Order and miscibility in the otavite--magnesite solid solution

The effects of cation substitution and ordering in the otavite (CdC[O.sub.3])-magnesite (MgC[O.sub.3]) solid solution have been investigated on samples synthesized at 1 GPa in the temperature range 500-800 [degrees]C for run durations up to 120 h. A complete, disordered solid solution, with R3c symmetry, was obtained at 800 [degrees]C, whereas Mg and Cd show partial ordering within the cadmium dolomite stability field, with R[bar.3] symmetry, at intermediate compositions in the temperature range 500-650 [degrees]C. Rietveld refinements for X-ray diffraction data show that variation of the a-axis is linear as a function of composition, independent of the degree of order, whereas the c-axis shows a positive deviation from linearity as a function of composition, decreasing with increasing degree of order. Octahedral bond distances of the 800 [degrees]C series vary linearly with composition. Site occupancies were used to determine the long-range order parameter, Q, for samples with R[bar.3] symmetry. [Q.sup.4] vades linearly as a function of temperature, suggesting a tricritical phase transition with a critical transition temperature [T.sub.c] of 702(10) [degrees]C. EDXTEM compositional microanalyses of samples within two-phase regions are in good agreement with Rietveld refinements, and allow better constraint of phase boundaries. Keywords: X-ray powder diffraction, otavite--magnesite solid solution, carbonates, phase transition, cation ordering