Critical phenomena at the spin-Peierls transition in MEM(TCNQ)₂

We have performed a detailed x-ray scattering study of the critical phenomena associated with the spin-Peierls phase transition in the organic compound MEM(TCNQ)₂. Analysis of the superlattice reflection intensity indicates an order parameter with an associated critical exponent β=0.35±0.06 consistent with three-dimensional behavior, as seen in the inorganic compound CuGeO₃, 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 Lorentzian^x with x varying or with a Lorentzian+Lorentzian² 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 γ and ν, obtained from the Lorentzian component, consistent with the results obtained for the order parameter.