The miscibility gap between the rock salt and wurtzite phases in the MgO–ZnO binary system to 3.5 GPa

Abstract. At ambient pressure, MgO crystallizes in the rock salt (B1) structure, whereas ZnO crystallizes in the wurtzite structure (B4). The asymmetric miscibility gap between these two structures in the MgO–ZnO binary system narrows with increasing pressure, terminating at the wurtzite-to-rock-salt phase transition in pure ZnO, which occurs at approximately 5 GPa at 1000 ∘C. Despite their essential simplicity, the pressure–temperature–composition (P–T–X) relations in the MgO–ZnO binary system have been sparsely studied experimentally, with disparate results that are inconsistent with available thermodynamic data. Here we report the experimental determination of the P–T–X relations of the miscibility gap from 940 to 1500 ∘C and 0 to 3.5 GPa, which we combine with calorimetric and equation-of-state data from the literature and on the transition in endmember ZnO, to build a thermodynamic model that resolves many of the inconsistencies. The model treats the rock salt phase as an ideal solution (no excess Gibbs free energy of mixing), while in the wurtzite phase the MgO component follows Henry's law and the ZnO component Raoult's law in the range of compositions accessed experimentally. However, there is an inconsistency between the partial molar volume of wurtzite-structured MgO deduced from this model and that inferred from lattice parameter measurements by X-ray diffraction in the quenched samples. This discrepancy may be caused by unquenchable disordering of some significant fraction of the substituting Mg2+ into normally vacant octahedral interstices of the wurtzite structure.