Ultrasonic investigations on elastic properties of (Bi,Pb)-Sr-Ca-Cu oxide ceramic superconductors

Ultrasonic velocities have been measured in single phaseBi_(1.7)Pb_(0.3)Sr_2 CaCu_2O_(8+x) and Bi_(1.7)Pb_(0.3)Sr_2Ca_2O_(10+x) polycrystalline samples.Anomalous changes of both longitudinal and transverse velocities wereobserved near 200K,which indicates that some structural phase transitiomight occur.The elastic constants of these samples have been determinedfrom the measured ultrasonic velocity data,which are much smaller thanthose of YBa_2Cu_3O_(7-y)and BaTiO_3,manifesting that the interlayer coup-ling will dramatically decrease when the c-axes of the unit cells of theseperovstite-like multilayer ceramics increase.The values of Poisson ratio,however,of those ceramics are very close,implying similar interatomicbonding forces.Corrected to void-free state,the Debye temperature ofthese materials is estirnated as 270±20k.The temperature dependence ofDebye temperatures for these materials is also calculated by using thesame method and reported for the first time.