EJM European Journal of Mineralogy EJMEur. J. Mineral. 1617-4011 Copernicus Publications Göttingen, Germany 10.5194/ejm-32-575-2020Equation of state and high-pressure phase behaviour of SrCO3EoS and high-pressure phase behaviour of SrCO3 BiedermannNicole nicole.biedermann@xfel.eu BykovaElena MorgenrothWolfgang https://orcid.org/0000-0001-8921-0052 EfthimiopoulosIlias https://orcid.org/0000-0001-6542-8188 MuellerJan SpiekermannGeorg GlazyrinKonstantin PakhomovaAnna AppelKaren https://orcid.org/0000-0002-2902-2102 WilkeMax European XFEL, Schenefeld, Germany Institute for Geosciences, University of Potsdam, Potsdam-Golm, Germany Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., USA c/o European Synchrotron Radiation Facility ESRF, Grenoble, France GFZ German Research Center for Geosciences, Potsdam, Germany Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland Deutsches Elektronen Synchroton DESY, Hamburg, Germany Nicole Biedermann (nicole.biedermann@xfel.eu)3November2020 32 6 575586 6December2019 10October2020 16October2020 Copyright: © 2020 Nicole Biedermann et al. 2020 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://ejm.copernicus.org/articles/32/575/2020/ejm-32-575-2020.htmlThe full text article is available as a PDF file from https://ejm.copernicus.org/articles/32/575/2020/ejm-32-575-2020.pdf Abstract

The high-pressure phase transition of strontianite (SrCO3) was investigated at ambient temperature by means of powder and single-crystal X-ray diffraction. The samples were compressed in a diamond anvil cell to a maximum pressure of 49 GPa. Structure refinements confirm the existence of SrCO3 in the low pressure aragonite-type phase Pmcn (62) up to about 26 GPa. Above this pressure, SrCO3 transforms into a high-pressure phase with post-aragonite crystal structure Pmmn (59). Fitting the volume extracted from the compression data to the third-order Birch–Murnaghan equation of state for the low-pressure phase of SrCO3 yields K0=62.7(6) GPa and K0′=3.2(1), and for the high-pressure phase this yields K0=103(10) GPa and K0′=2.3(6). The unit cell parameters change non-uniformly, with the c axis being 4 times more compressible than the a and b axes. Our results unequivocally show the existence of a Pmmn structure in SrCO3 above 26 GPa and provide important structural parameters for this phase.

 Introduction

Carbonates play a key role in the chemistry and dynamics of our planet. They are directly connected to the CO2 budget of our atmosphere and have a great impact on the deep carbon cycle . Moreover, it is believed that more than 90 % of the planet's carbon content is stored in the Earth's deep interior . Indirect evidence for the presence of a deep carbon cycle is given by the existence of carbonatite melts causing metasomatism in the upper mantle , by CO2 in peridotitic and eclogitic systems with implications for deep melting of subducting slabs , by mantle minerals (e.g. clinopyroxene, olivine, garnet) hosting carbon-bearing inclusions , and the formation of diamonds. Several studies performed under the conditions of the Earth's mantle showed that carbon is incorporated in carbonates, which are stable phases at such conditions in equilibrium with other mineral phases . Findings of carbonate inclusions in diamonds from the upper mantle and transition zone further substantiate the existence of carbonates in the deep Earth .

The most abundant carbonates entering the subduction zone are Ca, Mg, and Fe carbonates. At the pressure–temperature (PT) conditions expected for subduction zones and the Earth's mantle, carbonates undergo pressure- and temperature-induced structural changes: CaCO3, for instance, undergoes several phase transitions, some of them becoming more relevant to Earth's mantle conditions ; other studies have shown the stability and phase transition of dolomite , which is thought to be the main carbonate phase in subducting slabs. Regarding CaCO3, numerous studies on the phase behaviour of aragonite-type CaCO3 at high pressure reported a phase transition from orthorhombic aragonite (space group Pmcn) into monoclinic CaCO3-VII at around 30 GPa and into post-aragonite structure Pmmn at around 40 GPa .

