DIFFUSE-X-RAY SCATTERING EVIDENCE FOR PEIERLS AND SPIN-PEIERLS LIKE TRANSITIONS IN THE ORGANIC CONDUCTORS (PERYLENE)2[M(MNT)2] (M=CU, NI, CO AND FE)

The phase transitions of the low dimensional conductors (Per)2[M(mnt)2] (M = Cu, Ni, Co and Fe) were studied by X-ray diffuse scattering techniques. The metal-insulator transitions observed at 33, 25, 73 and 58 K for the Cu, Ni, Co and Fe compounds respectively, were found to correspond to a tetramerization on the perylene chains in the case of the Ni and Cu derivatives and to a dimerization for the Fe and Co derivatives. In all cases, the periodic lattice distortion can be associated to a 2k(F) Peierls instability of the perylene chains. The Ni compound shows in addition to the 2k(F) distortion also a lattice instability at twice this wave vector, which corresponds to a dimerization of the Ni(mnt), chain of localized 1/2 spin towards a non magnetic Spin-Peierls like ground state. The results are compared with previous studies of the Au, Pt and Pd analogs and the subtle nature of the lattice instabilities occurring in these compounds is discussed.