Emergence of polar monoclinic phase in heterohalogen substituted CsGeX$_3$

The occurrence of ferroelectricity in inorganic germanium-based halide perovskites has provided an alternative to oxide counterparts. Using first-principles methods, we have studied CsGeX$_3$ materials with heterohalogen substitution at the X site in a 2:1 configuration. Structurally, such variation alters the octahedral environment more strongly than in pristine materials, giving rise to a polar monoclinic phase at room temperature. The occurrence of the monoclinic phase is also confirmed through the energetics of structures generated by the displacements of atoms in accordance with soft mode eigenvectors of the dynamical matrix along various directions. In the chemically tuned phase, the polarization is along [101] and increases by 10-15% compared to pristine ones. The electronic structure analysis reveals that spin-splitting energy ranges from 25 to 250 meV in the valence band and from 9 to 80 meV in the conduction band in chemically tuned structures. In addition, these structures exhibit Rashba and persistent spin textures, which are coupled to the polarization direction. The parameters of the symmetry-dependent \textbf{k.p } Hamiltonian provide insights into the strength of spin-splitting and the nature of spin-texture. The semiconducting and spin-polarized nature of CsGeX$_3$ materials makes them strong candidates for Datta-Das spin transistors.

💡 Research Summary

The authors investigate the effect of heterohalogen substitution on the structural, ferroelectric, and spin‑orbit properties of inorganic germanium‑based halide perovskites CsGeX₃ (X = Cl, Br, I). Using density‑functional theory (PBE and the meta‑GGA r²SCAN functional) they replace the X‑site halogen in a 2:1 ratio (e.g., CsGeBr₂I, CsGeBrI₂, CsGeCl₂Br). The substitution acts as an internal uniaxial/biaxial strain, breaking the high‑symmetry cubic (Pm ¯3 m) and rhombohedral (R3m) phases and stabilizing a new polar monoclinic phase with space group Cm at room temperature.

Phonon calculations reveal soft modes at Γ, M, and X in both the pristine and chemically tuned structures. In the heterohalogen‑substituted compounds the triply degenerate negative frequency at Γ is lifted; the largest remaining instability is doubly degenerate (or non‑degenerate) depending on composition. By displacing atoms along the eigenvectors of these modes, the authors map the potential energy surface along