Low-temperature elastic constants of Y1Ba2Cu3O7

Abstract Using ultrasonic methods, we studied the 4–295 K elastic constants of a “good” Y 1 Ba 2 Cu 3 O 7 polycrystal. We report the bulk, shear, Young moduli and the Poisson ratio. Except for the Poisson ratio, all elastic constants show smooth temperature behavior. Near T c , during cooling, the Poisson ratio decreases irregularly. Focusing on the bulk modulus B , we use simple thermodynamics to show that the harmonic-observed decrement at T =0 corresponds to expectation, but the temperature effect d B /d T is too large by about a factor of two. We attribute the irregular slope to oxygen-atom reordering. Against many reports, our bulk-modulus value is approximately that found in monocrystals or that predicted by an ionic-crystal model calculation.