Crystal Structures, Surface Stability, and Water Adsorption Energies of

Article pubs.acs.org/JPCC

Crystal Structures, Surface Stability, and Water Adsorption Energies of La-Bastnäsite via Density Functional Theory and Experimental Studies Sriram Goverapet Srinivasan,† Radha Shivaramaiah,∥ Paul R. C. Kent,‡,§ Andrew G. Stack,† Alexandra Navrotsky,*,∥ Richard Riman,⊥ Andre Anderko,# and Vyacheslav S. Bryantsev*,† †

Chemical Sciences Division, ‡Center for Nanophase Materials Sciences, and §Computer Science and Mathematics Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ∥ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California−Davis, 1 Shields Avenue, Davis, California 95616, United States ⊥ Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08855, United States # OLI Systems, Inc., 240 Cedar Knolls Road, Suite 301, Cedar Knolls, New Jersey 07927, United States S Supporting Information *

ABSTRACT: 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 P6̅2c space group, whereas the common structure listed in the Inorganic Crystal Structure Database structure belonging to the P6̅2m 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, [101̅0], [0001], [101̅1], [101̅2], [101̅4], and [112̅2]. Among these, the [1010̅ ] 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 [101̅0] and [0001] facets, chiseled at its ends by the [101̅1] and [101̅2] 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.

1. INTRODUCTION

their ores must be devised to maximize REE production and improve the economics of mining REE. The largest mineral source of LREEs is bastnäsite, a fluoro-carbonate mineral, containing mostly Ce, La, and Y. Large deposits of bastnäsite are found in Bayan Obo, China, and Mountain Pass, California, USA.3 The process of extraction of REE from their ores begins with the beneficiation of the ore, which refers to the separation of gangue from the mineral. In the case of bastnäsite, the primary gangue materials include calcite, barite, and celestite.3

Rare earth elements (REE) consist of a group of 17 elements including the 15 lanthanides, La to Lu, plus Y and Sc. REE are used in a wide variety of applications ranging from phosphors to automobile exhaust catalysts and permanent magnets.1 For instance, Nd and Pr are widely used in permanent magnets, La and Ce are widely used in battery and metal alloys, petroleum refining, and catalysts, and Y is widely used in phosphors and ceramics.2 Among these, the elements from La to Eu are grouped together as light rare earth elements (LREE), and the elements Gd to Lu along with Y are grouped into heavy rare earth elements (HREE).3 With ever increasing demand for REE in daily applications,4 efficient means of extraction of REE from © XXXX American Chemical Society

Received: May 10, 2016 Revised: July 8, 2016

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DOI: 10.1021/acs.jpcc.6b04747 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

hydrogen-bonding interaction with the surface anions. The results of this study lay an important foundation for the design of improved flotation agents that can discriminate bastnäsite from the other carbonate materials based on selective binding to the most exposed mineral surface.

Though a number of procedures for ore beneficiation exist, such as gravity, magnetic, and electrostatic separations, froth flotation is the most widely used beneficiation technique for bastnäsite due to the ability to design the process to tailor the surface characteristics of a specific mineral.2 Both the REE ore and gangue material have carbonate as a common anion; thus, their isoelectric points somewhat overlap on the basic side (pH 8−9.5 for calcite5 and pH 4.6−9.5 for bastnäsite).6−10 The isoelectric point varies significantly for bastnäsite, depending on its source. As the purity of Ce-bastnäsite11 increases, the isoelectric point converges to a pH of 7.8. Collector molecules used in flotation must selectively bind to those surfaces that are dominantly exposed in bastnäsite crystals but not in the gangue material. Fatty acids have been widely used as flotation agents due to their low cost and widespread availability. However, they are not sufficiently selective for bastnäsite over calcite and barite.12 In a series of articles,13−15 Pradip and Fuerstenau showed that alkyl hydroxamates are much more selective for bastnäsite flotation as compared to fatty acids. Subsequent studies8,9,16 confirmed the effectiveness of hydroxamate collectors in microflotation experiments and provided spectroscopic evidence for the interaction of hydroxamates with bastnäsite.17 Though a number of works have examined various collector molecules for bastnäsite flotation, the approach to the development of effective collector molecules has largely been based on trial and error, along with knowledge gained from flotation of other minerals and empirical rules of thumb. Very little work has been done toward gaining an atomistically detailed understanding of the surface structure and chemistry of bastnäsite to aid in the rational design of ligands that specifically bind to bastnäsite over calcite and barite. To explain the selectivity of hydroxamates to bastnäsite, Pradip and Rai18 used molecular mechanics calculations to show that octyl hydroxmates adsorbed more strongly on the [101̅0] bastnäsite surface than on calcite and barite surfaces, and Zhang et al.19 used classical molecular dynamics simulations to demonstrate the hydrophilic to hydrophobic transition of the bastnäsite [1010̅ ] surface upon adsorption of octyl hyrdoxamates. To the best of our knowledge, no density functional theory (DFT) calculations have been reported for bastnäsite. Nor have the energies of different bastnäsite surfaces, crucial to molecular binding, been calculated or measured. In this article, we report the detailed investigation of bulk properties and surface and water adsorption energies of Labastnäsite using a combination of first-principles DFT calculations, powder X-ray diffraction (XRD) experiments, and calorimetry. The important finding of this work is that the structure of La-bastnäsite (P6̅2m) reported in the Inorganic Crystal Structure Database (ICSD)20 based on powder XRD is significantly less stable than the structure that is isomorphic to Ce-bastnäsite (P6̅2c).21,22 Theoretical results suggest that the structure of La-bastnäsite must be reexamined using single crystal experiments since powder XRD cannot resolve its structure unequivocally. Among several surfaces found in the natural mineral, the nonpolar [101̅0] surface is calculated to be the most stable, both in the gas phase and in aqueous solution. The surface energies calculated by DFT compare well with the average surface energy obtained by calorimetry. In agreement with calorimetry experiments, at low coverage water molecules are strongly adsorbed on the La-bastnäsite surface via coordination to the surface La3+ ions, whereas at higher coverage they are stabilized to a much lesser degree via

2. COMPUTATIONAL METHODS DFT-based calculations were performed using the VASP software.23−26 The valence electronic states were expanded on the basis of plane waves, and the core−valence interaction was described through the Projector Augmented Wave (PAW) approach.27,28 We used a plane wave kinetic energy cutoff of 600 eV. Positions of all the atoms were relaxed in our calculations. Geometry optimizations were deemed to have converged when the forces on each atom fell below 0.01 eV Å−1. The SCF convergence threshold was set to 10−5 eV for all calculations. A Pulay mixing scheme29 as implemented in VASP was used for charge density mixing during the SCF solution. For the bulk calculations, the Brillouin zone was sampled using a (4 × 4 × 4) Monkhorst−Pack k-point mesh. Use of a larger kpoint mesh resulted in less than 1 meV (0.096 kJ/mol) changes in the total energy (Table 1). The PBEsol30 GGA functional Table 1. Variation of the Total Energy of Bulk La-Bastnäsite (LaFCO3) with the Number of k-Pointsa gamma centered k-point Mesh

total energy (eV)

4×4x4 6×6x6 8×8x8

−298.60916 −298.60908 −298.60916

a

Calculations employed the PBEsol functional and DFT-D3 method. The plane wave energy cutoff was set to 600 eV.

was used to describe the exchange correlation interactions, combined with the DFT-D3 method of Grimme31 to include the van der Waals interactions between the LaF2+ and CO32− groups. Additionally, we tested the PBE GGA32,33 and PBEsol30 GGA functionals with no dispersion correction. Table 2 shows Table 2. Variation of the Lattice Parameters and the Formation Energy of La-Bastnäsite via Reaction 1 with Several DFT Methods method

a (Å)

c (Å)

