Ba₆Ge₂Se₁₂ and Ba₇Ge₂Se₁₇: Two Centrosymmetric Barium Seleno-Germanates with Polyatomic Anion Disorder

Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba₆Ge₂Se₁₂ and Ba₇Ge₂Se₁₇, are presented. Ba₆Ge₂Se₁₂ adopts the P2₁/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 Ba₇Ge₂Se₁₇ 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: [Se₂]^2- in the case of Ba₆Ge₂Se₁₂ and [GeSe₅]^4- in the case of Ba₇Ge₂Se₁₇. 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 Ba₆Ge₂Se₁₂ and Ba₇Ge₂Se₁₇, 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.