Cs2Ge3In6Se14: A Structure Transformation Driven by the Size Preference and Its Properties.

The new selenide Cs2Ge3In6Se14, featuring its own structure type with germanium in mixed-valence states, is discovered via a solid-state reaction at 1173 K. The compound crystallizes in the R3̅ m space group with a = 7.9951(6) Å and c = 41.726(4) Å. Two adjacent condensed layers of InSe4 tetrahedra are linked by a [Ge2+Se6] octahedron into a double slice that is further stacked along the c direction with a packing sequence of ··· abca··· through the [Ge3+2Se6] dimer via its Ge-Ge metallic bond. The coexistence of Ge2+/Ge3+ and Ge-Ge metallic bonding has been confirmed by XPS and ELF analyses, respectively. More interestingly, although sharing many structure similarities, Cs2Ge3In6Se14 and our previously reported Cs2Ge3In6Te14 reveal a R3̅ m to P3̅ m1 structure transformation with a tripled c parameter. Single-crystal diffraction data and a thorough structure survey of related compounds point out that such a transformation is driven by the size preference of the [Ge2Q6] dimer. The title compound possesses a band gap of 2.08 eV and shows photodegradation of RhB under visible light that is more efficient than that for the commercial P25.