Meanwhile, strontianite (SrCO3-I), which is isostructural to aragonite at room pressure and room temperature conditions and very common in natural carbonates, is considered to have similar phase transitions but is at a lower pressure compared to aragonite due to the larger ionic radius of Sr2+ (1.31 Å) in comparison with Ca2+ (1.18 Å) . Recent findings on mineral inclusions in transition zone diamonds showed significant amounts of strontium , which motivated our investigations of SrCO3 as a possible stable phase in the deep Earth.

Whilst most of the physical properties of SrCO3 are well known at ambient conditions , they have rarely been measured by powder X-ray diffraction at high pressure , with some of them being based on density functional theory (DFT) calculations . However, up to now, no single-crystal X-ray diffraction data were measured on SrCO3, thus making it difficult to analyse structural changes at the conditions of the Earth's mantle. At ambient pressure, SrCO3 has an aragonite-type crystal structure with space group Pmcn (62). This crystal structure consists of planar trigonal [CO3]2- oxyanions parallel to (001). The cations (e.g. Sr2+) are surrounded by six CO32- ions in a trigonal prismatic arrangement parallel to the c axis (see Fig. a), whereas the carbonate groups are octahedrally surrounded by six cations.The higher coordination number of the cation in the aragonite-group minerals correlates with a larger ionic radius (e.g. Sr2+, Ba2+, Pb2+) compared to the calcite-group minerals, where the cation is only coordinated six-fold by the oxygen atoms.

A few studies on the pressure-induced phase transitions in SrCO3 have been performed . They partially disagree regarding both the stability field and the structure of the high-pressure phases. suggested a phase transition of SrCO3 to post-aragonite between 32 and 35 GPa by Raman spectroscopy and proposed a space group setting of P2122 for this phase. In comparison, observed a post-aragonite phase by powder X-ray diffraction of SrCO3 already at 14.5 GPa and at 40 GPa for CaCO3 . In a later study, could show that the post-aragonite modification in BaCO3 has to be the same post-aragonite structure as CaCO3 and SrCO3 and had to be described in space group setting Pmmn. More recently, reported a possible high-pressure-induced transition at room temperature of SrCO3 from Pmcn to P21212 between 22.2 and 26.9 GPa and of BaCO3 from Pmcn to Pmmn between 9.8 and 11.2 GPa. A similar pressure range for a transition to the post-aragonite phase in SrCO3 was proposed by using experimental and computational Raman spectroscopy. Later on, the boundary of this phase transition was extended to high-temperature conditions using mid-infrared absorbance and Raman spectroscopy in combination with DFT-based calculations .

The variety of experimental conditions and techniques in the cited studies leads to results that are difficult to compare and sometimes even contradictory. To clarify these controversies we investigated the structure of pure SrCO3 up to 49 GPa at ambient temperature by powder and single-crystal X-ray diffraction. This method allows us to precisely determine the crystal structure and to finally resolve the disagreement concerning the correct space group symmetry for the post-aragonite phase.

 Methods Synthesis of sample material

For powder X-ray diffraction experiments, we used commercial SrCO3 powder from Sigma Aldrich Chemical Company (99.995 % purity). The single crystals of pure SrCO3 strontianite were grown in a Walker-type multi-anvil apparatus at 4 GPa and 1273 K for 24 h using the same SrCO3 powder as a starting material. The same synthesis has been used in previous studies . The chemical composition of the synthesized sample was determined using a JEOL Hyperprobe JXA-8500F with a field emission cathode at the GFZ Potsdam. Analysis was conducted with an acceleration voltage of 15 kV, a 10 nA beam current, and a <10 µm focused beam size. As reference-standard materials we used dolomite for CaO and strontianite for SrO. The chemical analysis indicated a concentration for Ca2+ below the detection limit of 130 ppm (see the Supplement for electron microprobe analysis). In addition, the morphology and chemical composition of the single crystals were studied with scanning electron microscopy. Most of the synthesized single crystals of SrCO3 were twinned, which is very common for aragonite-type carbonates, as they exhibit pseudo-hexagonal morphologies with a mirror plane (110) as a twin plane .

