First-Principles Discovery of Novel LiInP<sub>2</sub>S<sub>6</sub> Polymorphs with Promising Optoelectronic Responses.

Abstract

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 <i>C</i>2/<i>c</i> and trigonal <i>P</i>3̅1<i>c</i> (in-gap) space groups, in addition to examining the experimentally synthesized <i>P</i>3̅1<i>c</i> (in-layer) phase. Our studies identify the <i>C</i>2/<i>c</i> structure as the ground state, lying 9 meV per unit cell lower in energy than the experimentally realized trigonal <i>P</i>3̅1<i>c</i> (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 <i>P</i>3̅1<i>c</i> (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.