Article pubs.acs.org/cm
Uncovering the True Atomic Structure of Disordered Materials: The Structure of a Hydrated Amorphous Magnesium Carbonate (MgCO3·3D2O) Claire E. White,*,†,‡,§,∥ Neil J. Henson,§ Luke L. Daemen,‡ Monika Hartl,‡ and Katharine Page‡ †
Department of Civil and Environmental Engineering and Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States ‡ Lujan Neutron Scattering Center, §Physics and Chemistry of Materials, and ∥Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
ABSTRACT: Amorphous calcium/magnesium carbonates are of significant interest in the technology sector for a range of processes, including carbon storage and biomineralization. Here, the atomic structure of one hydrated amorphous magnesium carbonate (hydrated AMC, MgCO3·3D2O) is investigated using an iterative methodology, where quantum chemistry and experimental total scattering data are combined in an interactive iterative manner to produce an experimentally valid structural representation that is thermodynamically stable. The atomic structure of this hydrated AMC consists of a distribution of Mg2+ coordination states, predominately V- and VI-fold, and is heterogeneous due to the presence of Mg2+/ CO32‑-rich regions interspersed with small ‘pores’ of water molecules. This heterogeneity at the atomic length scale is likely to contribute to the dehydration of hydrated AMC by providing a route for water molecules to be removed. We show that this iterative methodology enables wide sampling of the potential energy landscape, which is important for elucidating the true atomic structure of highly disordered metastable materials.
■
INTRODUCTION
content, the structure of a hydrated AMC will depend on the amount and nature of this bound water. The atomic structures of natural and synthetic ACC have been rigorously debated in recent years. Goodwin et al. used Xray pair distribution function analysis (PDF) and reverse Monte Carlo (RMC) modeling to generate structural representations of ACC.12 These models consisted of H2O/CO32‑ interconnected channels supported by a Ca2+-rich porous framework. However, subsequent simulations of this model by Singer et al. using force field molecular dynamics (MD) revealed significant structural organization was undertaken during the MD simulation,8 which implied that the RMC model was not thermodynamically stable (i.e., not at a local minimum on the potential energy surface). Instead, Singer et al. reported that the CO32‑ molecules had an affinity to the Ca2+ ions during the MD simulation. There have been many simulation-based investigations on ACC in the literature carried out employing force field MD,18−20 including a force field specifically parametrized for aqueous calcium carbonate.22 Recently, Wallace et al. employed force field MD and lattice gas simulations to investigate the cluster populations prior to nucleation in
Nature-inspired human-made materials are increasingly being exploited in the technology sector due to their conforming abilities, including amorphous calcium/magnesium carbonates (ACC/AMC). These carbonate phases are precursors to crystalline CaCO3 and MgCO3 and therefore can influence the precipitation of the various crystalline polymorphs. Nature exploits this carbonate-based amorphous to crystalline transition in a variety of processes, including sea urchin spicules,1 crustaceans,2 earthworms,3 and plant cystoliths.4 The formation and stability of ACC and subsequent crystallization of CaCO3 polymorphs have received much attention in the research community,5−20 especially since CO2 capture and storage is being pursued as a means of reducing anthropogenic CO2 emissions. However, AMC has received much less attention, mainly being discussed in the context of mixed Ca/Mg amorphous carbonate phases.6,10,21 It is known that the incorporation of Mg2+ regulates the kinetics of the amorphous to crystalline transition of ACC, most likely due to the slow kinetics of Mg2+ dehydration.10 Hence, understanding the atomic structure of AMC, and how it differs from ACC is of paramount importance. Furthermore, due to the numerous hydrated crystal structures of magnesium carbonate that exist (e.g., barringtonite, nesquehonite, lansfordite, artinite, hydromagnesite and dypingite), each having a different water © 2014 American Chemical Society
