Trigonal-to-monoclinic structural transition in TiSe$_2$ due to a combined condensation of $\mathbf{ \textit{q} = (\frac{1}{2},0,0)}$ and $\mathbf{(\frac{1}{2},0,\frac{1}{2})}$ phonon instabilities - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Physical Review Materials Année : 2022

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

Alaska Subedi

Résumé

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.

Dates et versions

hal-03452360 , version 1 (26-11-2021)

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Alaska Subedi. Trigonal-to-monoclinic structural transition in TiSe$_2$ due to a combined condensation of $\mathbf{ \textit{q} = (\frac{1}{2},0,0)}$ and $\mathbf{(\frac{1}{2},0,\frac{1}{2})}$ phonon instabilities. Physical Review Materials, 2022, ⟨10.1103/PhysRevMaterials.6.01460⟩. ⟨hal-03452360⟩
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