STRUCTURAL EVOLUTION, THERMAL-EXPANSION ANOMALY AND THE GRUNEISEN CONSTANTS OF THE SUPERCONDUCTOR YBA2CU3O7

The analyses of the continuous structural evolutions of ceramics presenting a transition can be carried out using low temperature X-ray and neutron diffraction. A specific method is recalled here for evaluating the anisotropic elastic and Grüneisen constants required for the phase study. It also provides continuous characterization of the entire evolution of the thermo-elastic properties through the superconducting transition. This method is applied to the superconductor YBa2Cu3O7. The compressibility coefficients of the orthorhombic phase have been determined by neutron diffraction at 300 K between 0 and 10 kbar. Using structural approximations, average elastic constants and Grüneisen parameters are then fitted to experimental thermal expansion data: C11+C12=4.83; C13=1.24, C33=2.87 in 1011 Pa units; γ0=2.7 at T>200 K. These parameters are then used for interpreting thermal expansion anomalies through the Grüneisen parameter variations. Microstructure might involve local stresses thus affecting the thermal expansion anomalies in the transition range. © 1990.