Ultrasonic study of structural instability of monocrystalline and polycrystalline Bi-Sr-Ca-Cu-O

Two internal-friction (attenuation) peaks around 95 and 130 K in the polycrystal and single crystal of Bi-Sr-Ca-Cu-O are found to be related to the superconducting transition at 84 and 107 K, respectively. The jump of lattice parameter and shape-memory effect occurring at the peak temperatures reveal the characteristics of a martensitic transition. The velocity and attenuation versus temperature of ultrasonic longitudinal waves propagating along two directions in the a-b plane at 10° to the a and b axes and of [010] shear waves with [100] polarization were measured for the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystals. The anisotropic elasticity in the c plane is manifested. Only the longitudinal waves in a direction and the shear waves show an obvious softening minimum of velocity around 250 K, which is associated with an attenuation peak due to a phaselike transition, but other longitudinal waves reveal a monotonic stiffening from room temperature down to ${\mathit{T}}_{\mathit{c}}$. The origin of the overall trend of elastic stiffening that appears in the measurements so far for high-${\mathit{T}}_{\mathit{c}}$ oxides is discussed.