Structure and stability of Ba4CaCu3O8+delta and Ba6CaCu3O10+delta

International audience; The crystal structure and thermal stability of Ba(4)CaCu(3)O(8+delta) and Ba(6)CaCu(3)O(10+delta) has been investigated by neutron powder diffration, differential thermal analysis and thermogravimetry. Both phases are metastable at low temperature in p(O(2))=1 bar although they can be obtained with normal cooling conditions after sintering. Ba(4)CaCu(3)O(8+delta) has a cubic structure (Im3m, a = 8.1452(1) for delta = 0.7) for high oxygen contents as reported in the literature but undergoes a transformation into a tetragonal structure (P4/mmm, a = 8.1976(3), c = 8.0709(3) for delta = -0.8) when the oxygen content is lowered. The structure of Ba(6)CaCu(3)O(10+delta) is confirmed (I4/mmm, a = 4.0416(1), c = 21.7448(7) for delta = - 0.4 ) with however some differences with previously reported results.