Periodical Valence Ordering in Metallic MMX Chain Compounds and Their Dynamic Structures

New synthetic halogen-bridged mixed-valence diplatinum (II, III) compounds with linear chain structures, Pt2 (EtCS2) 4I (1) and Pt2 (n-BuCS2) 4I (2), show high electrical conductivity at room temperature and undergo metal-semiconductor transitions at TM-S = 205 K for (1) and 325 K for (2) . It has been revealed by the X-ray diffraction studies for (1) at SPring-8 that diffuse scattering with line shapes corresponding to 2-D periodical valence ordering are observed above TM-S and converted to diffuse planes near TM-S, and finally to superlattice reflections below 130 K. Alternate valence ordering structures -Pt2+-Pt3+-I-Pt3+-Pt2+-I- with lattice distortion have been determined by the crystal structure analyses for (1) at 48 K including superlattice reflections and for (2) at 167 K. The lattice parameter of (1) along the chain direction exhibit significant temperature dependence with drastic change in slope at 205-240 K. In variable-temperature crystal structure analyses for (1), a steep increase in U22 of the bridging I atom near TM-S is observed. These results suggest remarkable thermal vibration with large amplitude for the I atom at room temperature phase, which should be coupled with dynamic valence ordering modes.