12. Theory of MMX-Chain Compounds

Mechanisms of a variety of charge and lattice ordered phases observed in MMX compounds are theoretically studied by using a one-dimensional two-band three-quarter-filled extended Peierls–Hubbard model. In R₄[Pt₂(pop)₄I]nH₂O [R = Na, K, NH₄, (CH₃(CH₂)₇)₂NH₂, etc., pop = P₂O₅H ₂ ²⁻ ] 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 counterions are found to be crucial to produce the alternate-charge-polarization phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. A photoinduced transition has been found in a MMX compound, R₄[Pt₂(pop)₄I]nH₂O (R = (C₂H₅)₂NH₂). Its mechanism is theoretically studied by solving the time-dependent Schrödinger equation. Above a threshold in the photoexcitation intensity, a transition takes place from the charge-density-wave phase to the charge-polarization phase. The threshold-intensity dependence on the relative stability of these phases is explained qualitatively by their diabatic potentials. However, the transition in the opposite direction is hardly realized and needs careful consideration of different charge transfer processes.