LOW-TEMPERATURE ELASTIC-CONSTANTS OF Y1BA2CU3O7

Using ultrasonic methods, we studied the 4-295 K elastic constants of a ''good'' Y1Ba2Cu3O7 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 dB/dT 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.