SPIN-PEIERLS LATTICE FLUCTUATIONS OF PURE AND SI-SUBSTITUTED AND ZN-SUBSTITUTED CUGEO3

The pretransitional fluctuations of the spin-Peierls (SP) transition o f pure and Si- and Zn-substituted CuGeO3 have been investigated by x-r ay diffuse scattering. In pure CuGeO3 the critical fluctuations have b een measured from T-SP (14.2 K) to 35 K. Although they exhibit a modes t anisotropy, a crossover from a three- to a two-dimensional regime of structural fluctuations has been observed at about 1.3 K above T-SP. A quantitative comparison of the thermal dependence of the x-ray inten sity with that measured in the recent quasielastic neutron scattering investigation df Hirota er al., at the same reciprocal wave vector, sh ows that the SP critical lattice fluctuations of CuGeO3 are mostly of inelastic origin. Substitution by a fraction of a percent of Ge by Si or of Cu by Zn has a very important effect on the SP transition. In th e Si compounds the intrachain and interchain correlations are severely reduced and no superlattice reflections are observed at the temperatu re where a drop of spin susceptibility occurs in the 0.25% Si compound . Surprisingly, the substitution has a weaker effect in the 0.6% Zn co mpound where a pseudotransition towards a short-range SP order is obse rved at about 13 K. We discuss these differences with respect to the n ature of the substitution and we emphasize the key role played by the impurity-induced structural deformations in the case of the Si compoun ds to limit the SP order.