AN UNKNOWN PHASE-TRANSITION IN YBA2CU3O7-X AND PRBA2CU3O7-X CUPRATES

By means of differential scanning calorimetry, thermogravimetry, and h ightemperature x-ray diffraction in different atmospheres, an opposite thermal dynamic change in superconducting and semiconducting cuprates of YBa2Cu3O7-x (Y-123) and PrBa2Cu3O7-x (Pr-123) is observed. At abou t 300 degrees C the superconducting Y-123 polycrystals show an endothe rmal anomaly and a weight loss, while the semiconducting Pr-123 polycr ystals show an exothermal anomaly and weight gain. Similar phenomena a re also observed in the superconducting and semiconducting Y-123 singl e crystals at about 450 degrees C. High-temperature x-ray diffraction reveals that the lattice parameters of the Y-123 increase significantl y at this temperature. We argue that this can be attributed to an unkn own phase transition and a thermal instability, which is important for understanding the difference in properties between superconducting an d semiconducting cuprates.