

At standard temperature and pressure, synthetic AgTe3 begins to decompose into Ag5Te3 (stützite) and Te [5] almost immediately, which makes its thermodynamic properties very difficult to study using traditional methods.

Previously, the solid-state electromotive force (EMF) method was successfully used to determine the Gibbs energy of reactions involving silver [3,9,10], particularly for studying reactions at high pressures [11,12].

The aim of this paper is to synthesize AgTe3, refine the crystal lattice parameters of this phase, and first determine the thermodynamic properties of AgTe3 using the high-pressure solid-state EMF method.

Section snippets

Chemicals and compounds

Monocrystalline tellurium (99.9999%) for phase and mixture synthesis was provided by the Institute of Microelectronics Technology and High-Purity Materials, Russian Academy of Sciences (Chernogolovka). The synthesis also included silver powder (Alfa Aessar-325, 99.9%). A silver bar (99.99%) and a 0.2 mm platinum wire (99.97%) were used in the construction of the electrochemical cell. RbAg4I5, synthesized in the form of transparent monocrystalline blocks at the Institute of Microelectronics

Full profile XRD analysis of AgTe3

A fresh sample of AgTe3 stored in an airtight container for less than 10 days after the synthesis contained a small tellurium impurity (about 0.5 wt %). The full-profile fitting of the diffraction spectrum by three phases (AgTe3, Si, Te) has a stable convergence (Fig. 1).

Structural and fitting parameters are shown in Table 1. The diffraction lines of the phases AgTe3 show broadening at high angles of diffraction. Full-profile analysis of the diffraction spectrum shows that this broadening can

Discussion

Fig. 5 shows the diagram of the thermodynamic stability of phases in the Ag–Te system in the coordinates of reduced Gibbs energy (Jg−1atom−1) vs. composition under standard conditions. The phase diagram shows binary compounds with compositions: α- (hessite) and β-Ag2Te, Ag1.9Te, Ag5Te3 (stützite), AgTe (empressite), and AgTe3 (lingbaoite). The Gibbs energies of compounds for the diagram, other than AgTe3, were taken from Voronin et al. [3]. Each thermodynamically stable phase must be located

Conclusions

Our study has confirmed that AgTe3 obtained under elevated P-T conditions is not stable under standard conditions and decomposes relatively quickly into metallic tellurium and stützite, which makes its recent discovery in the form of lingbaoite noteworthy.

The thermodynamic properties of synthetic AgTe3 were first determined by EMF in a fully solid-state galvanic cell. Extrapolation of ΔfG°(T,p) to standard conditions also indicates the thermodynamic instability of this phase at 298.15 K and a

Funding source

The synthesis of the AgTe3 was done with the support of the state assignment of IEM RAS (Theme №AAAA-A18-118020590154-4). The powder-XRD analysis, Rietveld refinement, and EMF-measurements were supported by the Russian Foundation for Basic Research (grant №19-05-00482).

CRediT authorship contribution statement

Evgeniy G. Osadchii: Investigation, Writing - review & editing, Funding acquisition, Supervision. Veniamin B. Polyakov: Writing - review & editing. Valentin O. Osadchii: Formal analysis, Visualization, Writing - original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to Dr. S. Khasanov for the Rietveld refinement of AgTe3 powder-XRD spectra and a useful discussion.

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