Solid solution in the CaO—MnO system

Abstract

CaO–MnO samples have been prepared by thermal decomposition of mixed carbonates and annealing of the resulting oxide mixtures at 1273 and 1523 K in vacuo. Cooled samples have been ground in vacuo and transferred for X-ray powder diffraction analysis and e.p.r. analysis without exposure to the atmosphere. Lattice parameter measurements confirm the presence of a single phase of rock–salt structure (Ca^II_1–xMn^II_xO) across the whole composition range.

A deviation from ideality in respect of the volume change of the unit cell is observed for the solution of CaO in MnO. The deviation passes through a maximum near 35 mol % CaO. The result is interpreted in terms of a model involving the displacement of anions in particular directions when large Ca²⁺ ions are substituted for small Mn²⁺ ions in the partially covalent MnO structure. No deviation from ideality is observed for the solution of initial amounts of MnO in CaO. E.p.r. analysis of solid solutions very dilute in MnO (0.01%, 0.1% Mn) confirms substitutional replacement of Ca²⁺ by Mn²⁺.