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

High Resolution Image Download MS PowerPoint Slide 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, LiInP 2 S 6 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 LiInP 2 S 6 in the monoclinic C 2/ c and trigonal P 3̅1 c (in-gap) space groups, in addition to examining the experimentally synthesized P 3̅1 c (in-layer) phase. Our studies identify the C 2/ c structure as the ground state, lying 9 meV per unit cell lower in energy than the experimentally realized trigonal P 3̅1 c (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 P 3̅1 c (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 LiInP 2 S 6 phases for iontronic devices and optoelectronic applications.