Three structure types for strontium copper(I) lanthanide(III) selenides SrCuMSe3 (MÂ =Â La, Gd, Lu)

Quaternary strontium copper selenides with the trivalent lanthanides exhibit the simple formula SrCuMSe3, but occur in three orthorhombic structures (with Z = 4) depending upon the radius of the M3+ cation. For SrCuLaSe3 (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 4f0 La3+) coordination numbers of seven are realized for both high-charged cations (Sr2+ and La3+: CN = 7). For SrCuGdSe3 (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 4f7 Gd3+) Sr2+ shows the same (CN = 7), whereas Gd3+ reveals a diminished coordination number (CN = 6). For SrCuLuSe3 (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 4f14 Lu3+) the coordination number of Lu3+ remains unchanged (CN = 6), but Sr2+ gains an extra Se2− ligand to expand its coordination sphere (CN = 8). In all three selenides the Cu+ cations have only four Se2− anions as next neighbours, so the crystal structures can be described as being built up by vertex-sharing [CuSe4]7− tetrahedra (d(Cu+-Se2−) = 245-252 pm), which form infinite chains {∞1[CuSe2/1Se′2/2]5−} running along [0 1 0] in SrCuLaSe3 or SrCuGdSe3 and [1 0 0] in SrCuLuSe3, respectively. SrCuLaSe3 crystallizes with the PbCuLaS3-type structure and an equimolar distribution of the Sr2+ and La3+ cations over both monocapped trigonal prismatically coordinated sites. SrCuGdSe3 prefers the Eu2CuS3- (according to EuIICuEuIIIS3) and SrCuLuSe3 the KCuZrS3-type structure, which both contain layers of edge- and vertex-shared [MSe6]9− octahedra and display a Cmcm-Pmcn group-subgroup relationship.