Exploring novel structural phases and dynamical instabilities in LaSb₂

Rare-earth di-antimonides (RSb₂) form layered structures with a square net lattice of Sb, leading to a highly nested Fermi surface and intriguing phenomena like charge density wave and superconductivity. Despite the commonality of square net lattices, RSb₂ systems exhibit diverse structural phases and transitions. Recently, LaSb₂ was synthesized using molecular beam epitaxy (MBE),[1] resulting in a novel monoclinic phase rather than the well-known orthorhombic phase. Ab initio calculations revealed that the monoclinic phase has lower total energy than the orthorhombic phase and that the various structural phases in RSb₂ are linked to different stacking sequences of consecutive quintuple-layer building blocks. Furthermore, the calculations predicted another novel orthorhombic phase of LaSb₂, with a total energy comparable to that of the monoclinic phase.

In this study, we performed phonon calculations of the four phases of LaSb₂: two are the well-known structures similar to SmSb₂ and YbSb₂, one is the MBE-grown monoclinic phase, and the last is the theoretically predicted orthorhombic phase. The calculations reveal dynamical instabilities in the two well-known structures with or without strain. This work aims to deepen our understanding of the various structural phases and transitions in RSb₂ systems, associated with the intriguing phenomena.

[1] A. Llanos, et al., Nano Letters 24 (28), 8518-8524 (2024).