Aragonite-II and CaCO3‑VII: New High-Pressure ... - ACS Publications

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Aragonite-II and CaCO3‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO3 Pavel N. Gavryushkin,*,†,‡ Naira S. Martirosyan,†,‡ Talgat M. Inerbaev,§,∥ Zakhar I. Popov,∥ Sergey V. Rashchenko,†,‡ Anna Yu. Likhacheva,† Sergey S. Lobanov,†,⊥ Alexander F. Goncharov,⊥ Vitali B. Prakapenka,# and Konstantin D. Litasov†,‡ †

Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russia ‡ Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia § L.N. Gumilyov Eurasian National University, Satpayev 2, Astana 010008, Kazakhstan ∥ National University of Science and Technology MISIS, 4 Leninskiy pr., Moscow 119049, Russian Federation ⊥ Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, United States # Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: The importance for the global carbon cycle, the P−T phase diagram of CaCO3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new highpressure polymorphs of CaCO3. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P21/c-l, predicted theoretically earlier. The P−T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30−50 GPa and 0− 1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure−temperature range. to CaCO3-II, then to CaCO3-III, CaCO3-IIIb, and finally to CaCO3-VI. The metastability of the CaCO3-VI is disputable. Until recently, it was considered as stable polymorph and even as a possible host structure for carbon in the Earth’s mantle.8 However, theoretical calculations have shown that CaCO3-VI is less favorable than aragonite in all pressure ranges.9,10 It was long believed that aragonite and post-aragonite are the only stable CaCO3 polymorphs at mantle P−T conditions.5,11 Previous in situ diamond anvil cell (DAC) experiments and ab initio calculations have shown the stability of post-aragonite in the pressure range of 35−135 GPa, with further transformation to the C2221 polymorph with tetrahedrally coordinated carbon atoms.4,5,11,12 However, recently performed calculations9 find two new structures, P21/c-l and Pnma, which are more favorable than aragonite and post-aragonite in the pressure range of 32−48 GPa. In addition, these calculations reveal a new polymorph with

1. INTRODUCTION The phase diagram of CaCO3 is one of the most studied over wide pressure and temperature ranges. Its investigation was mainly motivated by the geological interest. Being among the major carbon-bearing species in the Earth’s interior, calcium carbonate plays a fundamental role in the deep carbon cycle and affects global tectonic processes.1−3 Accumulated experimental and theoretical data create the basis for the understanding of the crystal chemistry of carbonates and other inorganic compounds with rigid triangular atomic units (nitrates, orthoborates, etc.). In addition to stable at ambient pressure calcite, two stable high-pressure polymorphs of CaCO3 were synthesized, aragonite and post-aragonite. Transition between calcite and aragonite occurs at ∼4 GPa, and between aragonite and post-aragonite, at ∼40 GPa.4,5 Both transitions are reconstructive and realized only with heating up to ∼2000 K. Also five metastable polymorphs were synthesized in numerous experiments on cold compression. On cold compression of aragonite, it is preserved up to ∼50 GPa, gradually transforming to the phase with trigonal symmetry.6,7 On cold compression of calcite, it almost immediately transforms © 2017 American Chemical Society

Received: July 14, 2017 Revised: October 6, 2017 Published: October 9, 2017 6291

DOI: 10.1021/acs.cgd.7b00977 Cryst. Growth Des. 2017, 17, 6291−6296

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of a conventional Rietveld procedure for data analysis (another option is a “multigrain” approach not discussed here). For this purpose, we used a Pawley pattern decomposition algorithm embedded in the GSAS-II package.20 The starting unit cell parameters were obtained from density functional theory (DFT) structures optimized at experimental pressures. 2.2. Ab Initio Calculations. Density Functional Theory. Density functional theory (DFT) calculations were performed with VASP code21−23 using the plane wave basis set and the projector augmented wave method.24 Exchange-correlation effects were taken into account in the generalized gradient approximation by the Perdew−Burke− Ernzerhof functional.25 Crystal Structure Prediction. Crystal structure prediction were performed with evolutionary algorithms implemented in USPEX package26−31 based on DFT. For the crystal structure search, we used a plane-wave basis set cutoff energy of 400 eV and performed the Brillouin zone (BZ) integrations using uniform Γ-centered k-point meshes with a k-point grid of 2π × 0.035 Å−1 spacing. The iterative relaxation of atomic positions was stopped when all forces acting on atoms were smaller than 0.01 eV Å−1. Crystal structure predictions were performed at 20, 40, and 60 GPa with two, three, four, five, and six formula units in the unit cell. Phonopy package32 was used for calculation of phonon dispersion curves, VESTA333for structure visualization, ToposPro34,35for topological analysis. Phase Diagram Calculation. The BZ was sampled using Γ-centered k-points mesh with density of 0.2 Å−1. The calculations have been performed for aragonite,36 post-aragonite,11 CaCO3-VI8, P21/c-l9, P21/ch9 and also for structures argonite-II and CaCO3-VII found during present investigation. A 2 × 2 × 2 supercell of post-aragonite with 80 atoms was used. Plane-wave cutoff energy was set to 1000 eV. The Helmholtz free energy of crystal was calculated within the framework of lattice dynamics in the quasi-harmonic approximation as F = U + Fvib, where U is DFT calculated potential energy and Fvib is the vibrational contribution:

