Crystal structures, surface stability, and water adsorption energies of La-Bastnäsite via density functional theory and experimental studies

Bastnäsite is a fluoro-carbonate mineral that is the largest source of rare earth elements (REEs) such as Y, La, and Ce. With increasing demand for REE in many emerging technologies, there is an urgent need for improving the efficiency of ore beneficiation by froth flotation. To design improved flotation agents that can selectively bind to the mineral surface, a fundamental understanding of the bulk and surface properties of bastnäsite is essential. Unexpectedly, density functional theory (DFT) calculations using the PBEsol exchange correlation functional and the DFT-D3 dispersion correction reveal that the most stable form of La-bastnäsite is isomorphic to the structure of Ce-bastnäsite belonging to the P6-2c space group, whereas the common structure listed in the Inorganic Crystal Structure Database structure belonging to the P6-2m space group is ca. 11.3 kJ/mol higher in energy per LaFCO₃ formula unit. We report powder X-ray diffraction measurements on synthetic La-bastnäsite to support these theoretical findings. Six different surfaces are studied by DFT, namely, [101-0], [0001], [101-1], [101-2], [101-4], and [112-2]. Among these, the [101-0] surface is the most stable with a surface energy of 0.73 J/m² in vacuum and 0.45 J/m² in aqueous solution. The shape of a La-bastnäsite nanoparticle is predicted via thermodynamic Wulff construction to be a hexagonal prism with [101-0] and [0001] facets, chiseled at its ends by the [101-1] and [101-2] facets. The average surface energy of the nanoparticle in the gas phase is estimated to be 0.86 J/m², in good agreement with a value of 1.11 J/m² measured by calorimetry. The calculated adsorption energy of a water molecule varies widely with the surface plane and specific adsorption sites within each facet. The first layer of water molecules is predicted to adsorb strongly on the La-bastnäsite surface, in agreement with water adsorption calorimetry experiments. Our work provides an important step toward a detailed atomistic understanding of the bastnäsite-water interface and designing collector molecules that can bind specifically to bastnäsite.