Trends in thermodynamic parameters of phase transitions of lanthanide sulfides SrLnCuS3 (Ln = La–Lu)

Abstract

SrLnCuS3 (Ln = La–Lu) compounds melt incongruently. Their thermochemical parameters are determined. The melting temperatures and the enthalpies of melting are: for SrLaCuS3, T = 1513 K and ΔH = 6.9 kJ mol−1; for SrCeCuS3, T = 1468 K and ΔH = 5.2 kJ mol−1; for SrPrCuS3, T = 1459 K and ΔH = 13.2 kJ mol−1; for SrNdCuS3, T = 1429 K and ΔH = 16.8 kJ mol−1; and for SrSmCuS3, T = 1605 K and ΔH = 2.8 kJ mol−1. Three high-temperature polymorphic transitions are found to occur in SrLnCuS3 (Ln = Sm, Gd–Lu) compounds. The parameters of these transitions are determined: for SrSmCuS3, Tα ↔ β = 1452 K, ΔHα ↔ β = 3.0 kJ mol−1, Tβ ↔ γ = 1464 K, ΔHβ ↔ γ = 0.2 kJ mol−1, Tγ ↔ δ = 1476 K, and ΔHγ ↔ δ = 1.1 kJ mol−1; for SrDyCuS3, Tα ↔ β = 1530 К, Tβ ↔ γ = 1568 К, and Tγ ↔ δ = 1585 K; for SrTmCuS3, Tα ↔ β = 1580 K, Tβ ↔ γ = 1618 K, and Tγ ↔ δ = 1631 K; and for SrYbCuS3, Tα ↔ β = 1567 K, Tβ ↔ γ = 1608 K, and Tγ ↔ δ = 1621 K. The transitions are observed both upon heating and upon cooling. The high-temperature phases are not quenchable. Phase-transition temperature versus r(Ln3+) curves for SrLnCuS3 (Ln = La–Lu) feature the tetrad effect. The SrLnCuS3 (Ln = La–Nd) compounds are classified as thiocuprates; their melting temperatures decrease systematically from La to Nd. The SrCuLnS3 (Ln = Sm, Gd–Lu) compounds are classified as thiolanthanates; their melting temperatures increase in the order from Sm to Tm and from Tm to Lu.