First-Principles Discovery of Novel LiInP ₂ S ₆ Polymorphs with Promising Optoelectronic Responses

Recent advances in two-dimensional (2D) van der Waals (vdW) metal thiophosphates have attracted considerable attention due to their promising ionic conductivity, optical characteristics, and tunable physical properties. Within this material family, LiInP2S6 has emerged as an intriguing candidate, not only because of its sensitivity to air and moisture but also due to its suitable band gap within the UV–vis range, enabling potential optoelectronic and photocatalytic applications. In this study, through comprehensive first-principles investigations, we unveil two previously unreported polymorphs of LiInP2S6 in the monoclinic C2/c and trigonal P3̅1c (in-gap) space groups, in addition to examining the experimentally synthesized P3̅1c (in-layer) phase. Our studies identify the C2/c structure as the ground state, lying 9 meV per unit cell lower in energy than the experimentally realized trigonal P3̅1c (in-layer) phase. Further, we systematically examine the elastic, mechanical, thermodynamical, dynamical, electronic, and optical properties of all three polymorphs, confirming their mechanical, thermal, and dynamical stability. Notably, the P3̅1c (in-gap) phase exhibits enhanced stiffness, while the calculated indirect band gaps and strong photon absorption in the UV–vis range (∼3 eV) highlight the potential of the studied LiInP2S6 phases for iontronic devices and optoelectronic applications.