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

Abstract

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 P2 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.

Sr6Ge3OSe11: A Rationally Designed Noncentrosymmetric Oxyselenide with Polar [GeOSe3] Building Blocks

“…36 The experimental sound velocity was determined to be 1927 m s −1 at room temperature, and thus, the theoretical κ diff was calculated to be 0.22 W m −1 K −1 . This is 33% lower than the experimental κ values of the recently reported selenogermanates Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 , which exhibit positional disorder (0.33 W m −1 K −1 at 573 K), 39 and 62% lower than the experimentally determined κ of Sr 6 Ge 3 OSe 11 .…”

Crystal structure, phase width, and physical properties of the barium tetrel selenides Ba6Si2−xGexSe12 (x = 0, 0.5, 1, and 1.5) with ultralow thermal conductivity

“…31 The compound Ba 6 Ge 2 Se 12 , which also features disorded Se 2 2− pairs, was recently reported to have a similarly low thermal conductivity of 0.33 W m −1 K −1 at 573 K, which may in part be attributed to its larger average molar mass and stronger Umklapp scattering due to its greater unit cell volume. 11 Barium tin chalcogenides that possess S/Te mixing such as Ba 5 Sn 2 S 7.673 Te 1.327 and Ba 7 Sn 3 S 12.161 Te 0.839 have demonstrated extremely low lattice thermal conductivity ranging from 0.3 to 0.4 W m −1 K −1 between 300 and 773 K. 32 Ultra-low thermal conductivity in thermoelectric selenides have been observed in K 2 Bi 8 Se 13 , which achieved thermal conductivity between 0.42 and 0.20 W m −1 K −1 from 300 to 873 K due to anisotropic K/Bi mixed sites. 33 Similarly, low thermal conductivity has been observed in the Bi 2+2n O 2+2n Se n X 2 (X = Cl, Br) family of layered selenides, which are composed of Bi 2 O 2 X 2 and Bi 2 O 2 Se layers that efficiently scatter phonon modes in the transverse and longitudinal directions.…”

“…To study the optical properties, the Raman spectra were obtained for samples of Ba 6 Si 2−x Ge x Se 12 with x = 0, 0.5, 1, and 1.5. In addition, the thermal conductivity was determined for all members, as the related Ge variant Ba 6 Ge 2 Se 12 11 exhibited very low values, advantageous for the thermoelectric energy conversion. 12,13 Experimental Synthesis Single crystals of Ba 6 Si 2 Se 12 were first prepared by adding stoichiometric amounts of pure elements (Ba: pieces, 99.7%, Strem Chemicals; Si: pieces, 99.9%, Alfa Aesar; Se: powder, 99.999%, Alfa Aesar) into a carbon-coated silica tube inside a dry Ar filled glove box.…”

“…Interestingly, a comparable disorder is also observed in the monoclinic Ge variant, Ba 6 Ge 2 Se 12 ( P 2 1 / c ). 11 The Se 2 2− disorder was investigated via single crystal and Rietveld refinements. A thorough investigation of the phase width revealed that up to 75% of the Si atoms in the Ba 6 Si 2 Se 12 structure type can be replaced with Ge atoms without changing the structure type to that of Ba 6 Ge 2 Se 12 .…”

“…To study the optical properties, the Raman spectra were obtained for samples of Ba 6 Si 2− x Ge x Se 12 with x = 0, 0.5, 1, and 1.5. In addition, the thermal conductivity was determined for all members, as the related Ge variant Ba 6 Ge 2 Se 12 11 exhibited very low values, advantageous for the thermoelectric energy conversion. 12,13…”