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

oleh: N. Farmer, N. Farmer, H. St. C. O'Neill

Format: Article
Diterbitkan: Copernicus Publications 2023-11-01

Deskripsi

<p>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 <span class="inline-formula"><sup>∘</sup></span>C. Despite their essential simplicity, the pressure–temperature–composition (<span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>–<span class="inline-formula"><i>X</i></span>) 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 <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>–<span class="inline-formula"><i>X</i></span> relations of the miscibility gap from 940 to 1500 <span class="inline-formula"><sup>∘</sup></span>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 Mg<span class="inline-formula"><sup>2+</sup></span> into normally vacant octahedral interstices of the wurtzite structure.</p>