The new semiconducting sodalite-type BaGe8As14 compound exhibits a narrow band gap, low resistivity and partially disordered atom positions as good prerequisites for a thermoelectric material.
The new Zintl-compounds AEGe 8 As 10 (AE = Sr, Ba) and BaGe 7 P 12 were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGe 8 As 10 and BaGe 8 As 10 crystallize in the space group Cmce and show complex 3D networks composed of three different Ge−As motifs and As−As bonds with mixed valence of germanium in the oxidation states +2, + 3, and +4. Mixed valences of germanium +3 and +4 occur in BaGe 7 P 12 , which crystallizes in the space group R3̅ with a 3D network built up of Ge 2 P 6 dumbbells and P−P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP 6 trigonal antiprism. High temperature X-ray diffraction shows thermal stabilities up to 923−953 K. UV−Vis and resistivity measurements reveal a semiconducting nature with small indirect band gaps between 0.02 and 1.6 eV. Electronic band structure calculations confirm the semiconducting state and indicate covalent bonds within the Ge-Pn polyanions.
RbGe 7 As 15 and CsGe 7 As 15 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 I4̅ 3m isotypic to BaGe 8 As 14 . 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 × 10 4 S/m at 300 K and carrier densities of 1−2 × 10 20 cm −1 . First-principles DFT calculations give clear evidence of ultralow lattice thermal conductivity around 0.5 Wm −1 K −1 in BaGe 8 As 14 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 BaGe 8 As 14 even further. These effects are chemically switched off in RbGa 7 As 15 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 BaGe 8 As 14 , reaching the value of current record materials.
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