Lattice Dynamics of Polymorphic ZnIn₂S₄: An <i>Ab Initio</i> Study

The ternary chalcogenide, ZnIn₂S₄ is known to exhibit various polymorphic expressions: from the cubic spinel (high pressure) phase to various polytypic layered hexagonal structures, namely the α-, β- and γ-phases. Interestingly, these layer-type structures (α, β and γ) are speculated to exhibit relatively low thermal conductivity but a systematic theoretical description of the lattice thermal conductivity in these van der Waals (vdW) bonded layered of the polytypes is still lacking. Previous theoretical studies have not carefully considered vdW interactions which are dominant in layered materials and to date, only the simple α and spinel phases have been examined. In this work, we systematically perform first-principles density-functional perturbation theory calculations to understand the fundamental lattice properties for all ZnIn₂S₄ polytypes and propose their potential applications in the next generation thermoelectric devices via solving the Boltzmann transport equation.