COMPOSITIONAL PHASE-SEPARATION IN LA2-XBAXCUOY NEAR THE OPTIMUM COMPOSITION FOR SUPERCONDUCTIVITY

Compositional phase separation was observed in La2-xBaxCuOy near the o ptimum composition (x is similar to 0.16) for superconductivity with a decisive miscibility gap by means of X-ray diffraction measurements. The phase-separated samples do not show intrinsic superconductivity. T herefore, the superconductive compound in this system is not in a chem ical equilibrium state at low temperatures, but is a high-temperature phase obtained by quenching to room temperature. The critical temperat ure of the phase separation T(p) takes a maximum value of 950-degrees- C at x is similar to 0.16, suggesting that phase instability is crucia l for high-T(c), superconductors. The end composition of the miscibili ty gap in the Ba-poor composition region was found to be 0.12, which i s a singular point from crystallographic and band structural standpoin ts.