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

SrLnCuS 3 (Ln = La–Lu) compounds melt incongruently. Their thermochemical parameters are determined. The melting temperatures and the enthalpies of melting are: for SrLaCuS 3 , T = 1513 K and Δ H = 6.9 kJ mol −1 ; for SrCeCuS 3 , T = 1468 K and Δ H = 5.2 kJ mol −1 ; for SrPrCuS 3 , T = 1459 K and Δ H = 13.2 kJ mol −1 ; for SrNdCuS 3 , T = 1429 K and Δ H = 16.8 kJ mol −1 ; and for SrSmCuS 3 , T = 1605 K and Δ H = 2.8 kJ mol −1 . Three high-temperature polymorphic transitions are found to occur in SrLnCuS 3 (Ln = Sm, Gd–Lu) compounds. The parameters of these transitions are determined: for SrSmCuS 3 , Tα ↔ β = 1452 K, Δ Hα ↔ β = 3.0 kJ mol −1 , Tβ ↔ γ = 1464 K, Δ Hβ ↔ γ = 0.2 kJ mol −1 , Tγ ↔ δ = 1476 K, and Δ Hγ ↔ δ = 1.1 kJ mol −1 ; for SrDyCuS 3 , Tα ↔ β = 1530 К, Tβ ↔ γ = 1568 К, and Tγ ↔ δ = 1585 K; for SrTmCuS 3 , Tα ↔ β = 1580 K, Tβ ↔ γ = 1618 K, and Tγ ↔ δ = 1631 K; and for SrYbCuS 3 , 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 (Ln 3+ ) curves for SrLnCuS 3 (Ln = La–Lu) feature the tetrad effect. The SrLnCuS 3 (Ln = La–Nd) compounds are classified as thiocuprates; their melting temperatures decrease systematically from La to Nd. The SrCuLnS 3 (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.