Elastic anomalies in sodium-doped Bi-2212 single-phase HTSC materials

A series of sodium-doped Bi-2212 single-phase high-Tc superconductors have been prepared by the solid state reaction method. After the usual characterization by XRD, SEM, bulk density porosity etc, ultrasonic longitudinal velocity studies were undertaken over the temperature range 80-300 K using the pulse transmission technique. In the temperature range 300-230 K, in contrast to normal solids, the velocity of Bi-Na(0) and Bi-Na(3) samples is found to decrease with decreasing temperature (elastic softening) followed by a velocity maximum at around 150 K for all the four samples, signifying the presence of lattice instabilities. Plausible explanations are given for both the elastic softening and the velocity maxima on the basis of microstructure and ordering readjustments of the oxygen atoms among the Cu-O planes of the samples. (author)