TEMPERATURE DEPENDENCE OF POSITRON ANNIHILATION PARAMETERS IN HIGH Tc SUPERCONDUCTOR YBa_(2)Cu_(3)O_(7-x)

The temperature dependences of the positron annihilation lifetime and the Doppler broadening shape parameters have been measured in high Tc superconductors YBa_(2)Cu_(3)O_(7-x)=between 77.2 and 300K.The measured parameters show an abrupt change across the range of superconducting transition temperature,indicating a change of the electronic structure.The structural instabilities have also been observed in three higher temperature ranges of115-135,150-210 and 250-280K.This suggests that higher Tc superconductors might be found in these temperature ranges.