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

The crystal structure and stability of Ba{sub 4}CaCu{sub 3}O{sub 8+{delta}} have been investigated by neutron powder diffraction, differential thermal analysis and thermogravimetry. It is found that the phase is not stable below 1065K in p(O{sub 2})=1bar and decomposes according to the eutectoid reaction Ba{sub 4}CaCu{sub 3}O{sub 8+{delta}}+x O{sub 2}-bar Ba{sub 2}CuO{sub 3.4}+CaO+2BaCuO{sub 2}. However, the equilibrium with the outer gas is not reached for sintered ceramics so that Ba{sub 4}CaCu{sub 3}O{sub 8+{delta}} can be obtained in a metastable state after normal cooling conditions. In this case, the crystal structure is cubic (Im-3m, a=8.1452(1)A, {delta}=0.68, Z=2, R{sub wp}=2.5%, R{sub Bragg}=5.4%) as reported in the literature. In reduced oxygen partial pressure (p(O{sub 2})