Regularities of change in the structural parameters of EuLnCuS3 (Ln = La–Nd, Sm, Gd, Ho)

Abstract

Regularities of change in the structural parameters of EuLnCuS₃ (Ln = La–Nd, Sm, Gd, Ho) at an annealing temperature of 970 and 1170 K have been established. A decrease in the Ln³⁺ ionic radius results in the consecutive change of structural types (STs) for the compounds: α-EuLnCuS₃ (Ln = La, Ce, Pr, Nd; BaLaCuS₃ ST) → β-EuLnCuS₃ (Ln = Ce, Pr, Nd; Ba2MnS₃ ST) → γ-EuLnCuS₃ (Ln = Sm, Gd, Ho; Eu₂CuS₃ST). The change of structural types for EuLnCuS₃ leads to a jump-like change in their unit cell parameters and the transformation of coordination polyhedra shaped as a one-capped trigonal prism LnS₇ (α and β phases) into an octahedron LnS₆ (γ phases). The appearance of morphotropic changes correlates with the tetrad effect.