FACTORS GOVERNING THE CHARGE-DENSITY-WAVE PATTERNS OF LAYERED TRANSITION-METAL COMPOUNDS OF OCTAHEDRAL COORDINATION WITH D2 AND D3 ELECTRON COUNTS

Two different charge density wave (CDW) patterns are observed for layered transition-metal compounds ML2 (e.g., M = transition metal, L = oxygen, chalcogen) of octahedral coordination with d2 and d3 electron counts. Tight-binding electronic band structure calculations were carried out for several d2 ML2 layers, and factors controlling these patterns were discussed. In general, the IT-ML2 systems with short M-L bonds adopt a CDW pattern involving a weak distortion, while those with long M-L bonds adopt a CDW pattern involving a strong distortion. Our calculations show that the metal-atom trimerization in LiVO2 is energetically favorable and opens a band gap for a small displacement of the metal atoms. This supports the CDW model of weak metal-atom trimerization proposed for the square-root 3 x square-root 3 superstructure of LiVO2.