PBE PBE−DFT-D3 PBEsol PBEsol−DFT-D3 experiment−ICSD20 experiment−this work (single-layered structure) experiment−this work (double-layered structure) experiment36

7.224 7.180 7.107 7.073 7.094 7.168

9.902 9.865 9.797 9.768 9.718 (= 2c) 9.822 (= 2c)

7.182

9.838

formation energy (kJ/mol) −122.4 −138.5 −156.6 −169.7

−173.07 ± 2.4

the effect of the DFT method on the lattice parameters and the formation energy of La-bastnäsite via reaction 1: 1/3La 2O3(s) + 1/3LaF3(s) + CO2 (g) → LaFCO3(s) (1)

Structural relaxation of La2O3(s) and LaF3(s) was performed beginning from their ICSD structures in the P3̅m134 and P3̅c135 space groups, respectively, while a (8 × 8 × 6) and (4 × 4 × 4) B


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The Journal of Physical Chemistry C Monkhorst−Pack k-point mesh was used to sample their Brillouin zone. Optimization of a CO2(g) molecule was performed using a 15 Å × 15 Å × 15 Å box. The PBEsol functional combined with the DFT-D3 correction was chosen for the subsequent calculations because it closely matched lattice constants of the ICSD structure20 and provided the best accuracy in reproducing the experimental formation energy.36 Surface calculations were carried out on six different surfaces of La-bastnäsite. Five of these surfaces, namely, [101̅0], [0001], [101̅1], [101̅2], and [112̅2] surfaces, are found in natural bastnäsite.37 Calculations were also carried out for the [101̅4] surface, which is known to be the most stable surface for calcite gangue material. Among these surfaces, [101̅0], [101̅2], and [101̅4] are nonpolar, whereas the rest are dipolar. To minimize the dipole moment in the direction normal to the surface, half of the surface-terminating groups on the top end of the slab were shifted to the bottom of the unit cell, creating 50% vacancies at the top and the bottom ends. For the dipolar surfaces, we used dipole and quadrupole corrections38,39 to the total energies in the direction normal to the surface. Many possible surface cuts and terminations were examined (section 4.2) to identify the most stable surface structures. The DFT values of surface energy exhibit reasonably fast convergence with slab thickness, as shown for representative [1010̅ ] and [0001] surfaces in Table 3. For the most stable [101̅0] surface,

LaF2+ groups and water molecules on the [0001]-LaF2+terminated surface. The thickness ( Å) and the number of LaFCO3 formula units employed in each slab are provided in Table 4. The surface calculations utilized a vacuum spacing of 20 Å in the surface normal direction. The surfaces were cleaved using the MedeA modeling suite.40 Molecular dynamics (MD) simulations were used to anneal the bulk structure and surfaces of La-bastnäsite to search for additional states with low energy. These calculations utilized a lower plane wave energy cutoff of 400 eV, and the Brillouin zone was sampled using the Γ point approximation. An MD time step of 1 fs was used, and the temperature was controlled using the Nosé−Hoover thermostat. For surfaces, the annealing schedule involved equilibration at 300 K for 2 ps followed by a temperature ramp to 600 K at a rate of 0.15 K/fs and equilibration at 600 K for 2 ps. The system was then cooled to 300 K at a rate of −0.15 K/fs and equilibrated again at 300 K for 2 ps. For the bulk system, the maximum temperature in the annealing schedule was 1000 K and the heating and cooling rates were 0.35 K/fs and −0.35 K/fs, respectively. Each of the MD simulations lasted for a duration of 10 ps (10 000 time steps total). Following annealing, the system was energy minimized using the same simulation parameters as mentioned at the beginning of this section. The surface energies were then used to predict the shape of a La-bastnäsite nanoparticle through thermodynamic Wulff construction. The aqueous solvation energy of La-bastnäsite surfaces was calculated using an implicit solvation model, VASPsol,41 with default settings of 78.4 for the dielectric constant, 0.6 for the width of the dielectric cavity, and 0.525 meV Å−2 for the dielectric cavity surface tension. VASPsol has been shown to accurately predict the solvation energies of small molecules41 and condensed phase systems.42 Adsorption energy of a single water molecule on Labastnäsite surfaces was computed for the most stable termination of those facets exposed in the Wulff shape. For the [101̅0] surface, we also computed the adsorption energy of water molecules as a function of coverage ranging from one (2.89 H2O/nm2) to three (8.68 H2O/nm2) water molecules per surface metal ion. A water molecule in gas phase was used as the reference to compute the adsorption energies, calculated in a 15 Å × 15 Å × 15 Å box. Molecular graphics in this article were generated using the VESTA software.43

Table 3. Variation in the Surface Energy with the Thickness of the Slab surface thickness (Å)

#LaFCO3 formula units

surface energy (J/m2)

[101̅0]-(b) Surface (Nonpolar Surface) 10.792 12 0.77 16.918 18 0.79 23.043 24 0.78 [0001]-LaF2+-Terminated Surface (Dipolar Surface) 10.825 12 1.10 21.339 24 1.24 [0001]-CO32−-Terminated Surface (Dipolar Surface) 12.015 12 1.09 21.009 24 1.13

two layers with a thickness of 10.792 Å were sufficient to converge the surface energies to within 0.02 J/m2. A larger variation in the surface energy with slab thickness was observed for the [0001]-LaF2+-terminated surface. However, due to high computational cost, we employed the smaller slab containing 12 LaFCO3 formula units to investigate possible distribution of Table 4. Surface and Solvation Energies of La-Bastnäsite surface

#LaFCO3 formula units

slab thickness (Å)

symmetry

2 Evac surf (J/m )

Esol (J/m2)

Esurf (J/m2)

[101̅0]-(a) [101̅0]-(b) [101̅1]-CO32− [101̅1]-La3+ [101̅1]-F− [101̅1]-LaF2+ [101̅1]-LaFCO3 [101̅2] [0001]-CO32− [0001]-LaF2+ [112̅2] [101̅4]

14 12 24 24 24 24 24 18 12 12 24 30

15.111 10.792 12.412 11.457 11.457 11.892 12.412 13.641 12.015 10.825 10.812 12.889

Pma2 Pmn21 P1 P1 P1 P1 P1 P2 Pm P2 P1 P2

0.73 0.77 0.89 0.99 1.11 0.95 1.01 1.03 1.09 1.10 1.18 1.35

−0.27 −0.28 −0.33 −0.35 −0.40 −0.36 −0.38 −0.35 −0.26 −0.35 −0.40 −0.43

0.45 0.49 0.56 0.64 0.71 0.59 0.63 0.68 0.83 0.75 0.78 0.92

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like a liquid with a surface energy to 0.072 J/m2, as in liquid water. The surface energy of the anhydrous state was then calculated using eq 2

