TUNING OF THE CHARGE-DENSITY-WAVE IN THE HALOGEN-BRIDGED TRANSITION-METAL LINEAR-CHAIN COMPOUNDS

Local-density-approximation calculations are used to show that the met al-metal distance along the chains controls the charge-density wave (C DW) in halogen-bridged transition-metal linear-chain compounds. The st rength of the CDW can be understood in terms of a two-band Su-Schrieff er-Heeger model if a hard-core ion-ion repulsion potential is also add ed. We predict a second-order phase transition from an insulating to a semimetallic ground state and explain trends in dimerization, bond-le ngth ratios, band gaps, and Raman breathing modes in terms of the meta l-metal distance.