 High-pressure X-ray diffraction experiments Powder X-ray diffraction experiments

For high-pressure X-ray diffraction studies, SrCO3 powder was pressurized to 49 GPa in a membrane-driven Mao–Bell-type diamond anvil cell equipped with 400 µm culets. Drilled pre-indented rhenium gaskets with a hole diameter of 200 µm served as sample chambers. The pressure-transmitting medium (PTM) neon and a ruby sphere were loaded in addition to the sample. The pressure in the cell was measured with an online HR-2000 spectrometer (Ocean Optics) using the R1-line fluorescence band shift of ruby described by directly before and after each measurement, where the average is used as the experimental pressure. In addition, the equation of state of neon was used for additional pressure calibration . The powder and single-crystal XRD experiments were carried out at beamline P02.2 at PETRA III, DESY (Hamburg) . The X-ray wavelength was λ=0.2906 Å, and the beam size was 2×2 µm2 at full width at half maximum (FWHM). The calibration of the wavelength and the sample-to-detector distance was performed using standard CeO2 powder. For data collection, we used a fast flat panel detector XRD1621 from Perkin Elmer (2048 pixels × 2048 pixels with 200×200 µm2 pixel size). The two-dimensional X-ray images were integrated using the Fit2D programme . Refinements of the powder X-ray diffraction data were performed using the GSAS and EXPGUI software packages .

 Single-crystal X-ray diffraction experiments

In the case of high-pressure single crystal X-ray diffraction experiments, a small crystal of SrCO3 (25–30 µm in diameter and 10 µm thickness), a piece of tungsten (∼10 µm in diameter) and a ruby sphere (∼10 µm in diameter) were placed into a 150 µm wide cylindrical chamber drilled in a pre-indented rhenium gasket. We used the same type of membrane-driven diamond anvil cells as in the case of powder samples but using diamonds with a smaller culet size of 300 µm. Again, neon was loaded into the cell as a pressure-transmitting medium. The single crystal was pressurized up to pressures of 26.3 GPa. A picture of the single crystal loaded into the cell and compressed to 25.9 GPa is shown in Fig. .

Diamond anvil cell at 25.9 GPa loaded with a single crystal of SrCO3, with neon as a pressure-transmitting medium, a ruby sphere, and a piece of tungsten. The beam size was 2 µm × 2 µm FWHM.

The intensities of the reflections for the SrCO3 single crystal were integrated in steps of 0.5∘ over the entire opening angle of the cell of 54∘. At each pressure point, additional X-ray diffraction wide images (from -27 to +27∘ in the rotation axis) were collected to confirm the data quality. The instrument model of the experimental geometry (sample-to-detector distance, the detector's origin, offsets of the goniometer angles, and rotation of the X-ray beam and the detector around the instrument axis) was calibrated against an orthoenstatite reference crystal: (Mg1.93Fe0.06)(Si1.93Al0.06)O6, Pbca; a=18.2391(3) Å; b=8.8117(2) Å; c=5.18320(10) Å). The processing of XRD data (the unit cell determination and integration of the reflection intensities) was performed using CrysAlis PRO software . Indexing of the unit cell was carried out on about 30 reflections manually selected in the reciprocal space viewer (Ewald explorer implemented in CrysAlis PRO software). The reflections were selected in order to determine a 3D lattice in the reciprocal space. The identified unit cell parameters were then refined on the whole set of reflections at the end of the integration.

(a) Powder XRD patterns of SrCO3 at various pressures (T=300 K; λ=0.2906 Å). The different phases are indicated by black (SrCO3-I) and red (SrCO3-II) colours. (b) Refined XRD patterns for the aragonite-type SrCO3-I phase (12.6 GPa, Rietveld, bottom) and for SrCO3-II (43 GPa, Le Bail, top). Dots correspond to the measured spectra and the solid red lines represent the best refinements. The difference curves between the measured and the refined patterns are depicted as well (blue curves). Vertical ticks mark the Bragg peak positions.

Observed d spacings of the post-aragonite phase SrCO3-II at 26.3 GPa and room temperature in comparison with those from previous work .

This study hkl dobs [Å] dobs [Å] (26.3 GPa) (14.5 GPa) 001 4.2320 4.3106 110 3.3436 3.4197 011 3.2136 3.2750 101 3.0966 3.1644 111 2.6236 2.6803 020 2.4695 2.5210 200 2.2715 2.3260 021 2.1329 2.0477 121 1.9307 1.9720 102 1.9181 1.9545 211 1.5849 1.8964

Pressure dependence of lattice parameters of SrCO3 at room temperature derived from powder X-ray diffraction.