3. EXPERIMENTAL METHODS 3.1. La-Bastnäsite Synthesis. LaFCO3 was synthesized using the procedure reported earlier by urea hydrolysis in the presence of NH4F as a source of F−.36 In a typical synthesis, 0.02 mol of RE(NO3)3 was dissolved in 20 mL of water and was added to a 50 mL solution containing 0.02 mol of NH4F. After stirring for 30 min, 0.04 mol of urea was added to this mixture, and the resulting solution was aged at 90 °C for 20 h. The precipitate obtained was separated by centrifugation, washed with DI water, and dried at 60 °C. 3.2. La-Bastnäsite Characterization. Powder X-ray diffraction patterns (XRD) of LaFCO3 sample was recorded using a Bruker AXS D8 Advance diffractometer with CuKα radiation, Kα = 1.5418 Å. Data were recorded between 20 and 100° 2θ with steps of 0.02° and counting time of 10 s per step. The Rietveld technique was employed for structure refinement using GSAS software.44,45 The refinements were carried out using two different structure models obtained from theoretical calculations (section 4.1), one corresponding to the singlelayered structure consisting of alternate layers of LaF2+ cations and CO32− anions and the other corresponding to the doublelayered structure consisting of two alternating layers of LaF2+ cations and CO32− anions. Scanning electron microscopy of the sample was performed using a Zeiss Sigma Field Emission SEM with an Oxford INCA PentaFETx3 EDS system (model 8100). Surface area measurement was done by the Brunauer− Emmett−Teller (BET) method using a Micromeritics Gemini VII surface area analyzer. The samples were degassed at 110 °C for 10 h before the measurements to obtain the surface area using N2 adsorption isotherm. 3.3. Water Adsorption Calorimetry. Water adsorption calorimetry was used to measure the surface energy of Labasnäsite, where the heat of adsorption of water on to the surface of the sample was monitored as a function of surface coverage. The water adsorption calorimetric experiment was performed in two steps on the La-bastnäsite sample. First, the sample was degassed under vacuum for 10 h at 110 °C to remove all adsorbed water. Second, a coupled Micromeritics ASAP2020 analyzer and a Setaram DSC111 Calvet microcalorimeter operated at 25 °C were used to measure the water adsorption enthalpy on the La-basnäsite surface. The instrument and methodology details are described elsewhere.46−48 Briefly, a pelletized sample is taken in one side of a specially designed silica glass forked tube that is placed inside the twin chambers of the DSC111 Calvet microcalorimeter, and the empty side of the tube serves as a reference. This tube placed inside the calorimeter is plugged into the analysis port of Micromeritics ASAP 2020 analyzer. Water vapor was dosed into the forked tube in incremental values of 1 mmol of H2O per dose using ASAP 2020 analyzer. The amount of adsorbed water vapor on the sample surface in forked tube was determined from the pressure drop (relative pressure P/P0) as the water vapor is removed from the sample chamber onto the sample surface. Each water dose generates a distinct calorimetric peak due to heat effects associated with water adsorption. The integral of area under the peak provides the corresponding heat of adsorption (differential enthalpy). The amount of water chemisorbed was defined as the amount of water adsorbed that enabled the differential enthalpy to equal the condensation enthalpy (−44 kJ/mol or −0.46 eV). The remaining water adsorbed at −44 kJ/mol was considered to be physisorbed, and at this stage, the surface was assumed to be

0.072 = γSG + θ ΔHads

(2)

where θ is the minimal water coverage at which a liquid-like state is observed and γSG is the surface energy of the anhydrous surface. Further details regarding use of water adsorption for calculating surface energy can be found elsewhere.36 To determine the experimental uncertainties, experiments were repeated twice by degassing the sample at 110 °C for 10 h. In all the experiments, water vapor was the reference state whose enthalpy does not depend on pressure at constant temperature since H2O can be considered an ideal gas at these low pressures. A blank with an empty tube was run to correct the data for water adsorbed on the forked tube wall.

4. RESULTS AND DISCUSSION 4.1. Bulk Structure and Properties. Bastnäsite is a fluorocarbonate mineral consisting of alternate layers of MF2+ (M = La3+/Ce3+/Y3+) and CO32− groups. The Inorganic Crystal Structure Database (ICSD) reports La-bastnäsite to be hexagonal (P6̅2m space group, ICSD code: 26678) based on on Oftedal’s work,20 containing three formula units of LaFCO3 per unit cell, arranged as alternating layers of three LaF2+ cations and three CO32− anions. This structure will be referred to as the “single-layered” structure in the subsequent discussion. For Ce-bastnäsite, however, ICSD reports the crystal structure to consist of 6 formula units of CeFCO3 in P6̅2c space group (ICSD code: 81673),21,22 arranged in two alternating layers of three CeF2+ cations and three CO32− anions. This structure will be referred to as the “double-layered” structure. As evident from Figure 1, the single-layered structure obtained by powder XRD contains CO32− units that deviate significantly from the expected trigonal planar geometry and have unrealistically short C−O bond lengths (0.97 Å). However, the double-layered structure obtained from single-crystal XRD has no crystallo-

Figure 1. ICSD structure of (a) single-layered La-bastnäsite and (b) double-layered Ce-bastnäsite. La atoms are green, Ce atoms are yellow, F atoms are blue, C atoms are brown, and O atoms are red. D


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The Journal of Physical Chemistry C graphic disorder and no unresolved anomalies. Although ICSD reports different structures for La- and Ce-bastnäsite, it must be noted that in a later publication,49 Oftedal himself reported Laand Ce-bastnäsite structures to be isomorphic in the P6̅2c space group. However, the experimental samples in both of his references20,49 were (Ce, La)FCO3 in composition, with no specific ratio of the metal ions in the samples provided. Thus, while it may be reasonable to expect that La- and Ce-bastnäsite adopt the same structure, no conclusive evidence to this end is presently available. The major difference between the single- and double-layered structures lies in the orientation of the CO32− groups in adjacent anion layers. In the single-layered structure, the CO32− groups in adjacent anion layers are mirror images of each other, whereas in the double-layered structure, the CO32− groups in one layer are rotated by 120° about the c direction with respect to those in the adjacent layer. Also, in the single-layered structure, all the La3+ and F− ions lie in the same plane, whereas in the double-layered structure, the F− ions are located slightly above and below the plane of the La3+ ions. To identify the most stable structure of La-bastnäsite, we replaced the Ce atoms in the double-layered structure with La atoms and scaled the lattice parameters to match the experimental lattice parameters of La-bastnäsite (a = 7.094 Å and 2c = 9.718 Å). Full relaxation of atomic coordinates and cell parameters revealed that the double-layered structure is ∼700 meV (67.5 kJ/mol) more stable (117 meV (11.3 kJ/mol) per LaFCO3 formula unit) than a 1 × 1 × 2 supercell of the single-layered structure. Furthermore, the lattice parameters obtained for the double-layered unit cell (a = 7.073 Å and c = 9.768) are in better agreement with the experimental values reported in ICSD (Table 2) than those obtained for the single-layered unit cell (a = 7.157 Å and 2c = 9.777 Å). Significantly higher stability of the double-layered structure can be attributed to a more favorable orientation of the CO32− groups in the adjacent anion layers. Starting from the singlelayered structure, the rotation of the CO32− groups in the adjacent anion layers asynchronously by ∼60° about the c direction minimizes the repulsion between these groups, resulting in higher stability. In a systematic search for lowlying polymorphs with an alternative orientation of the CO32− groups, 20 different initial structures were generated, resulting in 9 new local minima. Among these conformers, two local minima, denoted as B and C in Figure 2, were found to be more stable than the single-layered structure. Starting from the double-layered structure as a reference, the initial structure B was obtained by rotating each of the CO32− groups in the top layer by 60° in the clockwise direction and the CO32− groups in the bottom layer by an equivalent amount in the anticlockwise direction. Upon structural relaxation, the CO32− groups have rotated such that each pair of CO32− groups in the adjacent layers approximately point in the same direction. This conformation is ca. 71 meV (6.9 kJ/mol) uphill in energy per LaFCO3 formula unit compared to the double-layered structure but ca. 46 meV (4.4 kJ/mol) downhill in energy compared to the single-layered structure. For conformer C, the initial structure was generated by rotating two CO32− groups in the bottom layer by 120° in such a way that all the CO32− groups in this layer pointed along the minor diagonal of the hexagonal unit cell. As a result of these modifications, however, the equivalence of carbonate oxygen positions in adjacent layers was broken, resulting in a loss of hexagonal symmetry. The parallel alignment of all the CO32− groups is the only other

