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HfSe2 is trigonal omega structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one HfSe2 sheet oriented in the (0, 0, 1) direction. Hf4+ is bonded to six equivalent Se2- atoms to form edge-sharing HfSe6 octahedra. All Hf–Se bond lengths are 2.69 Å. Se2- is bonded in a distorted T-shaped geometry to three equivalent Hf4+ atoms.
HfSe2(TiSe2)4 is trigonal omega-derived structured and crystallizes in the monoclinic C2/m space group. The structure is two-dimensional and consists of one HfSe2 sheet oriented in the (0, 0, 1) direction and four TiSe2 sheets oriented in the (0, 0, 1) direction. In the HfSe2 sheet, Hf4+ is bonded to six equivalent Se2- atoms to form edge-sharing HfSe6 octahedra. All Hf–Se bond lengths are 2.65 Å. Se2- is bonded in a distorted T-shaped geometry to three equivalent Hf4+ atoms. In each TiSe2 sheet, Ti4+ is bonded to six Se2- atoms to form edge-sharing TiSe6 octahedra. There are a spread of Ti–Se bond distances ranging from 2.56–2.61 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a distorted T-shaped geometry to three equivalent Ti4+ atoms. In the second Se2- site, Se2- is bonded in a distorted T-shaped geometry to three equivalent Ti4+ atoms.
Extensive crystal structure prediction searches provide evidence of two 3D structures with orthorhombic (Immm) and monoclinic (C2/m) space groups as reasonable candidates for the first pressure-induced phase of 1T-HfSe 2 . Our candidates are compared with two recent proposals that keep the 2D nature of the ambient conditions phase and display hexagonal (P6 3 /mmc) and monoclinic (C2/m) symmetry, although the latter has a different structure than our proposed C2/m phase. Both of these 2D-like structures are discarded based on simple thermodynamic and kinetic arguments that can be extended to explain the pressure-induced polymorphic sequence of other transition metal dichalcogenides. The computed observables of our orthorhombic phase are fully consistent with the experimental structural and Raman data observed at low and high-pressure.