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 SrLaCuS3, T = 1513 K and Delta H = 6.9 kJ mol(-1); for SrCeCuS3, T = 1468 K and Delta H = 5.2 kJ mol(-1); for SrPrCuS3, T = 1459 K and Delta H = 13.2 kJ mol(-1); for SrNdCuS3, T = 1429 K and Delta H = 16.8 kJ mol(-1); and for SrSmCuS3, T = 1605 K and Delta 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 SrSmCuS3, T alpha a dagger beta = 1452 K, Delta H alpha a dagger beta = 3.0 kJ mol(-1), T beta a dagger gamma = 1464 K, Delta H beta a dagger gamma = 0.2 kJ mol(-1), T gamma a dagger delta = 1476 K, and Delta H gamma a dagger delta = 1.1 kJ mol(-1); for SrDyCuS3, T alpha a dagger beta = 1530 De, T beta a dagger gamma = 1568 De, and T gamma a dagger delta = 1585 K; for SrTmCuS3, T alpha a dagger beta = 1580 K, T beta a dagger gamma = 1618 K, and T gamma a dagger delta = 1631 K; and for SrYbCuS3, T alpha a dagger beta = 1567 K, T beta a dagger gamma = 1608 K, and T gamma a dagger delta = 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.