A Polyselenide with a Novel Se78– Unit: the Structure of Sr19–xPbxGe11Se44 with x = 5.0 and 6.4

Abstract

An unprecedented, planar Se78– unit with a threefold rotation axis was discovered in the new polyselenide Sr19–xPbxGe11Se44. Sr12.6Pb6.4Ge11Se44 crystallizes in the non‐centrosymmetric hexagonal space group P63 with unit cell dimensions of a = b = 19.9867(2) Å, c = 12.0546(1) Å, V = 4170.28(7) Å3, with two formula units per cell. Its structure contains isolated [GeSe4] tetrahedra, [Ge2Se7] dimers, and a novel, planar propeller‐shaped Se78– unit. The latter is unprecedented, but reminiscent of the isoelectronic pyramidal Se78– and I7– units. It consists of a central Se atom, to which three Se2 pairs with a short Se–Se bond are attached through a longer Se–Se distance, resulting in a central Se atom featuring 120° bond angles, and bridging Se atoms featuring an approximately linear coordination with a 179° bond angle. It can thus be written as [Se(Se2)3]8–. We characterized this unit using Gaussian calculations in comparison to the pyramidal Se78– unit, which may be described also as [Se(Se2)3]8–. The solid‐state material is a black semiconductor with an optical band‐gap of 1.8 eV, which supports the assignment of this charge.