Structural and magnetic investigations of the Peierls transitionof ?-(Per)₂M(mnt)₂ with M = Fe and Co

In this work we present and discuss the temperature variation of X-ray, EPR and nuclear spin-lattice relaxation rate measurements for the two quasi-one-dimensional compounds alpha-(Per)2Fe(mnt)2 and alpha-(Per)2Co(mnt)2. From X-ray and EPR intensity data, a Peierls instability of the Perylene stacks is found to take place at 58 K and 75 K for the Fe and Co derivatives respectively. In contrast to other members of this series of two-chain compounds, the M(mnt)2 stacks are found to play no role in the Peierls instability. As for diffusive excitations of localized spins in dimerized Fe (mnt), stacks, their influence is shown to dominate the temperature variation of both the spin-lattice nuclear relaxation rate and the previously measured Faraday spin susceptibility.