Phonon instability and pressure-induced isostructural semiconductor-semimetal transition of monoclinic v O2

Recent experiments have revealed an intriguing pressure-induced isostructural transition of the low temperature monoclinic VO2 and hinted to the existence of a new metallization mechanism in this system. The physics behind this isostructural phase transition and the metallization remains unresolved. In this work, we show that the isostructural transition is a result of pressure-induced instability of a phonon mode that relates to a CaCl2-type of rotation of the oxygen octahedra, which alleviates, but does not completely remove, the dimerization and zigzagging arrangement of V atoms in the M1 phase. This phonon mode shows an increasing softening with pressure, ultimately leading to an isostructural phase transition characterized by the degree of the rotation of the oxygen octahedra. We also find that this phase transition is accompanied by an anisotropic compression, in excellent agreement with experiments. More interestingly, in addition to the experimentally identified M1′ phase, we find a closely related M1′′ phase, which is nearly degenerate with the M1′ phase. Unlike the M1′ phase, which has a nearly pressure-independent electronic band gap, the gap of the M1′′ drops quickly at high pressures and vanishes at a theoretical pressure of about 40 GPa.