Phonon softening and the metal-insulator transition in VO2

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Abstract
The phase transition in pure VO2 is ascribed to softening, in the high temperature semimetallic phase, of an optical phonon at the R point of the Brillouin zone. This softening arises from an electron-phonon interaction the strength of which is a manifestation of the tightly bound character of the d electrons. The insulating nature of the low temperature phase is considered to be due to band splitting and uncrossing produced by the crystalline distortion. This change in the bands is the static, large amplitude, limit of the electron-phonon interaction responsible for the phonon entropy of the semimetal. Detailed numerical calculations are made for one dimensional chains along the c axis. Good quantitative agreement is obtained with known experimental data.