Trigonal-to-monoclinic structural transition in TiSe${}_2$ due to a combined condensation of $\text{q}=(\frac{1}{2},0,0)$ and $(\frac{1}{2},0,\frac{1}{2})$ phonon instabilities

Abstract

I present first principles calculations of the phonon dispersions of 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 was able to stabilize 10 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.

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Cited by in corpus (5)

• Origin of chirality in transition-metal dichalcogenides
• Hidden excitonic quantum states with broken time-reversal symmetry
• Symmetry-mode analysis for local structure investigations using pair distribution function data
• Doping-induced nematic and stripe orders within the charge density wave state of TiSe${}_2$
• Revisiting the charge-density-wave superlattice of 1$T$-TiSe${}_2$