Formation and decomposition of LaBa2Cu3O7 –δ

Abstract

The formation and decomposition of LaBa2Cu3O7 –δ has been studied by X-ray diffraction and thermogravimetric analysis. The attempted preparation of LaBa2Cu3O7 –δ in air or oxygen gives a mixture of a solid solution of composition La1 +xBa2 –xCu3O7 –δ(x≈ 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 °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 Tc(onset)≈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 δ= 0.2. The stability of the single-phase material under air and oxygen has been studied by thermogravimetry. It is stable up to 800 °C in air; at higher temperatures, decomposition to La1 +xBa2 –xCu3O7 –δ(x≈ 0.2), and BaCuO2 occurs. On prolonged heating of single-phase tetragonal LaBa2Cu3O7 –δ at high temperatures, e.g. 875 °C for 64 h under argon, decomposition to give La4 – 2xBa2 + 2xCu2 –xO10 – 2x(x≈ 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 –δ may be considered to be a non-equilibrium phase. In orthorhombic samples of LaBa2Cu3O7 –δ prepared by quenching, Tcvaries linearly with δ samples cooled slowly show evidence of a plateau at ca. 94 K for δ≈–0.05 to –0.15.