Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 : Two Centrosymmetric Barium Seleno-Germanates with Polyatomic Anion Disorder.

Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 , are presented. Ba 6 Ge 2 Se 12 adopts the P 2 1 / c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and β = 90.303(1)°, whereas Ba 7 Ge 2 Se 17 crystallizes in the Pnma space group with a = 12.652(1) Å, b = 20.069(2) Å, c = 12.3067(9) Å. Both structures feature polyatomic anion disorder: [Se 2 ] 2- in the case of Ba 6 Ge 2 Se 12 and [GeSe 5 ] 4- in the case of Ba 7 Ge 2 Se 17 . The anion disorder is verified by comparing pair distribution functions of ordered and disordered models of the structures. These anions are split unevenly across two possible sets of atomic coordinates. The optical band gaps obtained from the powdered samples are found to be 1.75 and 1.51 eV for Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 , respectively. Differential scanning calorimetry experiments indicate that the compounds are stable under the exclusion of air up to at least 673 K. The thermal diffusivity measurements revealed thermal conductivities reaching values as low as 0.33 W m -1 K -1 in both compounds at 573 K.