Thermal expansion of superconduct

Temperature dependences of thermal strains of superconducting (Formula presented)(Formula presented)(Formula presented)(Formula presented) were measured by using the two-dimensional reciprocal-lattice method of x-ray diffraction in the temperature range between 15 and 300 K. Accuracies of measuring the lattice strains attained 1.6×(Formula presented). Very small anomalies of the lattice constants and lattice volume took place at (Formula presented) of the transition point. They were caused by definite discontinuities of the thermal-expansion coefficients. Using previously reported values of specific heat and isotropic compressibility a nearly temperature-independent equivalent Grüneisen parameter (Formula presented) was derived as 0.89±0.11. From the extrapolation of the lattice volume of the high-temperature phase into the superconducting temperature region, additive lattice strains were found to appear spontaneously in the superconducting phase. From this fact it was concluded that a structural phase transition of the second order took place with the onset of superconductivity.