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.
Grimm–Sommerfeld analogous II‐IV‐N2 nitrides such as ZnSiN2, ZnGeN2, and MgGeN2 are promising semiconductor materials for substitution of commonly used (Al,Ga,In)N. Herein, the ammonothermal synthesis of solid solutions of II‐IV‐N2 compounds (II=Mg, Mn, Zn; IV=Si, Ge) having the general formula (IIa
1−xIIb
x)‐IV‐N2 with x≈0.5 and ab initio DFT calculations of their electronic and optical properties are presented. The ammonothermal reactions were conducted in custom‐built, high‐temperature, high‐pressure autoclaves by using the corresponding elements as starting materials. NaNH2 and KNH2 act as ammonobasic mineralizers that increase the solubility of the reactants in supercritical ammonia. Temperatures between 870 and 1070 K and pressures up to 200 MPa were chosen as reaction conditions. All solid solutions crystallize in wurtzite‐type superstructures with space group Pna21 (no. 33), confirmed by powder XRD. The chemical compositions were analyzed by energy‐dispersive X‐ray spectroscopy. Diffuse reflectance spectroscopy was used for estimation of optical bandgaps of all compounds, which ranged from 2.6 to 3.5 eV (Ge compounds) and from 3.6 to 4.4 eV (Si compounds), and thus demonstrated bandgap tunability between the respective boundary phases. Experimental findings were corroborated by DFT calculations of the electronic structure of pseudorelaxed mixed‐occupancy structures by using the KKR+CPA approach.
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.
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