Doping Incompatible Elements into Calcite through AmorphousCalcium CarbonateSatoshi Matsunuma,† Hiroyuki Kagi,*,† Kazuki Komatsu,† Koji Maruyama,† and Toru Yoshino‡

†Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan‡Tokyo Metropolitan Industrial Technology Research Institute, 2-4-10 Aomi, Koto-ku, Tokyo 135-0064, Japan

ABSTRACT: Doping large amounts of a incompatible element into calcite,a polymorph of CaCO3, was achieved by pressurizing amorphous calciumcarbonate (ACC) containing a dopant. Strontium-doped ACC samples wereprepared from supersaturated solutions. ACC was transformed to calcite at0.8 GPa and 25 °C. The lattice volume of calcite obtained by pressurizing ACCincreased monotonically up to 372.5 Å3 (1.3% increase from a Sr-free sample)with increasing Sr content in ACC up to 0.15 in Sr/(Sr + Ca). For comparison,the lattice volumes of calcite samples precipitated from supersaturatedCaCO3 solution containing Sr were obtained, but the lattice volume of calciteprecipitated from supersaturated solutions showed no noteworthy increase.Results of this study indicate that pressure-induced crystallization is an efficientpathway to dope incompatible elements into crystals.

Calcium carbonate, an important resource for industrialmaterial, is a major natural mineral in surface environ-

ments and in the global carbon cycle.1 Recently, its applicationto nanoscale delivery system has been studied intensively.2,3

Properties of calcium carbonate with various polymorphs andmorphologies are the target of fundamental science. Calciumcarbonate has three polymorphs: calcite, aragonite, and vaterite.Calcite has a trigonal structure. Its Ca2+ coordination number issix. Aragonite has an orthorhombic structure. Its coordinationnumber is nine. Because of differences in their crystal structures,aragonite is much denser than calcite. In the P−T phase diagramof CaCO3, calcite is the stable phase at ambient conditions.Aragonite is a high-pressure phase.4 In contrast, vaterite is ametastable phase in the whole P−T region.Divalent ions of alkaline earth elements and transition metals

with ionic radius smaller than that of Ca2+ form a carbonate withthe calcite structure. Carbonates with the calcite structure tendto capture impurity elements of which the ionic radius is smallerthan that of calcium ion (e.g., Mg2+, Fe2+, Cd2+, Mn2+, Zn2+,Cu2+, Ni2+).5 In contrast, divalent ions with an ionic radius largerthan that of Ca2+ form a carbonate with the aragonite structure.Carbonates with the aragonite structure tend to captureimpurity elements of which the ionic radius is larger thanCa2+ (e.g., Sr2+, Pb2+, Ba2+, Na2+, U2+).5 For example, Sr2+ istaken selectively into aragonite, but not into calcite. A quantumchemical calculation study revealed that these large ions areincompatible with calcite.6

In addition to the three polymorphs of calcium carbonate,amorphous calcium carbonate (ACC) has recently attractedattention from researchers in biology,7 material science,8−11 andother disciplines. Actually, ACC is known as a precursor materialof biominerals. Taking the form of ACC for living organismspresents many benefits.7 In fact, ACC exists in larval spicules of

Echinodermata as a precursor phase and in crayfish as gastroliths,which serve as a temporary CaCO3 storage for exoskeleton at themolt.12,13 Many organisms form skeletons of magnesium-richcalcite with magnesium contents greater than 10 mol %. Thehigh magnesium content in biomineral calcite is explainable bythe high-magnesium content in the precursor ACC.14 Inspiredby biomineralization, other ions might be taken into the calcitelattice through ACC, which is thermodynamically metastable andwhich transforms to crystalline phases under high-temperatureand high-humidity environments.15,16 Recently, Yoshino et al.(2012) reported pressure-induced phase transition from ACCto calcite and vaterite.17

Pressure-induced crystallization of ACC might result in thedevelopment of a new method for doping incompatible but func-tional elements efficiently to calcium carbonate. In this study,we investigated the possibility of Sr-doping into calcite.Experimental procedures. In this study, ACC was synthesizedbased on a previously reported method.15 Strontium-dopedACC samples were prepared by mixing 0.1 M Na2CO3 and0.1 M blended solutions of CaCl2 and SrCl2 with varying Ca/Srratios at 0 °C. Precipitates were filtered immediately usinga membrane filter (ϕ 0.45 μm), washed with acetone, and driedfor 1 day at 25 °C in a vacuum desiccator that had beenevacuated with a diaphragm pump. This procedure is equivalentto the method described by Yoshino et al. (2012),17 except forSr-doping. Synthesized ACC samples were confirmed to containno crystalline phase from powder X-ray diffraction (XRD) patterns.Water contents in the obtained ACC samples were estimatedfrom weight loss in a temperature range from 25 to 250 °C in