Figure 2. (a) Structure B and (b) structure C energetically lying between the single- and double-layered La-bastnäsite structures. The energies are given with respect to the most stable conformation. La atoms are green, C atoms are brown, O atoms are red, and F atoms are blue.

topological arrangement possible for the single-layered structure with a primitive unit cell. After full relaxation, this form, however, is ca. 224 meV (21.6 kJ/mol) less stable than the single-layered ICSD structure. The single-layered structure with an arbitrary rotation of a single CO32− group relaxes back to the original structure. For the double-layered structure, we were able to find a local minimum after rotating a single CO32− group by 180° in the plane of the molecule, but this was 1.375 eV (132.7 kJ) higher in energy, suggesting that significant disorder in the positions of the CO32− groups is not expected. Finally, we performed MD simulations using the annealing protocol described in the Computational Methods section on the double-layered structure to search for alternative local minima. However, these simulations failed to yield any new energetically low-lying forms. Atomic positions from the single- and double-layered structures were used in the Rietveld fit of the XRD pattern of La-bastnäsite. Figure 3 shows the XRD pattern of La-bastnäsite along with the calculated profiles for both structures. From Figure 3, we can see that the Rietveld refinements with two different structures yielded a similar visual fit. Both singleand double-layered structures lead to similar peak positions in the calculated XRD pattern because the position of La3+, which is a strong scatterer, is the same in both the structures. As explained previously, the difference between the single- and double-layered structures lies in the positions of the F¯ and CO32− anions. The supercell reflections generated due to these differences in the CO32− and F¯ positions among the two structures are weak since all the three atoms C, O, and F are comparatively weak scatterers. As a result, it becomes increasingly difficult to distinguish between the two structures using XRD. However, as shown in Table 5, the goodness-of-fit parameters obtained with the double-layered structure are lower than those for the single-layered structure, suggesting that La-basnäsite adopts the double-layered geometry. This supports the theoretical finding that the double-layered E


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experiments on La-bastnäsite in order to accurately resolve its structure and confirm our theoretical predictions. Using neutrons instead of X-rays could also be useful as neutrons interact strongly with light atoms C, O, and F. Electronic band structure and density of states of Labastnäsite are shown in Figure 4. La-bastnäsite has a relatively flat band structure with a GGA DFT band gap of 4.44 eV, pointing to an ionic material with highly localized states. Since GGA DFT significantly underestimates band gaps, we expect the experimental band gap to be ∼1−2 eV higher. The edge of the valence band predominantly comprises oxygen p-type orbitals from the CO32− ions, whereas the bottom of the conduction band predominantly comprises the unoccupied f orbitals on the La3+ ions. 4.2. Surface Structures and Energies. Having established the most stable structure of La-bastnäsite, we can now generate surfaces with different orientations and terminations to investigate their stability in vacuum and in an implicit solvent. On the basis of the observed morphology of bastnäsite crystals,37 we have studied six different surfaces, namely, [101̅0], [0001], [101̅1], [101̅2], [112̅2], and [101̅4]. Among these surfaces, [101̅0], [101̅2], and [101̅4] are nonpolar surfaces, and the rest are dipolar surfaces. With the constraint that the surface dipole moment is zero, two stepped surfaces, [101̅2] and [101̅4], can only be cut in one way, whereas the [101̅0] surface can be cleaved in two different ways. All possible stoichiometric terminations for the [0001] and the [101̅1] surfaces were considered. It is now well-known that polar oxide surfaces can undergo complex surface reconstruction to nullify the dipole moment normal to the surface.50 Furthermore, exposure to humid environment can provide additional means of stabilization through the formation of surface hydroxyl groups upon the dissociation of water molecules. Presently, however, no experimental data on the surface structures of bastnäsite is available. Thus, in this work we have considered only a simple redistribution of atoms on the [0001] and [1011̅ ] surfaces wherein 2 × 1 × 1 supercells were created and 50% of La3+, LaF2+, F−, or CO32− ions were shifted from the top to the bottom of the unit cell as a way to alleviate or fully remove the dipole moment perpendicular to the surface. For the [1122̅ ] surface, as a result of a small separation of ions in each layer in the surface normal direction, we examined only one termination that gave the most compact arrangement of atoms at the surface. All surfaces were allowed to relax while keeping the cell parameters fixed. MD simulations were carried

Figure 3. Rietveld fit of the XRD pattern of La-bastnäsite with (a) single- and (b) double-layered structure models.

Table 5. Goodness-of-Fit Parameters Obtained for the Refinement of XRD Pattern of La-Bastnäsite with Singleand Double-Layered Structures space group lattice parameters (Å) goodness-of-fit parameters

single-layered structure

double-layered structure

P6̅2m a = 7.168 (9); c = 4.911 (5) Rwp = 9.1; R(F2) = 15.1; χ2 = 8.8

P6̅2c a = 7.182 (6); c = 9.838 (7) Rwp = 6.2; R(F2) = 11.3; χ2 = 4.5

structure is energetically preferred over the single-layered structure. On the basis of the extensive computational search for stable structures and experimental evidence based on XRD, we suggest that in the most stable form La-bastnäsite is isomorphic to Ce-bastnäsite, having the P6̅2c space group and consisting of six formula units of LaFCO3 arranged in two sets of alternating layers of three LaF2+ ions and three CO32− ions. The calculated energy difference between the single- and double-layered structures is significant enough to motivate single-crystal

Figure 4. Electronic structure of La-bastnäsite. The valence band maximum is set to 0 eV in panel a, whereas the Fermi level is set to 0 eV in panel b. F


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The Journal of Physical Chemistry C Table 6. C−O and La−F Bond Lengths in the Bulk and Different Surfaces of La-Bastnäsitea structure

min rC−O (Å)

max rC−O (Å)

avg rC−O (Å)

rmin La−F (Å)

rmax La−F (Å)

ravg La−F (Å)

ΔC−O (%)

ΔLa−F (%)

δC−O (%)

δLa−F (%)

bulk [101̅0]-(a) [101̅0]-(b) [101̅1]-CO32− [101̅1]-La3+ [1011̅ ]-F− [101̅1]-LaF2+ [101̅1]-LaFCO3 [101̅2] [0001]-CO32− [0001]-LaF2+ [112̅2] [101̅4]

1.289 1.276 1.278 1.260 1.259 1.246 1.269 1.264 1.257 1.255 1.256 1.239 1.212

1.292 1.312 1.325 1.340 1.348 1.362 1.340 1.351 1.353 1.389 1.358 1.353 1.422

1.291 1.290 1.291 1.294 1.294 1.295 1.294 1.294 1.293 1.296 1.295 1.296 1.295

2.394 2.284 2.278 2.239 2.242 2.066 2.228 2.209 2.276 2.307 2.083 2.274 2.299

2.403 2.451 2.476 2.570 2.582 2.591 2.579 2.589 2.492 2.569 2.434 2.591 2.555

2.397 2.392 2.382 2.388 2.396 2.383 2.389 2.389 2.395 2.429 2.350 2.357 2.415

0 −0.05 0.03 0.22 0.27 0.32 0.21 0.27 0.19 0.39 0.30 0.37 0.28

0 −0.22 −0.62 −0.37 −0.05 −0.61 −0.34 −0.35 −0.09 1.34 −1.95 −1.69 0.76

0.21 2.77 3.64 6.21 6.84 8.90 5.50 6.74 7.40 10.35 7.91 8.74 16.17

0.35 6.97 8.30 13.87 14.18 22.05 14.69 15.91 9.01 10.76 14.95 13.41 10.58

a The percentage change in the average C−O and La−F bond lengths with respect to the bulk, ΔC−O and ΔLa−F, is calculated using eq 5. The percentage of spread in the C−O and La−F bond lengths with respect to the average C−O and La−F bond lengths in each structure, δC−O and δLa−F, is calculated using eq 6.

