Pressure-induced phase transition and electronic structure evolution in layered semimetal HfTe₂

Motivated by the recent experimental work, the pressure-induced structural transition of well-known two-dimensional (2D) 1T-HfTe₂ was investigated up to 50 GPa through the advanced CALYPSO structure search technique combined with the first-principles calculations. Our calculations suggested that the 1T-HfTe₂ will first transform to C2/m phase at 3.6 GPa with a volume reduction of 7.6% and then to $P\overline{6}2m$ phase at 9.6 GPa with a volume collapse of 4.6%. The occurrences of 3D C2/m and $P\overline{6}2m$ phases mainly originated from the enhanced Te–Te interlayer coupling and the drastic distortions of Hf–Te polyhedrons in $P\overline{3}m1$ phase under compression. Concomitantly, the coordination number of Hf atoms increased from six in $P\overline{3}m1$ to eight in C2/m and eventually to nine in $P\overline{6}2m$ at elevated pressure. The metallic and semimetallic nature of C2/m and $P\overline{6}2m$ phases were characterized, and the evidence of the reinforced covalent interactions of Te–Hf and Te–Te orbitals in these two novel high-pressure phases were manifested by the atom-projected electronic DOS and Bader charge.