STRUCTURAL PHASE-TRANSITION OF LA4BACU5OY

By the heat treatment of tetragonal La4BaCu5Oy under flowing high-purity N2 gas followed by slow cooling, we could newly synthesize a single phase of monoclinic La4BaCu5Oy which is metallic but nonsuperconducting, as well as the tetragonal La4BaCu5Oy. From the carbon reduction method and thermogravimetric analysis, we have determined the values of the oxygen content y to be 13.0 and 12.5 for the tetragonal and new monoclinic phases, respectively. Electron diffraction patterns of the monoclinic phase show superlattice reflections corresponding to a 10ap × 10ap × ap unit cell. (ap means the unit dimension of fundamental perovskite structure.) In-situ XRD measurements at high temperatures have revealed the following two facts: (1) in air, La4BaCu5Oy has a tetragonal structure with a 5ap × 5ap × ap unit cell up to 1040°C, accompanying the oxygen loss up to 12.5 with the increase of temperature; and (2) the monoclinic phase, closed in an evacuated quartz capillary, transforms into the tetragonal phase above 690°C. © 1993 Academic Press, Inc.