Bonding and phase transitions in transition metal dichalcogenide layer compounds

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Abstract
The electronic structure of the transition metal dichalcogenide layer compounds is studied using hybridised orbitals appropriate to the transition metal atom coordination. Non-bonding orbitals on the transition metal atoms give the conduction electron bands which are responsible for the charge-density-wave phase transitions in the VB compounds, and the band structure of 1T- and 2H-TaS2 is found using tight binding. The phase transitions are studied by a tight-binding calculation of the conduction electron contribution to the dynamical matrix, Phi el. In 1T-TaS2 there is a Kohn anomaly in Phi el close to the displacement wavevector of the incommensurate phase, sufficiently large to make the ideal structure unstable. The ideal 2H structure is also unstable, though there is only a weak Kohn anomaly. As Phi el is fairly temperature independent, the stability of the high-temperature phase must be due to phonon entropy, and this is studied using a mean-field theory of anharmonic effects. The low-temperature commensurate phases are stabilised by chemical bonding effects.