P [GPa] a [Å] b [Å] c [Å] V [Å3] SrCO3-I in Pmcn 0.0001 5.126(1) 8.472(2) 6.061(1) 263.21(15) 3.6(1) 5.057(1) 8.358(2) 5.840(1) 246.81(15) 12.6(2) 4.978(1) 8.182(2) 5.419(1) 220.73(15) 15.1(2) 4.957(1) 8.121(2) 5.337(1) 216.94(15) 20.3(2) 4.929(1) 8.017(2) 5.186(1) 209.85(15) 25.9(3) 4.892(1) 7.878(2) 5.037(1) 202.32(15) SrCO3-II in Pmmn 28.5(3) 4.521(1) 4.918(1) 4.219(1) 93.79(8) 33(1) 4.459(1) 4.879(1) 4.194(1) 91.25(8) 38(1) 4.393(1) 4.845(1) 4.171(1) 88.79(8) 43(1.5) 4.325(1) 4.811(1) 4.151(1) 86.39(8) 49(2) 4.244(1) 4.782(1) 4.127(1) 83.75(8)

Errors in parentheses are a single standard deviation.

A representative XRD pattern of post-aragonite SrCO3-II obtained from single-crystal data at 26.3 GPa derived from continuous rotation around one axis from -27 to +27∘ (wide scan image). Reflections corresponding to SrCO3-II that are absent, less intense, or overlaid are marked in grey.

The selected grain of SrCO3 was a non-merohedral twin with following transformation: -0.46-0.5401.46-0.460001. The transformation corresponds to two oriented grains rotated by about 118∘ around the common c axis. Since the degree of overlap was small (less than 3 % of all reflections), no twin integration was applied for the SrCO3-I phase data, and only the most intense grain (e.g. larger volume) was used for extraction of the reflection intensities. Overlapping 00l reflections were missing in the data set due to the orientation of the crystal in the diamond anvil cell (DAC) and its restricted opening angle. However, after the phase transition above 26 GPa we observed three twin domains related to a three-fold rotation along the a axis in SrCO3-II. Due to a high degree of overlap between reflections (about 10 %), we applied simultaneous twin integration; however, further structure solution and refinement was performed using the data collected from the most intense twin component. Empirical absorption correction was applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, which is included in the CrysAlis PRO software. The crystal structures of aragonite-type SrCO3-I and post-aragonite SrCO3-II were determined by the dual space method using SHELXT software. After the structure solution most of the atoms were found and the remaining were located from a series of difference Fourier maps. The crystal structures were refined against F2 on all data by full-matrix least squares with the SHELXL software. The amount of the collected data allowed us to refine the structures in anisotropic approximation. Nevertheless, there is a pronounced elongation of anisotropic displacement parameters in SrCO3-II along the [100] direction, due to three-fold twinning about the a axis. Details of crystal structure refinements of SrCO3-I and SrCO3-II are given in Table . The X-ray crystallographic coordinates have been deposited at the Inorganic Crystal Structure Database (ICSD) under deposition no. CSD1944794. This data can be obtained from CCDC's and FIZ Karlsruhe's free service for viewing and retrieving structures (http://www.ccdc.cam.ac.uk/structures/, last access: 28 October 2020).

 Results and discussion Phase transition of <inline-formula><mml:math id="M135" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">SrCO</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> during compression

Powder XRD patterns in the 2θ range 3–22∘ were collected upon compression up to 49(2) GPa and are presented in Fig. . No other peaks except those of the sample and the pressure transmitting medium neon (Ne) were observed. Peaks corresponding to the sample at ambient conditions were well indexed to the low-pressure aragonite structure Pmcn with the lattice parameters a=5.126(1) Å, b=8.472(2) Å, and c=6.061(1) Å and are in agreement with the literature . As shown in Fig. , reflections of SrCO3 shift to higher angles with increasing pressure and no structural transformation occurred until 25.9(3) GPa. A Rietveld refinement for the low-pressure aragonite-type SrCO3-I phase up to a pressure of 15.1(2) GPa was used to calculate the lattice and structural parameters, including interatomic distances. Above 15 GPa, the broadening of the peaks does not allow reliable refinement of the structure; hence the method of Le Bail fitting was applied in order to derive lattice parameters from powder-XRD measurements between 15 and 49 GPa.

