I present first principles calculations of the phonon dispersions of ${\mathrm{TiSe}}_2$ in the $P\overline{3}c1$ phase, which is the currently accepted low-temperature structure of this material. They show weak instabilities in the acoustic branches in the out-of-plane direction, suggesting that this phase may not be the true ground state. To find the lowest energy structure, I study the energetics of all possible distorted structures corresponding to the isotropy subgroups of $P\overline{3}m1$ for the $M_1^{-}$ and $L_1^{-}$ phonon instabilities present in this high-temperature phase at $q=(\frac{1}{2},0,0)$ and $(\frac{1}{2},0,\frac{1}{2})$, respectively. I am able to stabilize ten different structures that are lower in energy relative to the parent $P\overline{3}m1$ phase, including two monoclinic structures more energetically stable than the $P\overline{3}c1$ phase. The lowest energy structure has the space group $C2$ with the order parameter $M_1^{-}(a,0,0)+L_1^{-}(0,b,b)$. This structure lacks inversion symmetry, and its primitive unit cell has 12 atoms.

• Received 26 July 2021
• Revised 7 December 2021
• Accepted 6 January 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.014602