Conformational Gap Control in CsTaS₃

Simple arguments based on orbital energies and crystal symmetry suggest the band gap of CsTaS₃ to be suitable for solar cell photovoltaics. Here, we combine chemical theory with sophisticated calculations to describe an intricate relationship between the structure and optical properties of this material. Orbital interactions govern both the presence and nature of CsTaS₃'s gap. In the first place, through a second-order Jahn-Teller (JT) distortion, which slides the Ta ion along the axial direction of TaS₃ chains. This displacement creates a gap that remains direct in the face of minor distortions. Using an advanced methodology, compressive sensing lattice dynamics, we compute the anharmonic interatomic force constants up to the fourth order and use them to renormalize the phonons at finite temperatures. This analysis predicts CsTaS₃ to undergo the JT metal-to-semiconductor transition at temperatures below 1000 K. At around room temperature, we find a second distortion that moves the Ta ion along the equatorial direction of the TaS₃ chains, giving rise to many possible supercell conformations. By relaxing all symmetry-inequivalent structures with Ta ion displacements, in supercells with up to 12 formula units, we obtain 204 symmetrically distinct conformations and sort them by energy and (direct) band gap magnitude. Since all structures with a gap lie within an energy range of 30 meV/Ta above the ground state, we expect CsTaS₃'s optical properties to be controlled by the full polymorphic ensemble of gapped conformations. Using the GW-Bethe-Salpeter approach, we predict a band gap of 1.3-1.4 eV as well as potent absorption in the visible range.