Some reflections of SrCO3-I disappear, and new peaks appear at 28.5(3) GPa that could not be indexed using the metric of SrCO3-I described in space group Pmcn. These new peaks increase in intensity at higher pressures and indicate that SrCO3 has transformed into a post-aragonite phase. Our observations are in good agreement with powder X-ray diffraction studies from where a phase transition in SrCO3 was observed between 22.2 and 26.9 GPa. In contrast, proposed the presence of a post-aragonite phase in SrCO3 already at 14.5 GPa and at high temperature, which was also indexed with the metric of SrCO3-II described in space group Pmmn. For CaCO3, a phase transformation into post-aragonite was observed at 40 GPa and for BaCO3 at 10 GPa . The pressure for a phase transition in SrCO3 determined here lies in between these values and complies with the pressure–homologue rule according to which isostructural compounds often exhibit similar phase transitions but at lower pressures with increasing ionic radius .

Details of single-crystal structure refinements of SrCO3-I and SrCO3-II between 0.5 and 26.3 GPa.

P [GPa] 0.5(6) 5.2(4) 14.6(6) 20.6(4) 22.7(5) 25.9(4) 26.3(5) Crystal data SrCO3-I SrCO3-II Space group Pmcn (no. 62) Pmmn (no. 59) a [Å] 5.1034(4) 5.0429(5) 4.9559(2) 4.9059(5) 4.8861(4) 4.8618(7) 4.543(2) b [Å] 8.4056(8) 8.2858(9) 8.1986(5) 8.1932(8) 8.2025(8) 8.2078(12) 4.939(2) c [Å] 6.0180(12) 5.7657(15) 5.3755(7) 5.1596(15) 5.0771(15) 4.985(2) 4.232(2) V [Å3] 258.15(6) 240.92(7) 218.41(3) 207.39(7) 203.48(7) 198.93(9) 94.97(8) Z 4 4 4 4 4 4 2 〈Sr1⋯Sr1〉 [Å] 4.1326(4) 4.0077(6) 3.8399(4) 3.7761(7) 3.7626(6) 3.7432(10) 〈Sr1⋯Sr1〉 in (001) [Å] 4.9174(4) 4.8507(5) 4.7908(4) 4.7752(6) 4.7739(5) 4.7699(8) Refinement θ range for data collection [∘] 2.358 to 16.704 2.413 to 17.989 2.501 to 16.767 2.553 to 16.882 2.574 to 17.052 2.599 to 16.950 2.595 to 15.947 Completeness to d=0.8 Å 0.492 0.602 0.536 0.538 0.570 0.541 0.541 Index ranges -6<h<9 -9<h<9 -8<h<9 -9<h<7 -8<h<9 -5<h<5 -4<h<5 -10<k<15 -15<k<15 -15<k<12 -15<k<12 -15<k<14 -15<k<13 -7<k<8 -7<l<7 -7<l<7 -6<l<6 -5<l<5 -5<l<5 -5<l<5 -5<l<6 Goodness of fit on F2 1.064 1.015 0.943 0.989 1.037 1.033 1.369 Reflections collected 485 687 542 400 640 501 256 Independent reflections 355 421 325 289 319 299 142 No. of parameters 29 29 29 29 29 29 19 Final R indices [I>2σ(I)], R1/wR2 0.0288/0.0696 0.0232/0.0611 0.0240/0.0629 0.0322/0.0951 0.0383/0.1050 0.0358/0.0944 0.0708/0.1922 R indices (all data) R1/wR2 0.0316/0.0739 0.0266/0.0654 0.0272/0.0677 0.0379/0.1020 0.0496/0.1087 0.0509/0.1112 0.0759/0.1955 Rint 0.013 0.016 0.020 0.017 0.047 0.022 0.014

Single-crystal X-ray diffraction experiments were performed from 0 to 26.3 GPa. In agreement with our results from powder X-ray diffraction, reflections of the single crystal can be unambiguously indexed with the same aragonite-type orthorhombic cell Pmcn between 0 and about 21 GPa. Results taken at 26.3 GPa clearly indicate that SrCO3 has fully transformed into post-aragonite phase with space group Pmmn, which is the stable phase of SrCO3 at this higher pressure. The XRD pattern derived from a wide scan of the single crystal at this pressure is shown in Fig. . Additional peaks are indexed for rhenium, tungsten and the pressure transmitting medium neon. Fitted d spacings from this study at 26.3 GPa are given in Table  and are in good agreement with previous results from at 14.5 GPa.

