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

Abstract

The new compound Ba₆Si₂Se₁₂ was synthesized, and the crystal structure and physical properties are reported here. Ba₆Si₂Se₁₂ adopts a new structure type in the triclinic P1̄ space group with the lattice parameters a = 9.1822(7) Å, b = 12.2633(14) Å, c = 12.3636(18) Å, α = 109.277(3)°, β = 104.734(2)°, and γ = 100.4067(16)°. Notably, the structure features disordered Se₂^2- dumbbells that have also been observed in the germanium selenide with the analogous stoichiometry (Ba₆Ge₂Se₁₂). Density functional theory calculations revealed that Ba₆Si₂Se₁₂ is a semiconductor with a calculated band gap of 1.74 eV. UV/vis/NIR absorption spectra indicated that the experimental band gap of Ba₆Si₂Se₁₂ is 1.89 eV. While exploring this compound's phase width, it was discovered that up to 75% of the Si could be substituted with Ge while retaining the structure type. Rietveld refinements were performed on the phase-pure samples of Ba₆Si_2-xGe_xSe₁₂ (x = 0, 0.5, 1, and 1.5) using data collected at the Canadian Light Source's High Energy Wiggler Beamline. The cell parameters, Si/Ge occupancies, and disordered Se₂^2- occupancies were studied. Raman spectra displayed the expected Si-Se and Ge-Se stretching modes from 215 cm^-1 to 280 cm^-1. The samples were also hot-pressed into pellets to determine their thermal conductivity values ranging from 0.5 to 0.4 W m^-1 K^-1 for the x = 0, 0.5, and 1.5 samples. The x = 1 sample stood out with a remarkably low thermal conductivity of 0.3 W m^-1 K^-1, consistent from room temperature up to 573 K.