First-principles prediction of structure, mechanical and thermodynamic properties of BixGeyOz ternary bismuth crystals

Abstract

The bismuth germanates are outstanding semiconductor materials due to excellent physical and chemical properties. The first-principles calculations are adopted to perform the insight to Bi-Ge-O ternary compounds, such as Cmc21-type-Bi2GeO5, Bi4Ge3O12, C1c1-type-Bi2GeO5, Bi2Ge3O9, Pnma-type -Bi2GeO5 and Bi12GeO20. The investigated compounds are the structural stabilities, elastic properties, anisotropy, thermodynamic and electron properties of these Bi-Ge-O ternary. According to criterions of the formation enthalpy and the total phonon density of states, all the six mentioned ternary compounds are thermodynamically and dynamically stable. The Cmc21-type-Bi2GeO5 may present ductile characteristics and the rest of Bi-Ge-O compounds are brittle. Moreover, the elastic hardness indicates that all the mentioned compounds possess relatively low hardness. The Cmc21-type-Bi2GeO5 compound possesses a more favorable performance of resistance to mechanical deformation. The subsequent order is competent to assess the elastic anisotropy: Pnma-type-Bi2GeO5>C1c1-type-Bi2GeO5> Bi12GeO20>Cmc21-type-Bi2GeO5> Bi2Ge3O9> B4Ge3O12. As for the electron properties, the band structures and the density of states are analyzed in details. The thermodynamic data are provided by the phonon calculations.