High Thermoelectric Properties in the Sodalite Compounds BaGe₈As₁₄ and AGe₇As₁₅ (A = Rb, Cs)

RbGe7As15 and CsGe7As15 have been synthesized and their structures were determined by single-crystal X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy. They crystallize with a cubic sodalite-type structure in the space group I4̅3m isotypic to BaGe8As14. Rubidium and cesium are highly coordinated by 16 arsenic or germanium atoms and fit better into the sodalite cage due to their bigger ionic radii compared to barium, which is displaced from the center. The compounds are narrow-band p-type semiconductors with electrical conductivities of 1.2–3 × 104 S/m at 300 K and carrier densities of 1–2 × 1020 cm–1. First-principles DFT calculations give clear evidence of ultralow lattice thermal conductivity around 0.5 Wm–1K–1 in BaGe8As14 due to the position disorder of the barium atoms and the anharmonicity of its thermal movement. Frozen phonon calculations and heat capacity data indicate that rattling probably decreases the lattice thermal conductivity of BaGe8As14 even further. These effects are chemically switched off in RbGa7As15 with a parabolic potential and no signs of rattling, leading to a four times higher lattice thermal conductivity. All calculated transport properties agree with the measured data, and their combination predicts a thermoelectric efficiency ZT up to 2.7 for BaGe8As14, reaching the value of current record materials.