THERMAL CONTRACTION AT THE SPIN-PEIERLS TRANSITION IN CUGEO3

We report the results of an x-ray-scattering study of the b-axis lattice constant of the quasi-one-dimensional spin-1/2 antiferromagnetic chain system CuGeO3 as a function of temperature and magnetic field. A spontaneous thermal contraction Δb along the b axis perpendicular to the chain direction is observed below the spin-Peierls transition temperature Tc near 14 K. The contraction, Δb, is well described by a simple power law, Δb(1-T/Tc)x, where the exponent x is found to be close to 0.5. Below the transition temperature, Δb scales with the intensity of the superlattice reflections with indices (h/2,k,l/2)(h,l: odd, k: nonzero integer) measured by neutron scattering. The shift of the transition temperature, ΔTcTc(0)-Tc(H), is found to scale as H2 in quantitative agreement with the results of magnetic susceptibility measurements and with theory. A small increase in the a-axis lattice constant is observed below Tc. © 1994 The American Physical Society.