over a lower energy surface is only a fraction of the difference in their surface energies in vacuum. For example, the [101̅4] surface is 0.62 J/m2 less stable than the [101̅0] surface, but stabilization of the [1014̅ ] surface by hydration is only 0.16 J/ m2 more than that of the [101̅0] surface. We find that the inclusion of solvation effects does not significantly impact the relative order of surface stabilities but makes the surface energies slightly more isotropic. To better understand the geometric relaxation upon the formation of a surface, Table 6 presents the shortest, longest, and average C−O and La−F bond lengths in bulk and in the slab models. The change in the bond length (ΔC−O and ΔLa−F) was computed as a percentage with respect to bulk structure using eq 6 (shown for the C−O bond length)

out for the lowest energy termination identified for each of the surface using the annealing protocol described in the Computational Methods section. Because of high computational cost, MD simulations were not carried out for the stepped [101̅4] surface with the largest surface area. For all surfaces other than [101̅1], geometry optimization after MD did not result in a more favorable arrangement of atoms in the slab. For the [1011̅ ] surface, geometry optimization after MD resulted in a lowering of the surface energy by 0.06 J/m2 due to the reorientation of a CO32− group at the surface as compared to the optimized structure prior to annealing. Finally, the solvation energies were calculated using an implicit solvent model to obtain an estimate of the surface stabilization in an aqueous environment. It must be noted that an implicit solvation model gives only an average value for the stabilization of a surface upon exposure to a continuum of solvent, without taking into account any of the microscopic details of the solute−solvent interface. The stabilization of the less stable surfaces could strongly depend on the atomic structure at the surface−solvent interface. In particular, hydroxyl groups formed upon the dissociation of water molecules could stabilize polar surfaces, but this is beyond the scope of the present work. The calculated surface and solvation energies for all the surfaces that we have studied are given in Table 4. The surface energy in vacuum, Evac surf, was calculated using eq 3 vac Esurf

E − NE bulk = slab 2A

ΔC − O =

sol Eslab − Eslab 2A

(6)

where is the average C−O bond length on a surface and rbulk C−O:avg is the average C−O bond length in bulk. The variation in the C−O and La−F bond lengths within a given structure was computed as a percentage of the mean C−O and La−F bond lengths in the same structure using eq 7 (shown for the C−O bond) δC − O =

(3)

rmax C−O

min (rCmax − O − rC − O) × 100 rCavg− O

(7)

rmin C−O

where is the maximum C−O bond length, is the minimum C−O bond length, and ravg C−O is the average C−O bond length in the structure. 4.2.1. Nonpolar Surfaces. The nonpolar surfaces we have studied are the[101̅0], [101̅2], and [101̅4] surfaces. The [101̅0] surface consists of layers of LaFCO3 that can be terminated in two different ways depending on the orientation of the CO32− groups: (a) one of the surface CO32− groups points into the surface, whereas the other CO32− group points away from the surface, and (b) one of the surface CO32− groups lies flat on the surface, whereas the other CO32− group points into the surface. Each surface La3+ ion on the cleaved [1010̅ ]-(a) and [1010̅ ](b) surfaces are six-coordinated. In comparison, La3+ ions in the bulk are nine-coordinated. Figure 5a,b shows the overlays of the cleaved (unrelaxed) and relaxed structure for the two surface terminations. It clearly shows a reorientation of the carbonate

(4)

where Esol slab is the total energy of the slab in the presence of an implicit solvent. Finally, the total surface energy was calculated using eq 5 vac E surf = Esurf + E sol

rCbulk − O:avg

rsurf C−O:avg

where Eslab is the total energy of the slab, Ebulk is the total energy of the double-layered structure, and A is the surface area. The solvation energy is calculated using eq 4 E sol =

bulk (rCsurf − O:avg − rC − O:avg ) × 100

(5)

In general, we observe that hydration stabilizes higher energy surfaces to a larger extent than the lower energy surfaces. However, the relative stabilization of a higher energy surface G


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for the [101̅4] surface in Table 6, together with a large number of broken bonds on the surface, is in line with its highest surface energy (1.35 and 0.92 J/m2 in the gas phase and aqueous solution, respectively). With the [1012̅ ] surface energy of 1.03 and 0.68 J/m2 in the gas phase and in solution, respectively, the order of stability of the nonpolar surfaces of La-bastnäsite is as follows: [101̅0] > [101̅2] > [101̅4]. 4.2.2. Dipolar Surfaces. The dipolar surfaces that we have studied are the [0001], [1122̅ ], and [1011̅ ] surfaces. The [0001] surface consists of alternating layers of LaF2+ and CO32− groups (A-B-A-B type arrangement). As such, this surface could be terminated either by the LaF2+ or the CO32− group. Such surfaces have a finite dipole moment perpendicular to the surface that diverges with the slab thickness.50,51 To minimize the dipole moment normal to the surface, we moved half of the surface terminating groups (LaF2+ or CO32−) to either side of the surface. To redistribute half of the terminating groups to the other side of the slab, the primitive unit cell of the [0001] surface consisting of three (an odd number) CO32− and LaF2+ groups per layer was expanded to a 2 × 1 × 1 supercell. This redistribution can be done in a number of ways, so we considered only those cases that resulted in some symmetry higher than C1. We have considered 16 and 9 different initial configurations for CO32− and LaF2+ terminations, respectively. The lowest energy structures of the CO32−- and LaF2+terminated [0001] surfaces are shown in Figure 7. The positions of the LaF2+ and CO32− groups from the cleaved structure are overlaid using white spheres.

Figure 5. Optimized structures of (a) [101̅0]-(a) surface and (b) [101̅0]-(b) surface. La atoms are green, C atoms are brown, O atoms are red, and F atoms are blue. The positions of atoms in the cleaved structure are overlaid in each panel using white spheres.

groups in the [101̅0]-(a) termination as compared to only a very slight relaxation of the surface atoms in the [101̅0]-(b) termination. Analysis of bond distances (Table 6) confirms that the percentage change in the average C−O bond length with respect to bulk (−0.05−0.03%), and the spread in the C−O (2.77−3.64%) and La−F (6.97−8.30%) bond lengths within the structure are the lowest among the studied surfaces. A small relaxation of the surface ions often implies relatively high surface stability. Indeed, [101̅0] is the most stable surface among the studied planes with the calculated surface energy of 0.73 and 0.77 J/m2, for terminations (a) and (b), respectively. The solvation effects stabilize this surface by 0.27−0.28 J/m2, yielding the surface energy of 0.45−0.49 J/m2 in aqueous solution. The [1012̅ ] and [1014̅ ] surfaces are stepped, or vicinal, surfaces consisting of alternate layers of LaF2+ and CO32− groups that are oriented at an angle of 51.4 and 68.2° with respect to the surface normal as shown in Figure 6a,b, respectively. The surface La3+ ions on the cleaved [1012̅ ] surface show a mixed coordination number five and six, whereas on the cleaved [101̅4] surface, they exhibit a coordination number five. Upon optimization, the CO32− groups on both surfaces undergo significant reorientation. Changes in the positions of the CO32− groups in the subsurface regions are more pronounced in [101̅4] than in [101̅2]. Consequently, the spread in the C−O bond length on the [1014̅ ] surface (16.17%) is larger than that on the [1012̅ ] surface (7.4%). The largest variation in the C−O bond length

Figure 7. Optimized structures of (a) CO32−-terminated [0001] surface and (b) LaF2+-terminated [0001] surface. La atoms are green, C atoms are brown, O atoms are red, and F atoms are blue. The positions of atoms in the cleaved structure are overlaid in each panel using white spheres.

