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 tec hniques. The metal-insulator transitions observed at 33, 25, 73 and 58 K for the Cu, Ni, Co and Fe compounds respectively, were found to cor respond 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 derivat ives. 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-Peier ls like ground state. The results are compared with previous studies o f the Au, Pt and Pd analogs and the subtle nature of the lattice insta bilities occurring in these compounds is discussed.