Systematics and anomalies in formation and crystal structures of RScSb and R3Sc2Sb3 rare earth compounds

Abstract

• New equiatomic RScSb compounds identified for R = La–Nd, Sm, Gd–Tm, Lu, and Y. • The RScSb compounds crystallize in two different crystal structures. • Lighter R (R = La–Nd, Sm) crystallize with the CeScSi-type (tetragonal tI 12, I 4/ mmm ). • Heavier R (R = Gd–Tm, Lu, Y) adopt the CeFeSi-type (tetragonal tP 6, P 4/ nmm ). • New R 3 Sc 2 Sb 3 phases identified for R = Gd–Tm, Lu, Y; β-Yb 5 Sb 3 -type (orth. oP 32, Pnma ). A systematic study of RScSb (R = rare earth) ternary alloys has been carried out by X-ray diffraction, optical and electron microscopy and microprobe analysis. As a result, the new equiatomic RScSb (R = La–Nd, Sm, Gd–Tm, Lu, Y) compounds have been identified. No formation of equiatomic 1:1:1 phases has been observed for Eu and Yb. It has been found the RScSb compounds crystallize in two different crystal structures. The phases formed by the lighter R (La–Nd, Sm) adopt the CeScSi-type (tetragonal tI 12, I 4/ mmm , an ordered variant of the La 2 Sb-type), while the ones containing the heavier R (R = Gd–Tm, Lu, Y) crystallize with the CeFeSi-type (tetragonal tP 6, P 4/ nmm , an ordered derivative of the Cu 2 Sb-type). The latter phases were expected to be dimorphic, thus suggesting they might be polymorphic having the CeScSi-type as the low-temperature form; however, no proof of this was found in the course of the present study. Besides the equiatomic compounds, the R 3 Sc 2 Sb 3 phases have also been identified. They form from Gd-Tm, Lu, included Y, and crystallize in the β-Yb 5 Sb 3 -type (orthorhombic oP 32, Pnma ). The observed lattice parameters , unit cell volume and volume contraction, for both the series of compounds, decrease on going from La to Lu following the lanthanide contraction trend. First principles calculations pinpoint that the differences in the electronic structure are directly related to the differences in the crystal structures of these compounds.