Figure 6. Optimized structures of the (a) [101̅2] surface and (b) [101̅4] surface. La atoms are green, C atoms are brown, O atoms are red, and F atoms are blue. The positions of atoms in the cleaved structure are overlaid in using white spheres. H


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The Journal of Physical Chemistry C The major change in the CO32−-terminated surface upon optimization lies in the reorientation of the surface CO32− groups initially directed perpendicular to the surface. Likewise, due to the vacant sites on the LaF2+-terminated surface, one of the three surface La3+ ions moved from surface to the subsurface CO32− layer to coordinate with three CO32− groups in a bidentate fashion. Of the three La3+ ions on the surface, two of them are six-coordinated, and the third La3+ ion is fourcoordinated. The percentage change in the average C−O bond length with respect to the bulk structure (Table 5) is nearly the same for both [0001]-CO32− (0.39%) and [0001]-LaF2+ (0.30%). However, the structural perturbations due to surface formation are more uniformly spread across the C−O and La− F bonds for the [0001]-CO32− surface (10.4 and 10.8%, respectively) compared to the [0001]-LaF2+ surface (7.9 and 15.0%, respectively). The surface energies of the most stable CO 3 2− - and LaF 2+ -terminated surfaces in vacuum are approximately equal at 1.09 and 1.10 J/m2, respectively. Hydration of the [0001] surface stabilizes the [0001]-LaF2 plane (−0.35 J/m2) to a larger extent than the [0001]-CO32− plane (−0.26 J/m2). The [101̅1] surface has no symmetry, so in order to minimize the dipole moment component perpendicular to the surface, we constructed a 2 × 1 × 1 supercell and moved half of the surface terminating group to the opposite side of the slab. This surface can have nine different cuts representing all possible surface terminations, namely, La3+, F−, LaF2+, CO32−, and LaFCO3. Upon geometry optimization of 24 different initial configurations for the [101̅1] surface, the CO32−terminated surface was found to be the most stable. Annealing of this surface followed by geometry optimization resulted in further reduction of the surface energy by 0.06 J/m2 due to a slight rotation of one of the CO32− group at the surface. A mixed coordination number of six and seven is exhibited by the surface La3+ ions. Compared to the [0001] plane, the [101̅1] plane is more compact, resulting only in small changes in the atomic positions upon geometry optimization. This is reflected in the relatively small changes in the average C−O and La−F bonds with respect to the bulk structure (Table 6). The [101̅1]CO32− termination with a surface energy of 0.89 J/m2 in vacuum and 0.56 J/m2 in aqueous solution is the most stable of the dipolar surfaces and the second most stable surface overall. As shown in Table 4, the stability of the [101̅1] planes with LaF2+, LaFCO3, and La3+ terminations is quite high, with surface energies of 0.95−1.01 J/m2 in the gas phase and 0.59− 0.71 J/m2 in aqueous solution. Among several options for surface cleavage along the [112̅2] plane, we have investigated the one that gives the most compact arrangement of atoms at the surface. No redistribution of ions has been considered because the thickness of one layer containing six units of the La3+, F−, and CO32− ions in the surface normal direction is only 2.70 Å. The surface La3+ ions in the cleaved structure show a mixed coordination of six to eight. An overlay of the optimized and cleaved structure of the [112̅2] surface given in Figure 8b shows no significant reorientation of the surface groups upon relaxation. However, a more detailed analysis of bond distances in Table 5 reveals a significant variation of the C−O (8.7%) and La−F (13.4%) bond lengths within the [1122̅ ] plane, likely a result of a significant number of broken bonds involving surface F− and CO32− ions. The [112̅2] surface has a fairly high surface energy of 1.18 and 0.78 J/m2 in the gas phase and solution, respectively. The order of stability of the dipolar surfaces is [1011̅ ] > [0001] > [1122̅ ].

Figure 8. Optimized structures of (a) CO32−-terminated [101̅1] surface and (b) [112̅2] surface. La atoms are green, C atoms are brown, O atoms are red, and F atoms are blue. The positions of atoms in the cleaved structure are overlaid in each panel using white spheres.

On the basis of the computed surface energies, we can predict the shape of an equilibrium La-bastnäsite nanoparticle through thermodynamic Wulff construction. The predicted morphology of La-bastnäsite is shown in Figure 9. The

Figure 9. Shape of a La-bastnäsite nanoparticle predicted by thermodynamic Wulff construction.

equilibrium morphology consists of a hexagonal prism with [101̅0] and [0001] facets chiseled at the ends by [101̅1] and [1012̅ ] facets. The calculated morphology agrees well with the experimental morphology of natural bastnäsite samples,52 expressing [101̅0] and [0001] faces with oscillatory [101̅1] and [101̅2] faces, although the predicted [0001] surface is too unstable. The discrepancy could be due to a chemical modification of the dipolar [0001] surface in the presence of H2O, CO2, and potential-determining ions that were not included in the calculations. A more detailed comparison of the nanoparticle shape with experimental electron micrographs can be made as they become available. The average surface energy of the nanoparticle, computed as an area weighted average of the crystal facet energies, is 0.86 J/m2 in vacuum, which is in good agreement with the experimental value of 1.11 J/m2 measured by calorimetry. 4.3. Adsorption of Water on La-Bastnäsite Surfaces. We have computed the adsorption energy of a single water molecule on the most stable termination of each of the four facets exposed in the Wulff shape of La-bastnäsite. The [101̅0], [101̅2], and [0001] surfaces are nonpolar. For these surfaces, one water molecule was added to each side of the slab symmetrically. The [101̅1] surface has C1 symmetry, so adsorption of only one water molecule was considered. Furthermore, while all the La3+ ions on the [101̅0] surface are equivalent to each other, the La3+ ions on the [101̅1], [1012̅ ], and [0001] surfaces are not. Thus, for the [1011̅ ], I


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The Journal of Physical Chemistry C [101̅2], and [0001] surfaces, we have computed the adsorption energy of a water molecule on two different surface La3+ sites. Finally, we have also studied the effect of surface coverage of water molecules on the adsorption energy for the [1010̅ ] surface, where the surface coverage was varied from 2.89 to 8.68 H2O/nm2. The adsorption energy of water was calculated using eq 8 n Eads =

surface CO32− groups resulting in the largest adsorption energy. A detailed description of water adsorption geometry and energetics at the other La3+ binding sites is provided in the Supporting Information. We have also performed a systematic search for dissociated states of water on the most stable [101̅0] surface and the most reactive [101̅2] surface (site A) with the highest water adsorption energy. Calculations beginning from five and three different initial configurations for the [101̅0] and [101̅2] surfaces, respectively, were performed wherein the proton was placed 1 Å away from a surface F− ion or an oxygen atom of the CO32− group, while the hydroxyl ion was bound to the surface La3+ ion. In all but one configuration for each surface (Figure 11), the proton hopped back onto the hydroxyl ion to form

n H 2O Eslab − Esurf − nE H 2O

(8)

n

2O EnH slab

where is the energy of the surface containing n adsorbed water molecules, EH2O is the energy of a water molecule in gas phase, and n is the number of adsorbed water molecules. Table 7 gives the adsorption energy of a water molecule on the Table 7. Adsorption Energy of a Water Molecule and La−Ow Distance on La-Bastnäsite Surfaces surface

adsorption energy (kJ/mol)

La−Ow distance (Å)

[1010̅ ] [101̅0] dissociated H2O [1011̅ ]-site A [101̅1]-site B [101̅2]-site A [101̅2]-site A, dissociated H2O [1012̅ ]-site B [0001]-LaF2+-site A [0001]-LaF2+-site B

−89.0 21.7 −87.9 −136.4 −141.9 −143.5 −115.4 −56.9 −125.9

2.59 2.11 2.55 2.47 2.53 2.12 2.35 2.66 2.52

Figure 11. Optimized structure of a dissociated water molecule on (a) [101̅0] and (b) [101̅2] surfaces. La atoms are green, F atoms are blue, C atoms are brown, O atoms of CO32− groups are red, Ow are purple, and H atoms are white.

