Homogeneity ranges and thermodynamic properties of the Te-rich phases in the Cr-Te system

Abstract

The Te-rich region of the Cr-Te system was investigated by using high temperature mass spectrometry. The composition range from ~60 to 82.7 at.% Te was covered by preferentially evaporating off tellurium at T < 700 K from the alloys of starting compositions 66.9, 70.1, 80.0 and 82.7 at.% Te. Evidence for the existence of a new polytelluride (77.0 ± 0.3 to 78.3 ± 0.2 at.% Te) was obtained while the existing uncertainty about the homogeneity range of CrTe₃, hitherto considered as the most Te-rich phase in the Cr-Te system, was removed. This phase exists from 70.5 ± 0.8 to 74.4 ± 0.5 at.% Te. In the case of Cr₅Te₈, only the Te-rich boundary (63.6 ± 0.6 at.% Te) could be deduced. The activities of tellurium across the single-phase regions were obtained from the p(Te₂) measured as a function of composition and those of chromium computed by a Gibbs-Duhem integration. Subsequently Gibbs free energies of formation, Δ_fG_m° at T = 650 K were also deduced. The values (in kJ mol^-1, given in brackets) for the formulae (given in parentheses) corresponding to the Te-rich and/or Cr-rich boundary compositions are: CrTe_4-y phase: - [90.7 ± 0.4] (CrTe_3.61) and -[90.5 ± 0.4] (CrTe_3.35); CrTe₃ phase: -[90.4 ± 1.0] (CrTe_2.91) and - [89.5 ± 1.5] (CrTe_2.39); Cr₅Te₈ phase: -[86.2 ± 2.2] (CrTe_1.75, the Te-rich boundary).