ELASTIC AND NONLINEAR ACOUSTIC PROPERTIES OF YBA2CU3O7-X CERAMICS WITH DIFFERENT OXYGEN CONTENTS

The effects of temperature and hydrostatic pressure on the velocities of longitudinal and shear ultrasonic waves propagated in ceramic samples of YBa2Cu3O6.3, YBa2Cu3O6.6, and YBa2Cu3O6.94 have been measured, to examine the effects of oxygen content and ordering on the elastic and nonlinear acoustic properties of YBa2Cu3O7-x. Corrections have been made by several methods for the effects of sample porosity and microcracking on the elastic properties. Temperature and pressure dependence of the ultrasonic characteristics are compared. The elastic moduli of YBa2Cu3O6.3 and YBa2Cu3O6.94 show normal behavior with temperature in accord with a phenomenological model of vibrational anharmonicity dawn to 40 and 100 K, respectively. Particularly marked elastic stiffening has been found in the temperature dependence of the elastic moduli of YBa2Cu3O6.6 with decreasing temperature from 270 to 200 K. It is suggested that the origin of this distinctive feature is a relaxation process of oxygen-atom hopping and formation of localized orthorhombic I phase within this orthorhombic II compound. The pressure derivatives of the natural velocities and bulk modulus of YBa2Cu3O6.3, YBa2Cu3O6.6, and YBa2Cu3O6.94 tend to decrease with increasing oxygen content. Using the results obtained in this work and some data published previously, we have shown that the pressure derivative (partial derivative B-s/partial derivative P)P=0 of the adiabatic bulk modulus of YBa2Cu3O7-x is correlated strongly with sample porosity and decreases approximately linearly with decreasing n/P-m, the ratio of porosity n to the maximum applied pressure P-m. We have also shown that values of the mean acoustic mode Gruneisen parameter gamma(el) of ceramic superconducting compounds, determined by ultrasonic measurements, have a linear relationship with the porosity n. The effects of oxygen contents on the pressure derivative (partial derivative B-S/partial derivative P)P=0 of YBa2Cu3O7-x are also discussed.