Temperature dependence of tellurium fugacity for the kotulskite (PdTe)–merenskyite (PdTe2) equilibrium determined by the method of a solid-state galvanic cell

Abstract

The thermodynamic properties of the kotulskite (PdTe)/merenskyite (PdTe2) equilibrium in the Ag-Pd-Te system were determined for the first time by the electromotive force (EMF) method. The thermodynamic properties were calculated from the temperature dependence of the EMF in a completely solid-state electrochemical cell with a common gas space: $$\left( - \right){\text{C}}_{{({\text{graphite}})}} \left| {{\text{Ag}}} \right|{\text{RbAg}}_{{4}} {\text{I}}_{{5}} \left| {{\text{Ag}}_{{2}} {\text{Te}},{\text{PdTe}},{\text{PdTe}}_{{2}} } \right|{\text{C}}_{{({\text{graphite}})}} \left( + \right),$$ following a virtual chemical reaction: $${\text{2Ag}} + {\text{PdTe}}_{{2}} \leftrightarrow {\text{Ag}}_{{2}} {\text{Te}} + {\text{PdTe}}.$$ The measurements were carried out in the temperature range of 371–488 K at the atmospheric pressure of pure argon. As a result, the thermodynamic properties for the reaction PdTe + Te = PdTe2 at a pressure of 1 bar (105 Pa) were determined as: ΔrGo/J·mol−1 = − 27,639; ΔrS/J mol−1 K−1 = − 21.98; ΔrHo/J·mol−1 = − 34,176. Fugacity of gaseous tellurium (Te2) over the PdTe + 1/2Te2 ↔ PdTe2 equilibrium was also calculated: log f Te2 = (9.205 ± 0.072) − (11.44 ± 0.03)·(1000/T), (371.7 < T/K < 493.6).