FORMATION AND DECOMPOSITION OF LABA2CU3O7-DELTA

The formation and decomposition of LaBa2Cu3O7-delta has been studied by X-ray diffraction and thermogravimetric analysis. The attempted preparation of LaBa2Cu3O7-delta in air or oxygen gives a mixture of a solid solution of composition La1+xBa2-xCu3O7-delta (x almost-equal-to 0.2), and BaCuO2 (J. M. S. Skakle and A. R. West, Physica C, 1994, 220, 187). However, heating this mixture in a less oxidising atmosphere such as flowing argon at temperatures above 850-degrees-C for a short time, ca. 16 h, yields a single-phase tetragonal product, which, when annealed in oxygen at lower temperature, gives an orthorhombic superconductor with T(c) (onset) almost-equal-to 94 K. The orthorhombic and tetragonal structures have been confirmed by Rietveld refinement of X-ray powder diffraction data; an orthorhombic-tetragonal transition occurs at delta = 0.2. The stability of the single-phase material under air and oxygen has been studied by thermogravimetry. It is stable up to 800-degrees-C in air; at higher temperatures, decomposition to La1+xBa2-xCu3O7-delta (x almost-equal-to 0.2), and BaCuO2 occurs. On prolonged heating of single-phase tetragonal LaBa2Cu3O7-delta at high temperatues, e.g. 875-degrees-C for 64 h under argon, decomposition to give La4-2xBa2+2xCu2-xO10-2x (x almost-equal-to 0.2), BaCuO2 and BaCu2O2 occurs. This is similar to the behaviour of YBCO under low oxygen pressure. Thus, under all conditions studied so far, LaBa2Cu3O7-delta may be considered to be a non-equilibrium phase. In orthorhombic samples of LaBa2Cu3O7-delta prepared by quenching, T(c) varies linearly with delta; samples cooled slowly show evidence of a plateau at ca. 94 K for delta almost-equal-to -0.05 to -0.15.