Received: February 7, 2014 Revised: March 24, 2014 Published: March 25, 2014 2693
dx.doi.org/10.1021/cm500470g | Chem. Mater. 2014, 26, 2693−2702
Chemistry of Materials
Article
out using the program Fit2D27,28 with CeO2 as the calibration material. Due to the insensitivity of X-ray PDF analysis to the water correlations in the presence of larger Z elements including Mg2+ and O2‑ in hydrated AMC, neutron PDF analysis was carried out in order to obtain local structural information on the O−D correlations in the sample. High-resolution time-of-flight neutron powder total scattering was carried out on the NPDF beamline at the Lujan Neutron Scattering Center, Los Alamos National Laboratory.29 The sample was measured in a standard vanadium can in a Displex cryostat at ambient temperature. Standard data reduction was performed using the PDFgetN software,30 including a background subtraction to remove incoherent scattering.31 The pair distribution function (PDF), G(r), is obtained by taking a sine Fourier transform of the measured total scattering function, S(Q), where Q is the momentum transfer, as outlined by Egami and Billinge.32 The PDF from the X-ray data was obtained using PDFgetX2,33 with a Qmax of 20 Å−1. The neutron PDF was produced using a Qmax value of 25 Å−1. Atomistic Modeling Methodology. DFT modeling was used to produce energy-minimized structural representations throughout the iterative DFT-PDF methodology As mentioned previously, the stoichiometry of the hydrated AMC sample was MgCO3·3D2O, which is the same as that of nesquehonite.25 Hence, the starting structure used for DFT modeling was a supercell generated from the crystal structure of nesquehonite (lattice parameters 7.701 Å, 5.365 Å, 12.126 Å; 90.0°, 90.41°, 90.0°).25 The starting structure contained 448 atoms and was generated by making the nesquehonite unit cell P1 followed by expanding to a 2 × 4 × 1 supercell. DFT modeling was performed using the plane-wave pseudopotential code VASP (version 4.6),34−37 and the GGA-PBE exchangecorrelation functional38,39 (using PAW potentials40,41) along with a 1 × 1 × 2 mesh for k-points (k-points at 0,0,0 and 0,0,1/2). Cell parameters were not refined during the DFT calculations except for one structure, as will be discussed shortly. For the energy minimizations ‘low’ precision was initially employed (Ecut = 300 eV), sometimes requiring a relatively large level of Gaussian smearing to aid convergence (0.4 eV width of smearing), after which the energy minimization was performed without smearing. Once the structure had converged using a ‘low’ precision setting, it was subsequently energyminimized using ‘accurate’ precision (Ecut = 400 eV). After the initial DFT calculation the structure was examined to ensure all D-atoms were located in water molecules. For several structures it was necessary to move up to 6% of D-atoms (up to ∼10 out of 192 D-atoms) to retain chemical feasibility of the water molecules. This was achieved by visually inspecting the atomic structure using O−D bond distances of 1.0 Å, and if a D-atom had departed from a nearby D2O molecule (leaving a deuteroxyl), it was manually moved back. Occasionally D3O+ molecules or bicarbonate groups formed, and if so the additional D-atom was shifted to a nearby deuteroxyl group. Once the structure was adjusted for feasibility, it was subjected to another round of DFT modeling, as outlined above. The structure used for the final DFT calculation in the iterative process was not manually adjusted and hence contained a small percentage of D-atoms associated with deuteroxyl ions and carbonate groups (∼6%). One of the structures in the iterative methodology was subjected to a DFT calculation (energy minimization) including allowing the cell parameters to change. This was carried out by performing a full DFT calculation on the structure without refinement of the cell parameters, using an ‘accurate’ precision, including manual alteration of the Datoms where necessary. A subsequent DFT calculation on the energyminimized structure was then performed using ‘accurate’ precision and allowing the cell parameters to refine. For the VASP DFT-molecular dynamics (MD) simulation (canonical ensemble, fixed NVT) the structure was subjected to 10,000 iterations (each iteration had a time step of 1 fs) at a temperature of 1500 K using a 1 × 1 × 1 mesh for k-points, after which the structure was quenched using the energy-minimization process (via the procedure mentioned previously). MD simulations using temperatures
