Structural studies of boracites. VI. The cubic phase of cobalt iodine boracite, Co₃B₇O₁₃I, and of copper bromine boracite, Cu₃B₇O₁₃Br

Abstract

For pt.V see Ferroelectrics, vol.13, p.357 (1976). Results are presented for full three-dimensional structural studies of the boracites Co3B7O13I (Co-I) and Cu3B7O13Br (Cu-Br) in their cubic phase at room temperature. In both experiments the data were collected with Mo K alpha radiation on a four-circle diffractometer to a resolution of sin theta / lambda approximately=1 AA-1. The final R indices achieved were 3.2% and 4.4% for Co-I and Cu-Br respectively. Standard statistical techniques have been used to test carefully the significance of (i) some features of the positional parameters and (ii) the inclusion of parameters to describe third- and fourth-order (anharmonic) terms in the thermal motion of some ions. The results obtained generally confirm the conclusions of earlier similar studies of cubic Cr-Cl, Ni-I and Cu-Cl boracites: in particular it remains the case that there is no clear evidence for the disordering of any ion as a necessary feature of the cubic phase of the metal-halogen boracites. A new conclusion is that it is the thermal anisotropy of one of the boron ions that varies systematically with the known anharmonicity of the system (generally increasing as I to Br to Cl), rather than the observed anharmonicity of the halogen itself as previously suggested. And this boron is believed to play an important role in cubic to orthorhombic transition.