Crystal structures of SrCO3-I (a) and SrCO3-II (b). Axis a, b, and c in SrCO3-I (Pmcn) correspond to b, c and a axis in SrCO3-II (Pmmn). Small-sized red spheres correspond to oxygen atoms, black spheres are carbon, CO32- are additionally highlighted as white and dark grey triangles, with different positions along the c axis in SrCO3-I and along the a axis in SrCO3-II. White and dark grey spheres are Sr2+. During the phase transition from aragonite to post-aragonite half of Sr2+ and CO32- ions highlighted by grey rectangles shift by 0.5[100] and then the ions line up in the (001) plane (the directions of displacements are shown by small blue arrows).

A comparison between the low-pressure phase and the post-aragonite phase is shown in Fig. . One can see that the a, b, and c axes in SrCO3-I (Pmcn) correspond to the b, c, and a axes in SrCO3-II (Pmmn). The phase transition from SrCO3-I (Pmcn) to SrCO3-II (Pmmn) is a first-order phase transition and is described by a shift of half of Sr2+ cations and CO32- anions along the [100] direction. At the same time, a shift by half of the translation of all Sr2+ cations and CO32- anions accompanied with a small displacement along [001] occurs such that the cations and anions line up in one plane parallel to (001). These shifts result in more dense packing of cations and anions and in the increase of the Sr2+ coordination number from 9 to 12.

Compressibility of the unit cell parameters βa, βb, and βc for SrCO3-I and SrCO3-II. Open symbols are data derived from powder samples, and solid symbols are from single-crystal X-ray diffraction. Note that the a, b, and c axes in SrCO3-I (Pmcn) correspond to the b, c, and a axes in SrCO3-II (Pmmn). The lattice parameters at ambient conditions are a0=5.126(1) Å, b0=8.472(2) Å, and c0=6.061(1) Å.

Evolution with pressure of the 〈Sr⋯Sr〉 distances in the (001) plane (distance between two grey Sr spheres) and out of the (001) plane (distance between grey and white Sr spheres) in SrCO3-I derived from single-crystal X-ray diffraction data. A schematic drawing of the crystal structure of SrCO3-I in the (100) plane is given as an inset.

 Compressibility of <inline-formula><mml:math id="M260" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">SrCO</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>-I and <inline-formula><mml:math id="M261" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">SrCO</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>-II

The refined unit cell parameters for the low-pressure phase strontianite (SrCO3-I) and for the high-pressure phase SrCO3-II obtained at different pressures are listed in Tables  and . The evolution of the lattice parameters with pressure is given in Fig. . Upon compression from 0.5 to 26 GPa, the axes of SrCO3-I change anisotropically with the highest compressibility found in the direction of the c axis, which is perpendicular to the carbonate groups. This anisotropic compression scheme is typical for aragonite-type carbonates and reflects the incompressibility of the CO3 groups in the (001) plane compared to the SrO9 polyhedra. The determined lattice parameters for SrCO3-I in this study exhibit quasi-linear pressure dependence with no obvious discontinuities. A linear fit of the given a/a0, b/b0, and c/c0 values against pressure yields linear compressibilities of βa, βb, and βc with 1.85(2)×10-3, 2.62(7)×10-3, and 6.4(5)×10-3 GPa-1, respectively, and agrees well with data from . Variations of the Sr⋅⋅⋅Sr distances in SrCO3-I upon compression further substantiate an anisotropic compression behaviour, indicating anomalous contraction along the c axis (Fig. ). Across phase transition at about 26 GPa, the length of the c axis (=^a in SrCO3-II) sharply decreases by ∼10 %, whereas the b axis (=^c in SrCO3-II) increases. The lattice parameters of SrCO3-II further decrease with pressure, albeit at lower rates, indicating that the structure of SrCO3-II is less compressible than SrCO3-I. The linear compressibilities of the high-pressure phase are found to be 2.223(7)×10-3 GPa-1 for the a axis, 1.29(8)×10-3 GPa-1 for the b axis, and 1.05(4)×10-3 GPa-1 for the c axis.

