High resolution and in situ neutron powder diffraction study of the crystal structure and the stability of Ba4CaCu3O8+delta

摘要

The crystal structure and stability of Ba4CaCu3O8+delta have been investigated by neutron powder diffraction, differential thermal analysis and thermogravimetry. It is found that the phase is not stable below 1065 K in P(O-2) = 1 bar and decomposes according to the eutectoid reaction Ba4CaCu3O8+delta + chi O-2 double right arrow Ba2CuO3.4 + CaO + 2BaCuO(2). However, the equilibrium with the outer gas is not reached for sintered ceramics so that Ba4CaCu3O8+delta can be obtained in a metastable state after normal cooling conditions. In this case, the crystal structure is cubic (1m-3m, a = 8.1452(1)angstrom, delta = 0.68, Z = 2, R-wp = 2.5%, R-Bragg = 5.4%) as reported in the literature. In reduced oxygen partial pressure (p(O-2) < 10(-6) bar), Ba4CaCu3O8+delta is stable down to room temperature and has a tetragonal structure with a significant lower oxygen content (P4/mmm, a = 8.1976(3)angstrom, c = 8.0709(3)angstrom, delta = -0.81, Z = 2, R-wp = 2.8%, R-Bragg = 5.1%). The difference between the two crystal structures is discussed in terms of oxygen content, copper formal valence and cation coordination. The influence of the oxygen pressure on the stability of Ba4CaCU3O8+delta is also discussed. (c) 2005 Elsevier Inc. All rights reserved.