Critical-Region Thermal Expansion in CuK2Cl4 center dot 2H(2)O

The Curie-point anomaly in the a-axis linear thermal expansion coefficient of CuK2Cl4 center dot 2H(2)O (T-c = 0.88 degrees K) has been observed using the three-terminal capacitance technique. Length changes of the 3.8-mm single-crystal sample were determined to within approximately 0.1 angstrom. In the critical region our data suggest a logarithmic singularity as found previously for the specific heat. However, imperfections in the sample limit the divergence of the expansion coefficient at temperatures closer than 0.01 T-c to the transition. From a comparison of the linear expansion coefficient with the specific heat in the critical region, the stress dependence of the Curie temperature is calculated. We find that the temperature derivative of the spin-correlation function describing nearest neighbor magnetic ions is not proportional to the temperature derivative of the spin-correlation function describing next nearest neighbors. Furthermore, the exchange parameters characterizing nearest and next nearest neighbor interactions do not have equal stress dependences. Between 1.5 and 2.5 degrees K the thermal expansion coefficient is proportional to the inverse square of the temperature. Comparison of the expansion coefficient with the specific heat in this temperature range indicates that the temperature derivative of both spin-correlation functions is proportional to T-2. The stress dependence of the Curie temperature calculated from data in this region agrees within experimental error with the value found from different considerations using data in the critical region.