(a) Isothermal compression of the molar volume plotted together with the best fit following the Birch–Murnaghan equation of state for SrCO3-I (dashed black line) and SrCO3-II (dashed blue line). Open symbols are data from (with methanol and ethanol as PTM), and solid symbols are from this study. Note that for SrCO3-II the depicted data points show the doubled values of the unit cell volume. (b) Normalized pressure (FV) as a function of the Eulerian strain (f) for SrCO3-I and SrCO3-II, respectively. The solid red lines are the weighted linear fits to the data at P>3.6 GPa.

A fragment of SrCO3-I (a) and SrCO3-II (b) crystal structure at ambient pressure and at 26.3 GPa, respectively, showing Sr2+ ions (light grey sphere) surrounded by six CO32- anions in the aragonite structure, whereas Sr2+ ions in the post-aragonite structure are surrounded by eight triangular CO32- anions (C: dark grey spheres; O: red spheres).

The pressure dependence of the molar volume for SrCO3-I and SrCO3-II is shown in Fig. a. A plot of the Eulerian strain against the normalized pressure (f-F plot) (Fig. b) indicates that a third-order Birch–Murnaghan equation of state is necessary to fit the pressure–volume (P-V) data of SrCO3, which is expressed as follows : P(GPa)=3K0fE(1+2fE)521+32(K′-4)fE, with fE as the Eulerian strain given by fE=12(V0/V)23-1, and where V0, K0, and K′ are the (molar) volume, the isothermal bulk modulus, and its pressure derivative at room pressure, respectively. The programme EosFit7c was used to fit the data. For the low-pressure phase strontianite (SrCO3-I), the following EoS parameters were calculated: V0=259.8(3) Å3, K0=62.7(6) GPa, and K0′=3.2(1). Values for K0 are in good agreement with previous studies on strontianite . The fit result for the high-pressure phase SrCO3-II yields V0=115(1) Å3, K0=103(10) GPa and K0′=2.3(6), respectively. As indicated in Table , these values are between the values for BaCO3-II, with K0=84(4) GPa , and for post-aragonite CaCO3, with 127(14) GPa , thus confirming a dependency on the cation radius.

Comparison of the literature data for aragonite-type (Pmcn) and post-aragonite (Pmmn or P21212) carbonates at ambient conditions.

V0 [Å3] K0 [GPa] KT0′ References Aragonite-type carbonates CaCO3 227.5(8) 65(4) 4 (fixed) 227.2(10) 67.1(63) 4.7(8) 227.2(10) 73.1(22) 4 (fixed) SrCO3-I 258.4(3) 62(1) 4 (fixed) 258.4(1) 64(4) 259.8(3) 62.7(6) 3.2(1) This work BaCO3 303.8(1) 50.4(9) 1.9(4) 304.1(3) 44.3(8) 4 (fixed) 319.3 47.2 2.4 304.8(3) 48(1) 4 (fixed) post-aragonite carbonates CaCO3 Post-aragonite 98(2) 127(14) 4 (fixed) 102(3) 90(13) 4.94 109.74 42(7) 7(1) SrCO3-II 115(1) 103(10) 2.3(6) This work 111(2) 101(16) 4 (fixed) BaCO3 post-aragonite 129.0(7) 84(4) 4 (fixed) 128.1(5) 88(2) 4.8(3)

Atomic coordinates and anisotropic displacement parameters Uij [Å2] for post-aragonite SrCO3-II at 26.3 GPa (space group Pmmn).

Site Wyckoff x y z U11 U22 U33 U23 U13 U12 position Sr 2b 0.25 -0.25 0.6186(4) 0.036(2) 0.0096(8) 0.0152(10) 0 0 0 C 2a 0.25 0.25 0.991(4) 0.04(2) 0.010(5) 0.007(8) 0 0 0 O1 2a 0.25 0.25 0.682(3) 0.029(12) 0.005(4) 0.011(6) 0 0 0 O2 4e 0.25 0.025(2) 0.146(2) 0.026(11) 0.012(3) 0.009(4) 0.000(2) 0 0 Crystal structure of post-aragonite <inline-formula><mml:math id="M362" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">SrCO</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>-II