tetrahedrally coordinated carbon atoms P21/c-h, which is more favorable than C2221. New, high-pressure experiments confirm the existence of the P21/c-h polymorph, showing its stability at pressures of 100 GPa.13 Analysis of available experimental data shows that a pressure range of 30−50 GPa is uncovered by high-temperature experiments, and the existence of P21/c-l phase remains hypothetical. In the present work, we comprehensively investigate this pressure range and show the existence of two new polymorphs.

2. METHODS 2.1. Diamond Anvil Cell Experiments. In situ X-ray diffraction (XRD) experiments were performed at 13ID-D of GSECARS at the Advanced Photon Source (APS), Argonne National Laboratory (Chicago, USA). High pressure was generated by a symmetric Maotype DAC equipped with 200 and 300 μm culets. Rhenium gaskets, preindented to a thickness of 30−35 μm, were drilled in the center of the indentation to create a sample chamber. Commercially available pure CaCO3 powder (99.9% purity, Alfa Aesar) was used as the starting material. The powder was first flattened between the two anvils to create a plate with a thickness of about 15 μm and then cut to fit the sample chamber. To provide efficient heat transfer to the sample, an iridium coupler (∼10 μm) with several holes of approximately 10 μm in diameter was placed in the center of the sample chamber between two initially prepared plates of CaCO3. The holes in the coupler, which formed a sample cavity, were filled with calcium carbonate upon compression. Experiments were conducted without an additional pressure-transmitting medium or thermal insulator as the coupler was insulated from the diamond anvils by CaCO3 layers. Two experimental runs were carried out at 35−50 GPa. The samples were first compressed gradually up to the target pressures at room temperature and then heated. Pressure was determined using the unit cell volumes of iridium and the known equation of state.14 High temperatures (up to 2600 K) were achieved using the double-sided Nd:YLF laser-heating system with a 25 μm diameter focused laser beam.15 Temperature was measured by spectroradiometry simultaneously with X-ray diffraction (XRD) measurements. Collected spectra were fit to Planck radiation using T-Rax software (developed by C. Prescher). The uncertainty in the temperature measurement is ∼150 K, which is typical for laser-heated DAC. Further details on the accuracy of temperature measurements at GSECARS are available in ref 15. The temperature gradient across the laser-heated coupler has been modeled by finite element calculations16 and are smaller than 100 K/μm across the coupler hole. Overall heating duration was about 10−20 min. After quenching, the pressure was increased, and all procedures were repeated. Every sample was heated in two areas. In the lower pressure set of experiments, the sample was heated between 1400−2200 K and 35−40 GPa in the first heating spot, and then in the second one at 45− 50 GPa and 1200−2500 K. Angle dispersive XRD spectra were collected in situ using a monochromatic beam (λ = 0.3344 Å) focused to a ∼3 × 4 μm spot on the sample. XRD data were acquired at each pressure increment before, during, and after heating to observe if any phase transformations occurred. Typical exposure time was ∼10 s. The sample to detector distance and the image plate orientation angles were calibrated using LaB6. Dioptas17 was used for integration of the two-dimensional (2D) images up to 20°. The obtained patterns were analyzed by the structureless (Pawley) refinement to find the best fit for the known CaCO3 structures. A characteristic feature of CaCO3 upon laser heating is fast grain growth, evident from 2D diffraction images, where broadened rings of coolcompressed sample transform into spotty patterns just after the start of the heating (Figure S1 in Supporting Information). Such images demonstrate diffraction spots from ca. 10−100 single crystals, which is definitely not enough to obtain one-dimensional (1D) powder pattern with reliable reflection intensities after integration. Herewith, a structureless Pawley18 or Le Bail19 refinement should be used instead