Received: June 30, 2014Revised: October 6, 2014Published: October 14, 2014

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thermogravimetric−differential thermal analysis (TG−DTA)measurements (TG-8120; Rigaku Corp.). Two representativesamples showed similar water contents: CaCO3·1.48H2O fornondoped ACC and Ca0.82Sr0.18CO3·1.40H2O for Sr-dopedACC precipitated from a solution with Sr/(Sr + Ca) = 0.1.Synthesized ACC samples were pressurized using a hydraulicpress in a tungsten carbide (WC) piston−cylinder of 4 mminner diameter, the same equipment as that used by Yoshinoet al.17 for 10 min at 25 °C. The applied pressure was 0.8 GPa.After decompression, the recovered samples were kept in avacuum desiccator for removal of water emitted after pressure-induced phase transition.For comparison, calcium carbonate samples were precipitated

from supersaturated CaCO3 solutions containing Sr compo-nents. Supersaturated solutions were obtained by mixing 0.1 MNa2CO3 and 0.1 M blended solutions of CaCl2 and SrCl2 withvarying Ca/Sr ratios. The precipitates were aged in solutions for3 days with stirring at 25 °C. Calcite was obtained under anequilibrated condition. Actually, ACC, a transient metastableprecipitates, is known to crystallize to calcite through vateritewithin 10 h in a supersaturated solution.18 Three days aresufficient to obtain the most stable phase, calcite, by aging.Obtained powders were treated in the same manner as the ACCsamples described above, filtered, washed with acetone, anddried for 1 day in an evacuated vacuum desiccator. Hereinafter,the precipitates are designated as aged precipitates.Powder X-ray diffraction (XRD) patterns of the calcium

carbonate samples were obtained on a silicon zero backgroundplate using an X-ray diffractometer (MiniFlexII, Rigaku Corp.).Potassium chloride powder as an internal standard for a latticeconstant was mixed with the samples. The measurement condi-tions for XRD were 0.02° step, scanned region from 10° to 70°in 2θ, a scan rate of 1° per minute, Cu Kα radiation operatedat 15 mA and 30 kV. Lattice constants of calcite wererefined using Rietveld analysis with GSAS software19 andEXPGUI.20

Scanning electron microscopy (SEM) images were obtainedwith an accelerating voltage of 5 kV after Au coating (JSM-6610LA; JEOL). The concentrations of Ca and Sr in ACCsamples were measured using an ICP inductively coupled plasmamass spectrometer (ELAN DRCII; PerkinElmer Inc.).Results and discussion. Figure 1 presents Sr concentrations instarting solutions versus of those of ACC determined fromICP-MS measurements. The relation shows that ACC cap-tures Sr2+ preferentially from the starting aqueous solutions.

Figure 1. Sr/(Sr + Ca) molar ratios of starting solutions vs those ofACC samples prepared from the solutions.

Figure 2. Powder X-ray diffraction patterns of calcium carbonatesamples with various Sr contents. Strontium molar concentrations instarting solutions are denoted as x, where x is Sr/(Sr + Ca) × 100: (a)pressurized ACC and (b) aged precipitation.

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Figure 2a displays powder XRD patterns of ACC samplesrecovered from pressurization treatment at 0.8 GPa. Afterpressurization, ACC crystallized to calcite. No other phasewas detected in the range of x = 0 to x = 9, where x is amolar percentage of Sr2+ in a starting solution described asSr/(Sr + Ca) × 100. Samples crystallized from solutions withhigher Sr concentrations (x = 10 and 15) contained a crys-talline phase of vaterite. The most intense reflection of calciteat 29.4° shifts slightly to a lower angle with increasing Sr con-centrations, which suggests an increase of lattice volume withincreasing Sr concentrations of starting solutions. Figure 2bportrays powder XRD patterns of aged precipitate samples ofcalcium carbonate. All samples include calcite. One sampleprecipitated from a high Sr concentration (x = 15) containedstrontianite (SrCO3 with aragonite structure).Lattice parameters of calcite samples recovered from the

high-pressure treatment on ACC and precipitated from thesupersaturated solutions were estimated from X-ray diffractionpatterns. The obtained lattice volume data are presented inFigure 3a. At least up to x = 15, the lattice volume of calciteobtained by pressurization of ACC increased monotonicallywith increasing Sr concentration in ACC. In contrast, thelattice volume of calcite obtained from aged precipitation fromsupersaturated solutions increased at least up to x = 8, sub-sequently reaching a plateau at x = 10. The increase in lattice

Figure 3. Plots of (a) lattice volume vs Sr concentration in the startingsolutions. (b) c/a ratio of calcite samples obtained from ACC vs Srconcentration in the starting solutions.

