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The positron lifetime spectra in Y-doped (Bao.6Sr0,4)Ti03 ceramics have been measured at room temperature for Y contents of 0-0.8 at. %. It was found from the experiments that with increasing Y content, the variation of the positron lifetime parameters is nonmonotonic, and the Y content corresponding to the knee is about 0.7 at. %, which is much larger than the La content corresponding to the knee in the La-doped BaTiO, (TANG 1994a). The experimental results show that the Y ions substitute predominantly for Sr ions, and this substitution induces a large number of Sr vacancies; when the Y content increases to about 0.7 at. %, the Y ions begin to substitute for Ba ions, and then the associated defects (2 YBaV&) and (2 YsrVi,) form gradually.

al. [l] reported the first positron-annihilation study of the YBaCuO(123) superconductor, many studies of positron annihilation in YBaCuO high-T, superconductors have been carried out. These studies are involved with three aspects : a) temperature dependence of the annihilation characteristics in the normal and superconducting states, b) the existence of a Fermi surface in these materials, and c) the structure and defect properties in the cuprate superconductors. Among the three aspects the temperature dependence of the annihilation characteristics is studied most. In the YBaCuO(123) superconductors many groups [l to 71 observed a decrease in the positron lifetime and Doppler-broadened annihilation radiation lineshape parameter below T,, but Harshman et al. [8] reported an increase in the lifetime below T, in a single-crystalline sample and Corbel et al. [9] reported different temperature dependences on samples with different preparation conditions. In the LaSrCuO superconductor an increase in lifetime below T, is observed [3]. In polycrystalline YBaCuO (124), which is structurally similar to YBaCuO(123) except for the presence of double Cu-0 chains, a decrease in the lifetime and lineshape parameter was observed for T < T, [lo]. In the TlBaCaCuO system [ll] different temperature dependences have been observed below T,: in TlBaCaCuO(2201) the lifetime seems to be independent of temperature, a decrease in lifetime has been observed in TlBaCaCuO(2212), and an increase in lifetime in TlBaCaCuO(2223). In the BiSrCaCuO(2212) system [12] no variation of the positron lifetime with temperature has been observed.As compared with the YBaCuO( 123) system, the studies of positron annihilation performed for other cuprate superconductors, specifically for the BiSrCaCuO and TlBaCa CuO systems, are very few. Thus further study of positron annihilation in BiSrCaCuO and TlBaCaCuO systems is necessary. In this note, we report the results of the temperature dependence of positron lifetime parameters in the two-mixed-phase system BiSrCaCuO and discuss the results. Experimental BiSrCaCuO samples were prepared by a solid-state reaction. The X-ray diffraction pattern showed that the samples have two bulk phases: a low-temperature phase BiSrCaCuO(2212) and a high-temperature phase BiSrCaCuO(2223). Resistivity measurements showed the samples to be superconducting below 83 K.Positron lifetime spectra were measured at different temperatures between 80 and 300 K by using a fast-fast coincidence spectrometer having a time resolution of 270 ps full width I) Wuhan 430074, People's Republic of China K52 physica status solidi (a) 136