ACC, reporting a liquid−liquid separation resulting in a dense liquid phase that coalesced and solidified into ACC.16 Although the structure, dynamics, and energetics of ACC formation and stability have been assessed rigorously in the community, mainly qualitative comparisons exist between experiments and classical and/or quantum chemical simulations. Furthermore, the structure and thermodynamics of AMC have not been explored. We recently exploited the complementarity of computational chemistry calculations and neutron scattering data in an iterative manner to explore the structure and thermodynamics of metakaolin (amorphous aluminosilicate), revealing the existence of III-coordinate aluminum (confirmed using X-ray absorption near edge spectroscopy)23,24 among other characteristics. Here we significantly advance this iterative theory-experiment methodology to enable elucidation of the atomic structure of a hydrated AMC, by employing density functional theory (DFT) combined with X-ray and neutron PDF analysis (denoted ‘DFT-PDF’). We specifically use ab initio calculations in place of traditional force field simulations for accuracy. Although such calculations are more computationally expensive, they are better suited for studying new systems for which accurate force fields are not established, such as hydrated AMC.
■
MATERIALS AND METHODS
Material Synthesis. Anhydrous magnesium chloride (>98% purity) and anhydrous sodium carbonate (>99.95% purity) were purchased from Sigma-Aldrich and used without further purification or drying. These reagents were weighed inside a glovebox with argon atmosphere. Then, 1.129 g of MgCl2 (11.9 mmol; 0.24 M solution) was placed in a 100-mL polyethylene bottle in the drybox and 50 mL of D2O (ACROS, > 99% isotopic enrichment) was added, after which the bottle was tightly capped. Similarly, 1.257 g of Na2CO3 (11.9 mmol) was placed in a separate polyethylene bottle inside the glovebox. The same volume of D2O (50 mL) was added, and the bottle was capped (0.24 M solution). The capped bottles were taken out of the glovebox and chilled to 4 °C in a refrigerator for 3−4 h or chilled on an ice bath until the solution temperature was under 5 °C. While the solutions were cooling, a jacketed Buechner funnel was set up in the glovebox and chilled to 0 °C. The chilled solutions were rapidly mixed together in a 250-mL polyethylene bottle. The bottle was capped and shaken for a few seconds, after which the slurry was immediately poured into the jacketed, cooled Buechner funnel and vacuum filtration was carried out as rapidly as possible. The precipitate was washed with four or five small portions (15−20 mL) of chilled D2O. The amorphous magnesium carbonate precipitate was transferred to a crystallization dish and dried at room temperature in a vacuum oven. The yield was essentially quantitative minus normal material transfer/container adherence losses. The equation for the synthesis reaction is given in eq 1:
MgCl2 + Na 2CO3 → MgCO3 + 2NaCl
(1)
Experimental Details. Thermogravimetric analysis was used to determine the water content of the hydrated amorphous magnesium carbonate (hydrated AMC) sample (Netzsch STA 449 C). The sample was heated from 30 to 300 °C at a rate of 20 °C/min in an argon atmosphere. The weight loss recorded (39.94%) revealed that the stoichiometry of the sample was MgCO3·3D2O (corresponding to crystalline nesquehonite).25 The hydrated AMC sample was loaded into a polyimide capillary and measured on the 11-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory, under ambient temperature conditions. The sample was analyzed using a wavelength of 0.2128 Å and a Perkin-Elmer amorphous silicon two-dimensional image plate detector.26 The wavelength was selected to provide a compromise between high flux (statistics), Q-resolution, and a sufficient maximum momentum transfer. The data conversion from 2D to 1D was carried 2694
