Ba2Cu(HCOO)6, THERMAL BEHAVIOR AND CRYSTAL STRUCTURE OF THE ROOM TEMPERATURE PHASE

摘要

Ba2Cu(HCOO)6 grown at RT crystallizes in space group P21; a=11.092 angstroms, b=7.079 angstroms, c=17.391 angstroms, beta=91.64 degrees; V=1365 angstroms3, Z=4, Dc=2.96 g/cm3. On heating, the following changes are observed: (1) a reversible, hysteretic, first-order phase transition at about 40-90 C; (2) partial decomposition at 190 C; and (3) an irreversible change at 230 C related to decomposition. Authors 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 C. A RT model structure of Ba2Cu(HCOO)6 is given. The Cu atoms are surrounded by seven O atoms, five of them defining an almost perfect square pyramid, and the remaining two, opposite to the apex, at hemicoordination distances of approx 2.85 angstroms. The Ba atoms display irregular, ninefold coordination polyhedra. 21 refs.