Delphine D. Le Pevelen

Structures and physical properties of BEDT-TTF salts containing channels of protons

Charge transfer salts with stoichiometry β″-(BEDT-TTF) [M(CO),](18-crown-6 ether)(9HO), where M= Cr, Ga or Fe and x= 2 or 3 are reported. Their structures suggest that proton migration is possible. Variable temperature resistivity measurements on the Cr salt show metallic behaviour down to 180K. Magnetic susceptibility of the Cr salt is paramagnetic down to 1.8 K. Preliminary impedance measurements suggest that the proton conductivities of the Cr and Ga salts are the order of 10 S cm at 300 K.

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

At 270 K, the charge transfer salt (TTF)9[Mo(CN)8]2·4H2O, I, crystallizes in the triclinic space group P with a = 9.9094(2), b = 10.6781(2), c = 23.6086(7) A, α = 75.7910(8), β = 88.6010(9), γ = 78.5250(8)°, V = 2372.5(1) A3 and Z = 2. At 120 K, the space group is unchanged with a = 9.7990(7), b = 10.6630(5), c = 22.9940(2) A, α = 79.981(4), β = 89.798(4), γ = 79.013(4)°, V = 2321.5 A3 and Z = 2. On comparing the two sets of data, we see significant changes in the cell parameters, most notably in the angle α. Variable temperature crystallographic studies indicate a first order phase transition accompanied by hysteresis, which corresponds to a change in the transport properties. I is a semiconductor and the high temperature activation energy of 0.06 eV changes sharply to 0.15 eV below 236 K. Bulk magnetic susceptibility and ESR measurements indicate that the TTF molecules are antiferromagnetically 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]2·4H2O consists of 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-red range. Preliminary magnetic susceptibility measurements under light irradiation with a multi-line (752.5–799.3 nm) laser were also performed, but no photomagnetic effect was noted.