Ba2Cu(HCOO)6, thermal behavior and crystal structure of the room temperature phase

Ba2Cu(HCOO)(6) grown at room temperature crystallizes in space group P2(1); a = 11.092 Angstrom, b = 7.079 Angstrom, c = 17.391 Angstrom, beta = 91.64 degrees; V = 1365 Angstrom(3), Z = 4, D-c = 2.96 g/cm(3). On heating, the following changes are observed: (1) a reversible, hysteretic, first-order phase transition at about 40-90 degrees C; (2) partial decomposition at 190 degrees C; and (3) an irreversible change at 230 degrees C related to decomposition. We have assigned to a new gamma-Ba(HCOO)(2) metastable phase one of the main components found when Ba2Cu(HCOO)(6) samples are quenched from 190 degrees C, A room temperature model structure of Ba2Cu(HCOO)(6) is given, The copper atoms are surrounded by seven oxygen atoms, five of them defining an almost perfect square pyramid, and the remaining two, opposite to the apex, at hemicoordination distances of ca. 2.85 Angstrom. The barium atoms display irregular, ninefold coordination polyhedra. (C) 1999 Academic Press.