X-ray scattering evidence for dimerization (4 kF) and spin-Peierls distortion (2 kF) in silver salts of dicyanoquinodiimine (2,5 MR-DCNQI )2 Ag (R = CH3, Cl or Br, M = CH3)

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Abstract
We have studied the structural instabilities of 2,5 (MR-DCNQI )2Ag (R = CH3, Cl or Br, M = CH3) using low-temperature X-ray diffuse scattering and diffraction techniques. In (DM-DCNQI)2Ag (DM = dimethyl) we observe quasi-1D 4 kF diffuse scattering at room temperature which transforms into satellite reflections at q4kF = (0, 0, 1/2) below T4 kF ≃100 K. A second transition with limited quasi-1D precursor scattering occurs at T 2 kF ≃ 83 K with satellite reflections at q 2 k F = (0, 0, 1/4). In relation with the transport and magnetic properties it is suggested that the upper transition localizes the charge carriers while the lower one leads to a spin-Peierls ground state. In (MCI-DCNQI )2Ag and (MBr-DCNQI )2Ag the quasi-1D 4 kF and 2 kF diffuse scatterings do not condense and there is no transition, probably because of the inherent disorder caused by the MC1 and MBr substituents. Supplementary diffuse scattering features and irradiation effects are also reported