[101̅0], [101̅1], [101̅2], and [0001] surfaces along with the distance of the water oxygen atom (Ow) from the La3+ ion on which it is adsorbed. Among the various adsorption sites studied in this work, site A on the [101̅2] surface is the most preferred location for the adsorption of a water molecule (Figure 10). At site A, the La3+ ion is coordinated to two F− ions and four oxygen atoms from the surrounding three CO32− groups. The water molecule at site A is strongly bonded to not only the surface La3+ ion (the La−Ow distance of 2.53 Å) but also the oxygen atoms of two

water. Although a local minimum on the [101̅0] surface with a proton attached to a surface CO32− group was found, this configuration is significantly less stable (by >110 kJ/mol) than the associated state of water. In contrast, the adsorption energies for the dissociated (Eads = −143.5 kJ/mol) and associated states (Eads = −141.9 kJ/mol) of water on the [101̅2] surface are very similar. This is not surprising given that a large number of bonds are broken upon the formation of the less stable and more reactive [101̅2] surface. Finally, for the dominant [101̅0] surface in the predicted Labastnäsite morphology (Figure 9), we have computed the effect of surface coverage on the water adsorption energies. Three valences are lost from a La3+ ion upon the cleavage of the [101̅0] surface, so the hydration of the [101̅0] surface by up to three water molecules per metal ion was considered. The adsorption of one, two, and three water molecules per metal ion corresponds to surface coverage of 2.89, 5.79, and 8.68 H2O/nm2 respectively. To avoid any dipole moment in the surface normal direction, the initial structure for all the systems were created symmetrically, i.e., for every water molecule on one side of the [101̅0] slab there was a symmetrically equivalent molecule present on the opposite side of the slab. The adsorption energy was calculated using eq 8, and the stabilization of successive water molecules on the [1010̅ ] surface was calculated using eq 9 Estab =

Figure 10. Optimized geometry of a water molecule adsorbed at site A on the [1012̅ ] surface. La atoms are green, F atoms are blue, C atoms are brown, O atoms of CO32− groups are red, Ow are purple, and H atoms are white.

(n + 4)H 2O n H 2O Eslab − Eslab − 4E H2O

(9)

4 3+

where 4 accounts for the total number of surface La sites on both sides of the slab. J


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Figure 12. Optimized structures of (a) 1 water, (b) 2 water, and (c) 3 water molecules per surface La3+ ion. La atoms are green, C atoms are brown, O atoms of CO32− groups are red, Ow are purple, F atoms are blue, and H atoms are white.

4.4. Water Adsorption Calorimetry of La-Bastnäsite. Water adsorption calorimetry was used to measure the surface energy of La-bastnäsite. Figure 13 shows a typical adsorption isotherm for water on La-bastnäsite and the corresponding calorimetric peaks. The area of the calorimetric peak decreases as the adsorption proceeds, indicating decrease in the magnitude of the exothermic differential enthalpy of water adsorption. Figure 13c shows the differential enthalpies of water adsorption as a function of surface coverage. The differential enthalpy of adsorption for the first dose at near-zero coverage is −163.2 kJ/mol, indicating strong chemisorption of water on the La-bastnäsite surface. With increasing surface coverage, the differential enthalpy becomes less exothermic with successive doses and finally reaches the value of enthalpy of condensation of bulk water at 25 °C (−44 kJ/mol) in about 8−10 doses. The average water coverage value on the bastnäsite surface at this point is 9.69 H2O/nm2. The water adsorbed up to this coverage is strongly bound and can be considered to be chemisorbed. The remaining water, with a differential enthalpy of −44 kJ/ mol, can be treated as physisorbed water. The integral enthalpy of adsorption, which is the sum of the differential enthalpies of adsorption, divided by the total water up to this coverage, corresponds to the enthalpy of chemisorbed water. The integral enthalpy for the La-bastnäsite degassed at 110 °C is −71.6 kJ/ mol. It can also be seen from Figure 13c that up to a coverage of ∼4 H2O/nm2 the differential enthalpies of adsorption are more exothermic and that between 4 and 9 H2O/nm2 coverage they decrease rapidly in magnitude to reach a plateau at −44 kJ/mol. This plateau corresponds to the enthalpy of condensation of water vapor, and all measured enthalpy prior to this plateau is due water molecules strongly bound to the surface that are chemisorbed. Using eq 2, the energy of the anhydrous surface of synthetic La-bastnäsite was found to be 1.11 ± 0.1 J/m2. Table 9 gives a summary of the water adsorption calorimetry data. Finally, Figure 14 shows the variation of the surface energy and water coverage as a function of the relative water vapor pressure. It can be seen that the surface energy values decay rapidly with increase in water coverage and eventually approach

The most stable configurations with different numbers of water molecules are shown in Figure 12. Table 8 summarizes Table 8. Predicted Adsorption Energies, Stabilization Energies, and La−Ow Bond Distances as a Function of Coverage on the [101̅0] Surface coverage (H2O/nm2)

Eads (kJ/mol)

Estab (kJ/mol)

La3+−Ow distance (Å)

2.89 5.79 8.68

−97.0 −93.7 −87.2

−97.0 −90.4 −74.0

2.55, 2.68 2.58, 2.59 2.48, 2.59

the adsorption and stabilization energies together with the La− Ow distances to the water molecules bound to the metal ion. A detailed description of different initial configurations and their optimized structures at different water coverage is given in the Supporting Information. Not surprisingly, water molecules at the surface are additionally stabilized through a network of hydrogen bonds. At 2.89 H2O/nm2 coverage, the water molecules bound to the neighboring metal ions tilt to form dimeric structures, which results in a more exothermic adsorption energy (−97.0 kJ/mol) compared to the adsorption of one water molecule (−89.0 kJ/ mol). At higher coverage, though the initial structures had two and three water molecules bound to the metal ion, upon optimization only one water molecule remained coordinated to the La3+ ion, while the other molecules were displaced into the space between two rows of metal ions. These water molecules formed hydrogen bonds among themselves and with the surface CO32− and F− ions. Thus, at the coverage above one water molecule per metal ion (2.89 H2O/nm2), the stabilization of additional water molecules occur through coordination to the anionic groups (CO32− and F− ions) present on the surface. The stabilization of water molecules by these groups is not as strong as that from coordination to the La3+ directly, as evidenced by the stabilization energy, which drops rapidly from −97.0 to −74.0 kJ/mol as the water coverage increases from 2.89 to 8.68 H2O/nm2. K


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Figure 13. (a) Typical water adsorption isotherm; (b) the corresponding calorimetric peaks for the La-bastnäsite sample. (c) Differential enthalpies of water adsorption as a function of surface coverage.