Fvib =

⎛ ⎛ ℏω ⎞⎞ 1 ∑ ℏωi + kBT ∑ ln⎜⎜1 − exp⎜− i ⎟⎟⎟ 2 i ⎝ kBT ⎠⎠ ⎝ i

where ωi is the ith frequency of crystal vibration, kB is the Boltzmann constant, and T is the temperature. All vibrational spectra were computed in Γ-point only using the finite-displacement method. For structural optimizations, a variable-cell-shape conjugate gradient method under constant unit cell volume condition was used. The optimization procedure converged when the maximal force acting on the atoms achieved the value less than 10−3 eV/Å. To obtain the force constants for the phonon calculations, an atomic displacement of 0.015 Å was employed. In the quasi-harmonic approximation, the free energy of crystal has the same form as in the harmonic approximation, but the structural parameters at fixed volume depend on the temperature. This dependence is determined self-consistently at calculation of the system’s free energy. To obtain the equation of state P(V) at constant temperature, the expression P = −(∂F/∂V)T was used. The thermodynamic stability was investigated using Gibbs free energies: G = F + PV.

3. RESULTS 3.1. Crystal Structure Prediction. For the test of the used methodology we perform calculations at ambient pressure with number of formula units equals to two, three, four, five, and six. Calcite was predicted as the most favorable structure, which is in agreement with experiment. In addition to aragonite, post-aragonite, and Pnma structure,9 calculations at 20, 40, and 60 GPa reveal the new structure, which can be considered as distorted aragonite (Figure 1) and was named aragonite-II: 6292

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high-degree of structural similarity between CaCO3-VI and P21/ c-l. The difference of these two structures is indistinguishable on visual inspection, and on Figure 3 we show only P21/c-l, in Supporting Information (Figure S3); both structures are shown for comparison. P21/c-l is monoclinic superstructure of triclinic CaCO3-VI. Their unit cell parameters, optimized by DFT at 30.4 GPa, relate as CaCO3-VI, a = 4.86 Å, b = 3.34 Å, c = 5.62 Å, α = 94.6°, β = 102.6°, γ = 89.5° P21/c-l, a = 4.84 Å, b = 3.36 Å, c = 11.14 Å, α = 90°, β = 103.15°, γ = 90° Also we found that, P21/c-l is the structural analogue of tetrahedrally coordinated P21/c-h. Both structures are characterized by the same cation array, and a slight shift of oxygen atoms transform one structure into the other (Figure 3c,d). 3.2. Experimental Results. Representative diffraction patterns collected at 35 and 50 GPa before, during, and after heating are shown in Figure 4. The X-ray patterns before heating

Figure 1. Columns of face-sharing Ca-octahedrons around CO3 groups in crystal structures of aragonite (a) and aragonite-II (b).

Name aragonite-II Pressure 40 GPa Space group P21/c Cell parameters a = 5.13 Å, b = 6.16 Å, c = 5.45 Å, α = 90°, β = 101.23°, γ = 90° Atomic coordinates Ca1 0.721 0.885 0.798 Ca2 0.278 0.385 0.702 C 0.77 0.193 0.414 O1 0.417 0.689 0.527 O2 0.74 0.509 0.896 O3 0.031 0.799 0.14 Aragonite-II is the most energetically favorable polymorph of CaCO3 at pressures of 32−46 GPa and 0 K (Figure 2). Phonon

Figure 4. Representative diffraction patterns collected at 35 and 50 GPa: (a, d) before; (b, e) during and (c, f) after heating. Iridium peaks are marked, and remaining peaks correspond to CaCO3 polymorphs.

contain distinct symmetrical peaks of iridium and only few carbonate reflections (Figure 4a,d). The carbonate phase identification in this case is impeded due to peak broadening, which is likely caused by deviatoric stresses accumulated during the compression at ambient temperature or partial amorphization. After the first minutes of heating at 1400−2200 K, the diffraction patterns change substantially (Figure 4b,e) and new peaks appear. The intensity of the new peaks increases during heating. The number or positions of peaks on the X-ray patterns did not change significantly during the heating or after quenching to room temperature (Figure 4, 1.1, b−f). At room temperature diffraction patterns were collected at different angles while rotating the DAC on up to 10°. For experiments at 35 and 50 GPa, three patterns collected after quenching at ω = −10°, 0°, and 10° were summed to have more peaks visible (Figure 5). Analysis of the obtained diffraction patterns presented below starts from the results of higher pressure experiments. The lower pressure region is more reach in new phases, and data obtained at 50 GPa were used for interpretation of the lower pressure diffraction patterns. 50 GPa. Diffraction pattern recorded at 50 GPa (Figure 5a) unambiguously shows the presence of post-aragonite in the mixture with additional phase or phases. All attempts of indexing based on known CaCO3 polymorphs (CaCO3-VI, aragonite-II, P21/c-l, P21/c-h) failed. However, we noticed that tetrahedrally coordinated P21/c-h reproduces quite well positions of experimental peaks. As the tetrahedral coordination of carbon