Figure 4. SEM images showing calcite transformed from ACC at 0.8 GPa for (a) x = 0 and (b) x = 10 and showing aged precipitation obtained froma supersaturated calcium carbonate solution for (c) x = 0 and (d) x = 10. SEM images of pressurized ACC were obtained on a cracked cross section.Aged precipitate samples were attached to a carbon tape. Scale bars of (a) and (b) are 50 μm and those of (c) and (d) are 10 μm.

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volume reflects the uptake of Sr ion into the calcite latticebecause the ionic radius of Sr2+, for which the coordinationnumber of six is 18% larger than that of Ca2+.21

The increase of the c-axis with increasing Sr content is moreprominent than the increase of a-axis with increasing Sr contentin the starting ACC samples (see Figure 3b). Incorporation oflarge cations into a calcite-structure compound increases thec/a axial ratio.22 Figure 3b shows the incorporation of Sr2+ intocalcite lattice.Figure 4 portrays representative SEM images of calcium

carbonate samples prepared by pressurizing ACC and agedprecipitation obtained from supersaturated calcium carbonatesolutions. As shown in Figure 4a,b, the texture of the pressurizedsamples can be grouped mainly into two regions: the edge ofthe sample attached to a WC piston and the internal part.Spherulitic grains, which aggregate inside of the samples, consistof crystal arrays radiating from a single nucleation site. Theirtexture is dendritic. Grain sizes differ between Sr-free samplesand Sr-doped samples (see Figure 4a,b); the grains of Sr-freespecimens are coarser than those of Sr-doped specimens. Thiscontrast suggests a difference in nucleation density. In theSr-free sample nucleation that occurred at the interface betweenACC and a WC piston, a prismatic layer grew normal to theboundary layer (see Figure 4a). In contrast, the interfacebetween ACC and the WC piston for the Sr-doped samplesconsists of spherulitic grains (see Figure 4b). These contrastiveresults suggest that Sr ion notably affects the crystallization ofcalcite from ACC.The grain size and morphology of pressurized ACC and aged

precipitates differ greatly from each other. The grain sizes ofcalcite crystallized from ACC at high pressure were much largerthan those of aged precipitations (cf., Figure 4, panel a vs c andFigure 4, panel b vs d). Results of this study suggest stronglythat the pressure-induced crystallization process from ACCdiffered from that from a supersaturated solution.During crystal growth of calcite from a supersaturated

aqueous solution, Sr2+ impurity is excluded from the calcitestructure because of the incompatibility and mobility of Sr2+ inthe solution. Calcite samples transformed from ACC at highpressure contained significantly higher concentrations of Srcompared with those of calcite precipitated from a super-saturated solution, which suggests that crystal growth of calcitekinetically predominates over the exclusion of Sr from calcite.High-pressure treatments on ACC induced calcite crystalliza-tion and caused the preferential incorporation of Sr2+ into thecalcite lattice. Figure 5 presents a schematic illustrationcontrasting the two crystallization processes.Amorphous materials can accommodate elements that are

structurally incompatible with the corresponding crystallinephases with high concentration. This study demonstratedthat Sr, which is incompatible with calcite, was structurallyincorporated into calcite through pressure-induced crystalliza-tion from Sr-doped ACC. Similar phenomena have beenreported for the uptake of salt species into a high-pressurephase of ice.23 In general, salt is extremely incompatible withice lattices. Reportedly, the significant concentrations of LiClwere captured structurally into ice VII (a high-pressure phase ofice) crystallized through an amorphous state. It is reasonablethat an amorphous state significantly affects the distribution ofincompatible elements into crystals. Results of this study areexpected to present new avenues for the development of newmaterials by doping incompatible ions or functional moleculesinto a crystal phase through an amorphous state.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: kagi@eqchem.s.u-tokyo.ac.jp. Phone: +81(0)-3-5841-7625. Fax: +81(0)-3-5841-4119.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Prof. Kazumasa Sugiyama for valuable discussion. Weare grateful to two anonymous reviewers whose commentsimproved the manuscript considerably. This study wasperformed under the Inter-university Cooperative ResearchProgram of the Institute for Materials Research, TohokuUniversity (Proposals No. 13K0056 and 14K0046). This studywas financially supported by a Grant-in-aid for ChallengingExploratory Research (25610162).

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Figure 5. Schematic processes of pressure-induced crystallization fromACC and precipitation from supersaturated solutions.

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