Elastic constants and Debye temperature of Y 1Ba 2Cu 3O x: effect of oxygen content

Abstract

Using ultrasonic methods, we measured the elastic constants of polycrystalline Y 1Ba 2Cu 3O x between 295 and 4 K. For a single specimen, using vacuum annealing and oxygen annealing, we varied the oxygen content in four steps from x=6.2 to x=6.9. Elastic stiffnesses that depend mainly on shear modes increase monotonically with increasing oxygen content. Stiffnesses that depend on dilatation modes increase up to about x=6.7 and then decrease. The elastic Debye temperature, which depends mainly on shear modes, increases monotonically, in agreement with the specific-heat Debye temperature. An ionic-model calculation for the bulk modulus and Debye temperature predicts a monotonic increase for both. We conjecture that the irregular oxygen-content dependence of the dilatation-mode-related elastic constants arises from changes in copper-oxygen valences.