Critical phenomena at the spin-peierls transition in (formula presented)

We have performed a detailed x-ray scattering study of the critical phenomena associated with the spin-Peierls phase transition in the organic compound (Formula presented) Analysis of the superlattice reflection intensity indicates an order parameter with an associated critical exponent (Formula presented) consistent with three-dimensional behavior, as seen in the inorganic compound (Formula presented) and clearly inconsistent with mean field behavior, as indicated in previous studies. Measurements of lattice constants indicate the presence of spontaneous strains below the transition temperature. The measured critical scattering is not well described by an Ornstein-Zernike, Lorentzian line shape. An adequate description is obtained with a (Formula presented) with x varying or with a (Formula presented) line shape. The latter descriptor is reminiscent of previous high-resolution x-ray scattering studies where a two-component line shape has been observed. Analysis using such a line shape indicates critical exponents (Formula presented) and (Formula presented) obtained from the Lorentzian component, consistent with the results obtained for the order parameter.