A detailed refinement of the high-pressure phase was carried out on single crystal X-ray diffraction data at 26.3 GPa. We were able to refine the structure in the anisotropic approximation. Fractional atomic coordinates and anisotropic displacement parameters are shown in Table . At this pressure, the post-aragonite phase of SrCO3 crystallizes in an orthorhombic unit cell with space group Pmmn (no. 59, origin choice 2) and with the following unit cell parameters: a=4.543(2) Å, b=4.939(2) Å, c=4.232(2) Å, and V=94.97(8) Å3. The atomic arrangement in post-aragonite SrCO3-II adopts the structure type of the orthorhombic high-pressure form of RbNO3-V . The Sr ion is located on a Wyckoff position 2b, C is on a 2a position, and O1 and O2 atoms occupy 2a and 4e positions, respectively (Table ). The same crystal structure was already proposed for high-pressure modifications of CaCO3 and for BaCO3 . However, there are also XRD data suggesting a trigonal symmetry for the post-aragonite phase in CaCO3 and in BaCO3 . When adapting the trigonal symmetry reported by some diffraction peaks of the high-pressure phase of SrCO3 could not be indexed. We conclude that for SrCO3 only an orthorhombic description of the crystal structure for the post-aragonite phase is possible.

The Pmcn to Pmmn transition of SrCO3 is characterized by an increase of the coordination of Sr2+ from 9 to 12. The relative change in density across this transition is about 5 %. In the post-aragonite phase the Sr to O interatomic distances vary from 2.418(10) to 2.718(6) Å. Six oxygen atoms with shorter distances (2.418(10), 2.4842(19), and 2.613(10) Å) are located on a plane intersecting Sr, parallel to the bc plane. Three oxygen atoms are located below the plane, and three atoms are located above it. C to O distances in the high-pressure phase of SrCO3 are 1.291(13) and 1.31(2) Å and identical within errors with data from for CaCO3.

 Conclusions

Powder X-ray diffraction in combination with single-crystal X-ray diffraction was used for the first time to determine the high-pressure phase behaviour of SrCO3 up to 49 GPa at ambient temperature. We observed a transformation from strontianite (Pmcn) to post-aragonite SrCO3-II (Pmmn) at around 26 GPa, which is in agreement with previous studies that used other techniques, e.g. Raman spectroscopy, to detect a phase transition in SrCO3 . We present reliable structural information for post-aragonite SrCO3-II, including interatomic distances and anisotropic displacement parameters, and finally resolve the discussion about the correct space group setting of the post-aragonite phase in SrCO3. Our results further confirm a bulk modulus of 62.7(6) GPa for strontianite (SrCO3-I) and provide the first experimental data for the equation of state for the high-pressure phase SrCO3-II with V0=115(1) Å3, K0=103(10) GPa and K0′=2.3(6).

 Data availability

The electron microprobe analysis of the sample material and the refinement of the single-crystal X-ray diffraction data on the post-aragonite phase of SrCO3 are included in the Supplement. Further data will be made available upon request for scientific research purposes.

The supplement related to this article is available online at: https://doi.org/10.5194/ejm-32-575-2020-supplement.

 Author contributions

KA and MW initiated the project. IE and JM processed and contributed powder X-ray diffraction data to this study. NB and EB processed single-crystal X-ray diffraction data and wrote the paper with input from WM and IE. EB refined single-crystal X-ray diffraction data. KA, GS, WM, KG, AP, and MW participated in X-ray diffraction data acquisition.

Competing interests

The authors declare that they have no conflict of interest.

Acknowledgements

This research was supported by funds from the German Science Foundation (DFG) through the CarboPaT research Unit FOR2125 (AP 262/1-1). We acknowledge the support of the Deutsche Forschungsgemeinschaft (German Research Foundation) and Open-Access Publication Fund of Potsdam University. We thank the Deutsches Elektronen-Synchrotron DESY for provision of beamtime (P02.2, ID11003684) and Hanns-Peter Liermann for additional technical assistance. Careful review from Fernando Cámara and one anonymous reviewer has substantially improved an earlier version of this paper. Nicole Biedermann thanks Monika  Koch-Mueller from Geo Research Centre (GFZ) in Potsdam for providing sample material and Thomas Preston from European XFEL for fruitful discussions.

Financial support

This research has been supported by the Deutsche Forschungsgemeinschaft (grant no. AP 262/1-1).

Review statement

This paper was edited by Carmen Sanchez-Valle and reviewed by Fernando Cámara and one anonymous referee.

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