PHASE-TRANSITIONS AND STRUCTURAL INSTABILITY IN HTSC COMPOUNDS AND RELATED PHASES

Lattice dimensions anomalies are investigated for different HTSC compo unds (123- and 124-YBCO, 2201-,2212- and 2223-BSCCO, 2223-TBCCO; LSCO, NCCO) and related phases (Y2BaCuO5 CuO) by acousic and X-ray powder d iffraction measurements in temperature range T-c - 300 K. Two kinds of anomalies have been established for superconducting compounds only at T-0 similar to T-c + 15 K and T-1 similar to 190-270 K, with each hav ing a quite different nature. It is shown that T-0 increases and T-1 d ecreases as critical temperature increases. The values of T-0 and T-1 as functions of the distance between CuO2- planes are obtained. Locali zation processes in the charge carrier subsystem are suggested to be a ssotiated with the T-1 anomaly.