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

Abstract

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) Å, α = 75.7910(8), β = 88.6010(9), γ = 78.5250(8)°, V = 2372.5(1) ų and Z = 2. At 120 K, the space group is unchanged with a = 9.7990(7), b = 10.6630(5), c = 22.9940(2) Å, α = 79.981(4), β = 89.798(4), γ = 79.013(4)°, V = 2321.5 ų 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.