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 <i>R</i>3̅<i>m</i> space group with <i>a</i> = 7.9951(6) Å and <i>c</i> = 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 <i>c</i> direction with a packing sequence of ···<i>abca</i>··· 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 <i>R</i>3̅<i>m</i> to <i>P</i>3̅<i>m</i>1 structure transformation with a tripled <i>c</i> 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