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By means of differential scanning calorimetry, thermogravimetry, and high-temperature x-ray diffraction in different atmospheres, an opposite thermal dynamic change in superconducting and semiconducting cuprates of (Y-123) and (Pr-123) is observed. At about the superconducting Y-123 polycrystals show an endothermal anomaly and a weight loss, while the semiconducting Pr-123 polycrystals show an exothermal anomaly and weight gain. Similar phenomena are also observed in the superconducting and semiconducting Y-123 single crystals at about . High-temperature x-ray diffraction reveals that the lattice parameters of the Y-123 increase significantly at this temperature. We argue that this can be attributed to an unknown phase transition and a thermal instability, which is important for understanding the difference in properties between superconducting and semiconducting cuprates.