Phase Equilibria in the Ag–Ge–Bi–Te System and Thermodynamic Properties of the nGeTe•mBi₂Te₃ (n, m = 1–4) Layered Compounds

Phase equilibria of the Ag–Ge–Bi–Te system in the part GeTe–Ag₈GeTe₆–Te–Bi₂Te₃ (I) were established by the electromotive force (EMF) method with a solid Ag⁺ conducting electrolyte. It was shown that the concentration space of (I) in the part 4GeTe⋅Bi₂Te₃–Ag₈GeTe₆–Te–Bi₂Te consists of 8 four-phase regions, formed of the layered compounds of the nGeTe⋅mBi₂Te₃ (n, m = 1–4) homologous range, as well as Ag₈GeTe₆, Bi₂Te₃, and Te. Equations of overall potential-forming reaction of the decomposition and synthesis of compounds of the homologous range were written for each region. Reactions were performed in the following electrochemical cells (ECCs) (−)IE | Ag | SE | PE | IE(+), where IE is the inert electrode (graphite), Ag is the negative (left) electrode, SE is the solid-state Ag⁺ ion-conducting electrolyte, PE is the positive (right) electrode. PEs of ECCs were prepared by melting of a mixture of the high-purity elements Ag, Ge, Bi, and Te. The component ratios in samples were determined based on the equations of the potential-forming reactions in respective phase Phase equilibria regions. The finely grounded samples were used as PEs of ECCs. The synthesis of an equilibrium set of phases was performed in the part of PE that is in contact with SE of ECC at T = 580 K for 50 h. Silver cations that shift from the left to the right electrode acted as the nucleation centers of equilibrium compounds in the corresponding phase regions. Linear dependences E versus T of ECCs in the range of T = (440–500) K were used to calculate values of the Gibbs energies, enthalpies of formation, and entropies of compounds GeTe⋅4Bi₂Te₃, GeTe⋅3Bi₂Te₃, GeTe⋅2.5Bi₂Te₃, GeTe⋅2Bi₂Te₃, GeTe⋅Bi₂Te₃, 2GeTe⋅Bi₂Te₃, 3GeTe⋅Bi₂Te₃, and 4GeTe⋅Bi₂Te₃. The differences in the values of the thermodynamic functions of GeTe-rich and Bi₂Te₃-rich compounds with respect to GeTe⋅Bi₂Te₃ correlate well with the literature data on the differences of their crystal structures.