An unusual phase transition in the crystal structure and physical properties of (TTF)(9)[Mo(CN)(8)](2)center dot 4H2O(2) where TTF = tetrathiafulvalene

At 270 K, the charge transfer salt (TTF)(9)[Mo(CN)(8)](2). 4H(2)O, I, cryst allizes in the triclinic space group P (1) over bar with a = 9.9094(2), b = 10.6781(2), c = 23.6086(7) Angstrom, a = 75.7910(8), beta = 88.6010(9), ga mma = 78.5250(8)degrees, V = 2372.5(1) Angstrom (3) and Z = 2. At 120 K, th e space group is unchanged with a = 9.7990(7), b = 10.6630(5), c = 22.9940( 2) Angstrom, = 79.981(4), beta = 89.798(4), gamma = 79.013(4)degrees, V = 2 321.5 Angstrom (3) and Z = 2. On comparing the two sets of data, we see sig nificant changes in the cell parameters, most notably in the angle. Variabl e temperature crystallographic studies indicate a rst order phase transitio n accompanied by hysteresis, which corresponds to a change in the transport properties. I is a semiconductor and the high temperature activation energ y of 0.06 eV changes sharply to 0.15 eV below 236 K. Bulk magnetic suscepti bility and ESR measurements indicate that the TTF molecules are antiferroma gnetically coupled. The temperature dependence of the EPR spectrum changes from 300-200 K, in approximate agreement with the transport and structural results. The optical spectrum of (TTF)(9) [Mo(CN)(8)](2). 4H(2)O consists o f several broad bands assigned to TTF charged molecules, to [Mo(CN)(8)](4-) and to charge transfer from the donors to the acceptor in the near infra-r ed range. Preliminary magnetic susceptibility measurements under light irra diation with a multi-line (752.5-799.3 nm) laser were also performed, but n o photomagnetic effect was noted.