ORDER-DISORDER IN T-PHASE, T'-PHASE, AND T-ASTERISK-PHASE - SUPERCONDUCTORS AND RELATED MATERIALS

After reviewing microstructural studies on superconducting materials s howing T, T', and T- structural types, results are presented on the m icrostructure of some n-type superconductors and related materials pre pared with accurate control of the oxygen stoichiometry. Electron micr oscopy is used to describe the ordering of interstitial oxygen defects in T-type La2NiO4+delta leading to the formation of the n = 2 term of a homologous series with the general formula La8nNi4nO16n+1. Structur al transitions and superstructure formation in the Pr2-x-yCexSryCuO4-d elta system are reported, where T, T', and T- phases are isolated as a function of both Ce and Sr content. (C) 1995 Wiley-Liss, Inc.