the surface energy of bulk water (0.072 J/m2) at relative pressures of 0.25−0.4 with a coverage of ca. 9.5 H2O/nm2. These findings support the theoretical prediction that the first layer of water molecules on the La-bastnäsite surface is strongly bound through coordination to the La3+ ions and hydrogen bonding to the surface anions. A direct comparison of the measured water adsorption energies with theoretical predictions is complicated by the fact that the experimental sample consisted of fine grains that did not exhibit a clear faceted morphology, as seen in the SEM image in Figure S2. Surface energy calculated using water adsorption experiments corresponds to an average value over all the exposed surfaces. Moreover, the experimental adsorption enthalpies at low coverage correspond to the adsorption at highly reactive edges, kinks, and terraces, among others, on the surface of the sample. Theoretical predictions however are based on water adsorption at a well-defined and defect-free facet of Labastnäsite, which may not reflect the real material. Furthermore, within a given crystal facet, theoretical predictions show that the adsorption energy of water molecules is strongly dependent on the specific adsorption site. Given these differences, the calculated adsorption energy of a single water molecule at site A on the [101̅2] surface (−141.9 kJ/mol) is in good agreement with the experimental adsorption enthalpy of −163.2 kJ/mol at low coverage. Likewise, the calculated water adsorption energy of −87.2 kJ/mol at 8.68 H2O/nm2 coverage is in reasonable agreement with the measured integral adsorption enthalpy of −71.6 kJ/mol at 9.69 H2O/nm2 coverage.

Table 9. Summary of Water Adsorption Calorimetry Data surface area (m2/g)

differential enthalpy for first dose (kJ/mol)

integral enthalpy of adsorption (kJ/mol)

coverage (H2O/nm2)

chemisorbed water (mol)

67.13

−163.2

−71.6

9.69

0.142

Figure 14. Surface energy and surface water coverage as a function of relative pressure of water vapor.

L


Article

The Journal of Physical Chemistry C 4.5. Implications for the Design of Bastnäsite Floatation Agents. The major challenge in improving the beneficiation efficiency of bastnäsite through froth flotation lies in the development of flotation agents that are sufficiently selective for bastnäsite over calcite gangue. The morphology of natural crystals of bastnäsite and calcite are expected to be dominated by their most stable surface terminations, namely, [1010̅ ] for bastnäsite and [1014̅ ] for calcite. Both these surfaces are chemically similar, with bastnäsite [101̅0] containing La3+ ions surrounded by the F− and CO32− ions and calcite [101̅4] containing Ca2+ and CO32− ions. On both surfaces, only one water molecule binds directly to the cation whereas other water molecules are stabilized by coordination to the surface anionic groups via hydrogen. Furthermore, the adsorption energy of water molecules at 100% coverage on the calcite [101̅4] surface (−87.853 to −96.1 kJ/mol54 depending on the DFT method used) is almost equal to that at bastnäsite [101̅0] surface (−97.0 kJ/mol). Thus, any ligand that binds to the surface through a single headgroup by displacing the water molecule coordinated to the metal ion is not expected to be sufficiently selective for bastnäsite over calcite. Indeed, fatty acids have been shown to be poorly selective for bastnäsite in flotation of the Mountain Pass ore.12 Attempts to improve selectivity by using ligands that bind to the surface anions instead might also prove futile since both the surfaces contain a common anion. Thus, the way forward in developing flotation agents with high selectivity for bastnäsite is to exploit the structural differences in the calcite [1014̅ ] and bastnäsite [1010̅ ] surfaces. On the bastnasite [101̅0] surface, two La3+ ions are separated by ca. 4.80 and 7.07 Å along the two lattice directions, whereas the separation between two Ca2+ ions on the calcite [101̅4] surface is ca. 4.03 and 4.99 Å. Thus, ligands containing two chelating groups that are separated by ca. 7.1 Å may bind to bastnäsite preferentially over calcite since the resultant strain in the molecule upon coordination to two Ca2+ ions on the calcite [1014̅ ] surface could make it unfavorable for the molecule to bind to the surface cations. An ongoing research effort in our group is to use the results from the calculations presented here to study the nature of adsorption of organic molecules in the presence and absence of water on bastnäsite surfaces and to design candidate organic collector molecules to improve the beneficiation efficiency of bastnäsite ore.

The inclusion of solvent effects using a polarizable dielectric continuum model does not alter the order of surface stability but makes the surface energies more isotropic. The equilibrium morphology of a La-bastnäsite crystal is predicted via Wulff construction to be a hexagonal prism with [101̅0] and [0001] facets chiseled at the ends by [101̅1] and [101̅2] facets. The adsorption energy of a water molecule is found to depend strongly on both the surface orientation and the specific adsorption site on that surface. In agreement with water adsorption calorimetry experiments, DFT calculations predict that water molecules at low coverage are strongly adsorbed on the surface through a network of hydrogen bonds and coordination with the surface La3+ ions. The computed water adsorption energy at high coverage is in reasonable agreement with the integral adsorption enthalpy measured by calorimetry. The results of this work provide the necessary structural and energetic information to study the interaction of organic ligands with La-bastnäsite surface and to design collector molecules to enhance the beneficiation efficiency of bastnäsite ore.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04747. Detailed description of the adsorption of a single water molecule on the surface of La-bastnäsite, effect of water coverage on the [101̅0] surface, and the SEM image of synthesized La-bastnäsite (PDF) Crystallographic information file for the CO32−-terminated [0001] surface obtained from VASP calculations (CIF) Crystallographic information file of LaF-terminated [0001] surface obtained from VASP calculations (CIF) Crystallographic information file of [101̅0]-(a) surface obtained from VASP calculations (CIF) Crystallographic information file of [101̅0]-(b) surface obtained from VASP calculations (CIF) Crystallographic information file of the [101̅1] surface obtained from VASP calculations (CIF) Crystallographic information file for the [101̅2] surface obtained from VASP calculations (CIF) Crystallographic information file for the [101̅4] surface obtained from VASP calculations (CIF) Crystallographic information file of the [112̅2] surface obtained from VASP calculations (CIF) Crystallographic information file of one-layered bulk structure obtained from VASP calculations (CIF) Crystallographic information file of two-layered bulk structure obtained from VASP calculations (CIF)

5. CONCLUSIONS We have employed density functional theory (DFT) calculations, powder X-ray diffraction (XRD), and water adsorption calorimetry to investigate bulk structure, surface stability, and water adsorption energies of La-bastnäsite. The results of this study reveal that the ICSD structure which consists of alternate layers of LaF2+ and CO32− groups in P6̅2m space group is not the most stable form of La-bastnäsite. DFT based calculations, supported by XRD data, indicate that the most stable form of La-bastnäsite is isomorphic to the structure of Ce-bastnäsite belonging to the P6̅2c space group and consists of two alternate layers of LaF2+ and CO32−, differing by the orientation of the CO32− groups in adjacent anion layers to minimize the electrostatic repulsion between these groups. Labastnäsite is a wide band gap insulator with a GGA DFT band gap of 4.44 eV. The edge of the valence band predominantly arises from the p orbitals on oxygen atoms, and the bottom of the conduction band consists of unoccupied f orbitals from the La3+ ions. A systematic examination of various surfaces identified the nonpolar [1010̅ ] surface to be the most stable.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +1 530-752-3292. *E-mail: [email protected]. Tel.: +1 865-576-4272. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. This research used resources M


Article


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of the National Energy Research Scientific Computing Center and the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, both of which are supported by the Office of Science of the U.S. Department of Energy under contract Nos. DE-AC02-05CH11231 and DE-AC0500OR22725, respectively.

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