Thermodynamic properties and crystal growth behavior of the hashemite (BaSO4-BaCrW) solid solution

Thermodynamic properties of the BaSO4-BaCrO4 hashemite solid solution were calculated using molecular principles. Force-field potentials and physical properties of the end member BaCrO4 were derived using the program package GULP, Subsequently, (SO42- - Cro(4)(2-), SO42- -, CrO42-, - CrO42-) anion-anion interaction energies were fitted to a number of random and ordered anion distributions, which were energy-optimized using these empirical potentials. With these interaction energies, diagrams for the enthalpy and free energy of mixing could be computed for the entire range of the hashemite solid solution between the BaSO4 and BaCrO4 end members and for a number of annealing temperatures. These thermodynamic data show that the solid solution is close to ideal and ordering is not observed at low temperatures for any composition. However, according to our calculations, exsolution may occur at the nanoscale due to a slight tendency of sulfate-chromate avoidance. Growth experiments of chromate-rich hashemite on barite using atomic force experiments (AFM) in a fluid cell indicate that hashemite does not show the same degree of growth anisotropy as barite. In the case of barite, this anisotropy leads to a structural self-inhibition of spiral growth. In contrast, hashemite exhibits a spatially more extended spiral growth, which can contribute more to overall volume growth than in the case of barite. This finding is confirmed by a higher growth rate of hashemite than of barite in macroscopic batch-reactor and flow-through experiments. Molecular simulations indicate that there is a less pronounced step energy difference between bounding steps of growth islands for hashemite than for barite, which may partially explain the difference in growth anisotropy of the two minerals. Differences in adsorption energies to the corners of growth islands, which are the limiting steps of growth, do not seem to be high enough to explain differences in growth anisotropies. Therefore, kinetic effects or differences in the interfacial free energies of the two minerals may serve as an additional explanation for differences in the growth behavior. (c) 2005 Elsevier B.V. All rights reserved.