Ground state phases and optical properties in extended Peierls–Hubbard models for halogen-bridged binuclear metal complexes

Abstract

Mechanisms of a variety of charge and lattice ordered phases observed in halogen-bridged binuclear metal complexes are theoretically studied by applying the exact diagonalization and strong-coupling expansion methods to one- and two-band extended Peierls–Hubbard models. In R4[Pt2(pop)4I]nH2O [R = Na, K, NH4, (CH3(CH2)7)2NH2, etc., pop = P2O5H22−] containing charged MMX chains, three electronic phases are suggested by experiments. We find that the variation of the electronic phases originates not only from competition between site-diagonal electron–lattice and electron–electron interactions but also from competition between short-range and long-range electron–electron interactions. On the other hand, in Pt2(RCS2)4I (R = CH3, n-C4H9) containing neutral MMX chains, a site-off-diagonal electron–lattice interaction and the absence of counter ions are found to be crucial to produce the recently found, ordered phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. Their dependence on the electronic phase and on model parameters is clarified from the strong-coupling viewpoint.