THERMODYNAMIC PROPERTIES OF THE SPIN-PEIERLS TRANSITION IN CUGEO3

We present an investigation of the spin-Peierls transition at T(sp) = 14.5 K in polycrystalline CuGeO(3) through specific-heat and thermal-expansion measurements. Clear second-order phase-transition anomalies are found in both properties at T(SP), although only a small entropy of S approximate to 0.1 Rln2 is released at the transition. Most of the entropy is released at T(SP) < T < 150 K, where the temperature dependence of the magnetic contribution to the specific heat as well as the thermal expansion exhibit extrema at T* approximate to 40 K. These are caused by one-dimensional antiferromagnetic fluctuations along the Cu chains, possibly accompanied by structural fluctuations. Using Ehrenfest's relation, a hydrostatic pressure coefficient (partial derivative T(SP)/partial derivative)(p))(p) --> O approximate to (0.45 +/- 0.06) K/kbar is derived.