Diversity of structural phases in AGeX3 halides

Utilizing first-principles and particle swarm optimization methods, we performed a systematic investigation of crystal structural stability and electrical properties of AGeX(3) compounds (A = Li, Na, K, Rb, Cs; X = F, Cl, Br, I). Our work not only found the perovskite phases, but also discovered the post-perovskite, post-post-perovskite, and octahedra face-sharing structure phases, which are usually stable under high pressure, large strain, or exist in the inorganic-organic compound. These diverse structural phases provide abundant and promising properties. One striking phase is the P2(1)2(1)2(1) perovskite in KGeCl3 and KGeBr3, which possesses intrinsic small polar vortices and antivortices that may lead to a gyrotropic phase transition. The ferroelectric phase of R3c (LiGeX3, NaGeX3), R3m (CsGeX3) and Pna2(1) (KGeX3, RbGeX3) were also found. The new Pna2(1) phase possesses both proper and hybrid improper polarization, and the latter is evidenced by two original energetic couplings: one coupling polarization with the antipolar distortion from Ge atoms and antipolar distortion from A-site atoms, and another coupling polarization with out-of-phase halide octahedral tilting, in-phase halide octahedral tilting, and antipolar distortion from Ge atoms. The diverse structural phases and electrical properties make Ge-based halide a promising ABX(3) structure family in ferroics and piezoelectrics.

Automated summary

Using first-principles and particle swarm optimization methods, this study systematically investigated the crystal structural stability and electrical properties of AGeX(3) compounds (A = Li, Na, K, Rb, Cs; X = F, Cl, Br, I). Various new structural phases and their corresponding electrical properties were discovered, such as the P2(1)2(1)2(1) perovskite phase with intrinsic small polar vortices and antivortices in KGeCl3 and KGeBr3, which may lead to a gyrotropic phase transition. Additionally, ferroelectric phases were found in R3c (LiGeX3, NaGeX3), R3m (CsGeX3) and Pna2(1) (KGeX3, RbGeX3), including the new Pna2(1) phase with both proper and hybrid improper polarization. These findings highlight the promising applications of Ge-based halide compounds in ferroics and piezoelectrics.