Figure 2. Enthalpies per f.u. of CaCO3 polymorphs, normalized on enthalpy of aragonite.

dispersion curves show its dynamical stability (Figure S2). Despite the same symmetry and close values of enthalpies of aragonite-II and P21/c-l (Figure 2), these two structures are sufficiently different (Figure 3). Calculated enthalpies of CaCO3-VI confirm8 metastability of this phase in the whole pressure range (Figure 2). We found

Figure 3. Comparison of crystal structures of aragonite (a), aragonite-II (b), P21/c-l (c), P21/c-h (d). 6293

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Figure 5. Results of Pawley refinement for quenched sample after heating at 50 GPa (a) and at 35 GPa (b). The blue crosses, green line, and cyan line represent observed pattern, calculated pattern, and their difference, respectively. Red, green, black, cyan, and blue strokes show peak positions of CaCO3VII, post-aragonite, iridium, aragonite-II, and aragonite (respectively).

Table 1. Experimentally Determined Unit Cell Parameters of CaCO3 Polymorphs P, GPa

phase

a, Å

b, Å

c, Å

α,°

β, °

γ, °

V, Å3

35

aragonite CaCO3-VII aragonite-II

5.070(2) 4.792(8) 5.122(7)

7.568(1) 3.354(2) 6.156(1)

4.749(0) 10.95(3) 5.567(8)

90 90 90

90 103.96(10) 100.02(4)

90 90 90

182.24(6) 170.77(10) 172.87(4)

50

post-aragonite CaCO3-VII

4.207(2) 4.807(2)

4.633(0) 3.354(0)

4.030(2) 10.405(6)

90 90

90 105.06(1)

90 90

78.577(19) 162.01(6)

Obtained experimental results show the presence of aragoniteII at 35 GPa and its absence at 50 GPa together with the presence of CaCO3-VII at both 35 and 50 GPa. This assumes that aragonite-II is a lower pressure polymorph relative to CaCO3VII. 3.3. Pressure−Temperature Phase Diagram. The calculated phase diagram of CaCO3 is presented in Figure 6. According to the diagram, phase transition from aragonite to post-aragonite occurs at 35−40 GPa and 0−1500 K, which is in good agreement with experimental value of 40 GPa at 1500− 2000 K.5 Observed in our experiments phase transitions between aragonite, aragonite-II, CaCO3-VII, and post-aragonite are also correctly reproduced on the phase diagram. The calculations of phonon dispersion curves (Figure S2), enthalpies (Figure 2) and Gibbs energies (Figure.S4) show dynamical and thermodynamical stability of the structural model suggested for CaCO3-VII. Gibbs energy of CaCO3-VII is slightly higher than that of aragonite-II. The difference in Gibbs energies between these two phases is negligible and fall within the error of the DFT, but increases with pressure. Hence, we can suggest that CaCO3-VII will form together with aragonite-II in the lowerpressure region of its stability field and in the higher-pressure region alone. This is in strict agreement with experimental data, showing the presence of CaCO3-VII together with aragonite-II at 35 GPa, and CaCO3-VII alone at 50 GPa. Additional presence of aragonite and post-aragonite in these experiments is due to the temperature gradient inside the DAC chamber. Stability fields of CaCO3-VII and aragonite-II lie close to but slightly lower than the coldest subduction geotherm (Figure 6). However, because of the increase of anharmonicity of atomic oscillations with temperature, accuracy of the data obtained based on quasi-harmonic approximation decreases with temperature and difference between theoretical and experimental values is sufficient in high-temperature region. Phase boundaries

