ULTRASONIC STUDIES OF STRUCTURAL PHASE-TRANSITIONS AND SUPERCONDUCTIVITY IN LA2-XBAXCUO4-DELTA AND LA2-XSRXCUO4-DELTA

Attenuation and velocity of 7 MHz longitudinal ultrasound in polycrystalline ceramic La2−xBaxCuO4−δ and La2−xBaxCuO4−δ have been measured in the temperature range from 300 K to 1.5 K. On cooling a pronounced softening of the lattice is observed for 0.09 ≤ × ≤ 0.20 just above the transition temperature Tdl from the tetragonal phase to the orthorhombic phase. A large attenuation peak is observed at Tdl . The further low-temperature transformation from the orthorhombic phase to the tetragonal low-temperature phase is observed, with partial lattice stiffening and a small attenuation peak at Td2, not only in La2−xBaxCuO4−δ but also in La2−xSrxCuO4−δ The mechanism of the unusual TC reduction around x = 0.12 is proposed on the basis of the distortion of CuO2 basal planes from a square to a rhombus below Td2. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.