dx.doi.org/10.1021/cm500470g | Chem. Mater. 2014, 26, 2693−2702
Chemistry of Materials
Article
below 1500 K resulted in partially crystalline structures, as determined by simulation of the X-ray PDFs (not shown here). For all X-ray PDF refinements and DFT calculations/simulations, the D-atoms were converted to H-atoms. Refinement against neutron PDF data was performed using D-atoms, to ensure that the O−D bond distances in the structural representation were consistent with the experimental data. Least-Squares Refinements. Least-squares refinements of the PDF data were performed using the DISCUS software42 and the reverse Monte Carlo (RMC) module,43 by setting the acceptance parameter (‘sigma’) to zero. Therefore, refinement moves (displacement of the atoms) were accepted only if the moves caused the agreement with data to increase. For each iteration, an atom is chosen at random and displaced using a Gaussian distribution with sigma of 0.005 lattice units (i.e., 0.005a, 0.005b, 0.005c, where a, b, c are the cell parameters). For the X-ray PDF data, the refinement was carried out for 78,400 iterations, by which time convergence was achieved. The iteration number includes trialed moves that were not accepted according to the least-squares criterion. At the beginning of the refinement approximately 50% of moves were accepted, but by 70,000 iterations the acceptance rate was only ∼2%. For the neutron PDF data, the refinement was limited to 22,400 iterations (to achieve convergence) and only D-atom positions were allowed to participate in the refinement. The refinement range for the simulated X-ray PDFs was 1.04 ≤ r ≤ 12.0 Å, which was chosen to limit the impact of the data reduction errors seen at low-r (12.0 Å). In the RMC community the refinement range is restricted to be no larger than half of the smallest box (i.e., 12.126 Å ÷ 2);43,44 however, since we are modeling an amorphous material with limited correlations at r > 6.063 Å, having a wider r-range for refinement will not lead to artifacts in the simulated models.24 In fact, the wider refinement range aids development of an amorphous structure model since it forces the atom−atom correlations at high-r to replicate the amorphicity seen experimentally. The refinement range for the simulated neutron PDFs was set at 0.65 ≤ r ≤ 12.0 Å to account for the O−D correlation at ∼1 Å. The simulated PDFs (X-ray and neutron) were produced using Gaussian-based atomic displacement parameters set at B = 0.6 Å2. Periodic boundary conditions were used during the refinements. The instrumental contributions to the X-ray PDF data were determined by refining the Q-dependent instrument resolution (Qdamp) and Qdependent instrumental peak broadening (Qbroad) in PDFgui for a nickel calibration sample (Qdamp = 0.061 Å−1, Qbroad = 0.019 Å−1). The neutron instrument parameters were obtained using a silicon calibration sample (Qdamp = 0.00623 Å−1, Qbroad = 0.00201 Å−1). Relaxed minimum distance constraints (to enable a wide range of bond lengths to be sampled) were used during refinement of structures against experimental data, as shown in Table 1. These distances were determined from either (i) the experimental PDFs for distinct correlations (e.g., O−D, and C−O) or (ii) the existing
inorganic structures available in the Inorganic Crystal Structure Database and literature, including the structure of nesquehonite.25 PDFgui45 was used to refine the density of the hydrated AMC supercell during one iteration step (as noted in Figure S.2 in Supporting Information) by allowing the box dimensions (supercell lattice parameters) to change. Prior to this refinement, the box angles were set to 90°, 90°, 90° for simplicity, since the structural representation by this stage was amorphous and therefore did not contain space group symmetry.
■
RESULTS AND DISCUSSION X-ray and Neutron Total Scattering Data. The experimental X-ray total scattering function for hydrated AMC (with stoichiometry MgCO3·3D2O) is displayed in Figure 1a. It is clearly visible that hydrated AMC is predominately amorphous, as seen by the weak relative intensity of Bragg peaks compared with the evident diffuse scattering. The weak Bragg peaks are due to a small amount of crystalline sodium chloride (