is unlikely at 50 GPa, we suggest the new polymorph with unit cell parameters of P21/c-h and atomic arrangement of P21/c-l. Such a suggestion is reasonable as P21/c-h and P21/c-l are structurally related and differ only in the coordination of carbon (Figure 3c,d). The polymorph with unit cell parameters of P21/ch and atomic arrangement of P21/c-l was named CaCO3-VII. In the next section, we will show thermodynamic arguments confirming stability of CaCO3-VII at experimental conditions. Structural data of CaCO3-VII are shown below, the results of Pawley refinement, on Figure 5a. Name CaCO3-VII Pressure 50 GPa Space group P21/c Cell parameters a = 4.807(2), b = 3.354(0), c = 10.405(6), α = 90°, β = 105.06(1)°, γ = 90° Atomic coord. Ca1 0.302 0.668 0.601 C1 0.180 0.563 0.863 O1 −0.082 0.682 0.833 O2 0.385 0.789 0.846 O3 0.196 0.285 −0.049 35 GPa. As in the case of diffraction pattern at 50 GPa, all attempts of indexing based on single CaCO3 polymorph failed at 35 GPa. The combinations of any two polymorphs also did not give a positive result, and we had to adopt the simultaneous presence of three phases. Among combinations of three polymorphs the best fit gives mixture of CaCO3-VII, aragoniteII, and aragonite (Figure 5b, Table 1). Although the indexing based on two monoclinic and one orthorhombic phases is ambiguous, there are strong thermodynamic arguments (which will be shown in the next section) confirming the simultaneous formation of CaCO3-VII, aragonite-II, and aragonite at 35 GPa. 6294

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Among the intermediate structures, only aragonite has the simple topology with numerous examples on binary compounds. Topologies of other intermediate structures are quite specific, and we did not find their analogues among the main structural types of AB compounds. To answer the question whether topologies of aragonite-II and CaCO3-VII are unique or have some representatives among AB compounds, the automated scan of inorganic structural database with modern topological algorithms, like one performed by V. Blatov,43 is necessary.

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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.cgd.7b00977. Representative 2D diffraction image recorded after the start of the heating, Phonon dispersion curves of aragonite-II and CaCO3, comparison of crystal structures of P21/c-l, CaCO3-VI, CaCO3-VII, dependencies of Gibbs energies on pressure for CaCO3-VI, CaCO3-VII, and aragonite-II (PDF) Accession Codes

Figure 6. Calculated phase diagram of CaCO3 with experimental points. Reference point of post-aragonite,4,5 aragonite,37 mantle adiabat.38 Numbers show the PT-profiles of hot (1) medium (2) cold (3) and coldest (4) subduction slabs stagnant in the transition zone (solid lines) and penetrating into the lower mantle (gray dashed lines) according to.39−41 UM - upper mantle, LM - lower mantle.

CCDC 1580988−1580989 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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aragonite → CaCO3-VII and CaCO3-VII → post-aragonite intersect at a steep angle, and even small deviation drastically changes upper temperature limit of the CaCO3-VII stability. Phase boundary between post-aragonite and tetrahedrally coordinated P21/c-h is quite steep and lies sufficiently below relevant mantle geotherm (Figure 6).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Pavel N. Gavryushkin: 0000-0002-9419-2167 Sergey V. Rashchenko: 0000-0003-2936-0694

4. DISCUSSION It is well-known, that calcite’s cation array is of NaCl-type (B1) and aragonite’s of NiAs-type (B8).42 Recently performed topological analysis has shown that post-aragonite’s cation array is of CsCl-type (B2).43 Thus, transition from calcite to postaragonite corresponds to the most typical for AB compounds B1 → B2 transition.44 Uniqueness of the CaCO3 polymorphism is in the number of structures realized between B1 and B2 types. Considering found aragonite-II and CaCO3-VII, there are seven such intermediate structures. In Figure 7 we have summarized their stability fields. On transition from calcite to post-aragonite coordination numbers (CNs) of C by Ca gradually increase from 6 to 8. CN 6 6, 7 7 8 phase I, II, arag. III, IIIb VI, VII, arag.-II p.-arag.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Information Technology Centre of Novosibirsk State University for providing access to the cluster computational resources. The research was supported by Ministry of Education and Science of Russian Federation (Grant Nos. 14.B25.31.0032 and MK-3417.2017.5). T.M.I. gratefully acknowledges financial support of the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST MISIS (No. K3-2017-026) implemented by a governmental decree dated 16th of March 2013, N 211. Z.I.P. acknowledges the financial support of the RFBR, through the Research Project No. 17-42-190308 r_a. A.F.G and S.S.L. acknowledge the support of DCO. S.S.L. was partially supported by the state assignment Project 0330-2016-0006. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR 1634415) and Department of Energy- GeoSciences (DE-FG0294ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357.

Figure 7. Phase transitions of CaCO3. Stable phases are shown in color, metastable, in gray. Cat., arag., p.-arag−calcite, aragonite and postaragonite (respectively); HT, LT(cat.), LT(arag.) − high-temperature and ambient temperature data on compression of calcite8,10,45 and aragonite6 (respectively). 6295

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DOI: 10.1021/acs.cgd.7b00977 Cryst. Growth Des. 2017, 17, 6291−6296