BEDT-TTF Salts with a-Keggin polyoxometallates: Synthesis and magnetism of (BEDT-TTF)8[PMo3NbW8O40]: a possible Spin-Peierls system

Electrical and magnetic properties of the new radical-cation salt (BEDT-TTF)8[PMo3NbW8O40] are reported. The compound is a semiconductor, σπ = 0.037Scm−1 with an activation energy Ea=160meV. The magnetic susceptibility (30–300K) has been fitted according to a uniform (S=l/2) 1D Heisenberg model of nearly ionised BEDT-TTF molecules, by using the g-value of 2.005 and the exchange constant J/kB = −60K. Below 20K, a Curie-like tail is present and the C-value is approximately 1/3 of that corresponding to one impaired electron for formula-unit. If the tail is subtracted to the total magnetic susceptibility, a sharp fall in susceptibility at about 7K is observed, and this is probably associated with a Spin-Peierls distortion.