Three structure types for strontium copper(I) lanthanide(III) selenides SrCuMSe 3 (M = La, Gd, Lu)

Abstract

Quaternary strontium copper selenides with the trivalent lanthanides exhibit the simple formula SrCuMSe 3, but occur in three orthorhombic structures (with Z = 4) depending upon the radius of the M 3+ cation. For SrCuLaSe 3 ( a = 848.82(5) pm, b = 423.04(3) pm, c = 1671.23(9) pm, space group: Pnma) with the largest lanthanide ion (closed-shell 4f 0 La 3+) coordination numbers of seven are realized for both high-charged cations (Sr 2+ and La 3+: CN = 7). For SrCuGdSe 3 ( a = 1068.10(7) pm, b = 410.79(3) pm, c = 1349.21(8) pm, space group: Pnma) with an intermediate lanthanide ion (half-filled shell 4f 7 Gd 3+) Sr 2+ shows the same (CN = 7), whereas Gd 3+ reveals a diminished coordination number (CN = 6). For SrCuLuSe 3 ( a = 404.98(3) pm, b = 1345.67(8) pm, c = 1040.19(7) pm, space group: Cmcm) with the smallest lanthanide ion (closed-shell 4f 14 Lu 3+) the coordination number of Lu 3+ remains unchanged (CN = 6), but Sr 2+ gains an extra Se 2− ligand to expand its coordination sphere (CN = 8). In all three selenides the Cu + cations have only four Se 2− anions as next neighbours, so the crystal structures can be described as being built up by vertex-sharing [CuSe 4] 7− tetrahedra ( d(Cu +–Se 2−) = 245–252 pm), which form infinite chains { ∞ 1 [ CuS e 2 / 1 S e ′ 2 / 2 ] 5 − } running along [0 1 0] in SrCuLaSe 3 or SrCuGdSe 3 and [1 0 0] in SrCuLuSe 3, respectively. SrCuLaSe 3 crystallizes with the PbCuLaS 3-type structure and an equimolar distribution of the Sr 2+ and La 3+ cations over both monocapped trigonal prismatically coordinated sites. SrCuGdSe 3 prefers the Eu 2CuS 3- (according to Eu IICuEu IIIS 3) and SrCuLuSe 3 the KCuZrS 3-type structure, which both contain layers of edge- and vertex-shared [MSe 6] 9− octahedra and display a Cmcm– Pmcn group–subgroup relationship.