Crystal Structures, Surface Stability, and Water Adsorption Energies of La-Bastnäsite via Density Functional Theory and Experimental Studies

Abstract

Bastnasite 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 bastnasite 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-bastnasite is isomorphic to the structure of Ce-bastnasite belonging to the P6̅ 2c space group, whereas the common structure listed in the Inorganic Crystal Structure Database structure belonging to the P6̅ 2m space group is ca. 11.3 kJ/mol higher in energy per LaFCO 3 formula unit. We report powder X-ray diffraction measurements on synthetic La-bastnasite to support these theoretical findings. Six different surfaces are studied by DFT, namely, [101̅ 0], [0001], [101̅ 1], [101̅ 2], [101̅ 4], and [112̅ 2]. Among these, the [101̅ 0] surface is the most stable with a surface energy of 0.73 J/m 2 in vacuum and 0.45 J/m 2 in aqueous solution. The shape of a La-bastnasite nanoparticle is predicted via thermodynamic Wulff construction to be a hexagonal prism with [101̅ 0] and [0001] facets, chiseled at its ends by the [101̅ 1] and [101̅ 2] facets. The average surface energy of the nanoparticle in the gas phase is estimated to be 0.86 J/m 2 , in good agreement with a value of 1.11 J/m 2 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-bastnasite surface, in agreement with water adsorption calorimetry experiments. Our work provides an important step toward a detailed atomistic understanding of the bastnasite−water interface and designing collector molecules that can bind specifically to bastnasite.

Dynamic observation of manganese adatom mobility at perovskite oxide catalyst interfaces with water

“…The formation of a few monolayer thick water layer on oxide surfaces down to pressures of~10 −5 Pa much below the equilibrium vapor pressure of water of p vap = 3.17 kPa at T = 25°C has been previously demonstrated [50][51][52] . It is driven by the large adsorption enthalpy of H 2 O on oxide surfaces of the order of ΔH ad,ox -160 kJ/mol 51,53 . Although this value might vary between different oxides, ΔH ad,ox < ΔH ad = −44 kJ/mol for H 2 O on bulk water reflects exothermic water adsorption on oxide surface at much reduced pressures.…”

A Molecular-Scale Approach to Rare-Earth Beneficiation: Thinking Small to Avoid Large Losses

“…Periodic-DFT-calculated adsorption energies for SHA, BHA, and SA on bastnäsite-[10 0], the most prevalent bastnäsite surface ( Srinivasan et al, 2017 ; Srinivasan et al., 2016 ), with acidic protons proximal to the adsorbate, are provided in Table 2 , and optimized structures are shown in Figure 1 and detailed in Figure S1 with interactions contributing to adsorption specified.…”

Molecular Recognition at Mineral Interfaces: Implications for the Beneficiation of Rare Earth Ores

“…Herein, we apply our approach to identify bis-phosphinic acid ligands capable of selective and strong binding to target sites on the bastnäsite (100) interface, which are the single largest mineral source for light rare earth elements (REE). 16,17 Focusing on this particular model REE mineral, we are motivated by the major role of REE in many current high-technology applications put at risk by the uncertain supply of REE. 18,19 As the demand for REE and accompanying supply risk is expected to grow, 20 the REE have been identified as critical materials.…”