Carbonates and their high-pressure behavior have attracted significant interest due to their potential role as carbon-bearing phases in the deep Earth. Recent discoveries of novel compounds that contain tetrahedral CO44- units (e.g., Merlini et al. 2015, Cerantola et al. 2017) increase the relevance of such studies, as the new high-pressure phases may be stable at conditions prevalent in the deep part of Earth’s lower mantle. In addition, theoretical modelling predictions imply potential structural analogues of CO4-bearing carbonates and silicates, and thus tetracarbonates may be important to understanding the complex geochemistry of Earth’s mantle.
Tetrahedrally-coordinated carbonates are not well characterized, despite their potential significance, as structural studies have to be carried out at high pressure and are therefore challenging. A reliable structural characterization is, however, a prerequisite for determining phase stabilities and to understand, for example, why the p,T-phase diagram of MgCO3 is relatively simple compared to the dense phase diagram of CaCO3 (see summary in Bayarjargal et al., 2018).
It is generally accepted that magnesite transforms to MgCO3-II at 80 – 115 GPa (Isshiki et al., 2004, Boulard et al., 2011, 2015, Maeda et al., 2017). Models based on density functional theory (DFT) (Oganov et al. 2008) and interpretation of X-ray diffraction data and IR spectra imply that magnesite-II contains carbon in tetrahedral coordination (Boulard et al. 2011, 2015). While structure-prediction techniques are undoubtedly useful for preliminary surveys of phase stabilities, they provide a range of possible new phases, derived under constraints such as unit cell contents. Powder diffraction data obtained at pressures around 100 GPa generally do not yield accurate structure determinations and typically do not allow unambiguous assignment of the space group or site occupancies. In contrast, single-crystal X-ray diffraction is a powerful and unique tool that can provide accurate structure refinements (Boffa Ballaran et al. 2013). Well-established statistical parameters allow an assessment of the reliability of the structural model. Other tetracarbonate structures at extreme conditions have been previously reported using this method, such as the novel Fe4C3O12 (𝑅3𝑐), (Mg,Fe)4C4O13 (𝐶2/𝑐) (Cerantola et al. 2017) and Ca(Fe,Mg)2C3O9 (Merlini et al. 2017) phases. These results lead to two conclusions. Firstly, the stability fields of carbonates strongly depend on their composition. Secondly, CO44- units have the ability to form polymerized networks, and thus are potential analogues to silicates.
190 8.2. Structural commentary
At ambient conditions (Fe0.15Mg0.85)CO3 crystallizes in the calcite-type structure with space group 𝑅3̅𝑐. Iron and magnesium share the same crystallographic site and are coordinated by six oxygen atoms, while 𝐶𝑂32− units form planar equilateral triangles. After compression to 98(2) GPa at ambient temperature, X-ray diffraction data of (Fe0.15Mg0.85)CO3 can still be indexed in the 𝑅3̅𝑐 space group (Table 1). However, the unit cell volume is decreased by nearly 32 % compared to ambient conditions. This result challenges a recent suggestion based on DFT-based calculations that predicted a structural transformation of MgCO3 to a triclinic phase at 85-101 GPa and 300 K (Pickard and Needs, 2015). After annealing at 2500 K and 98 GPa, we observed a phase transition to a polymorph in which carbon is tetrahedrally coordinated
Figure 1. Crystal structure of (Mg2.6Fe0.4)C3O9 according to a) this study and b) Oganov et al. (2008). The three cation sites that host Mg/Fe atoms are shown in c. d) C3O96- rings are formed from three edge-shared CO4
tetrahedra. Atomic positions are shaded according to colors in c and oxygen atoms appear as small white spheres.
191 by oxygen. The newly formed phase with chemical formula (Fe0.4Mg2.6)C3O9 (as determined from structural refinements, see below) has monoclinic symmetry and the diffraction pattern indicates space group 𝐶2/𝑚 (Figure 1 , Table 1).
We identify this phase as the MgCO3-II structure that was previously predicted (Oganov et al.
2008; Boulard et al. 2015). In contrast to earlier studies, we provide an accurate structure solution and refinement. The atomic coordinates obtained from our structure solution are presented in Table 2. The structure consists of three-membered C3O96- rings formed by corner-sharing CO4 tetrahedra (Figure 1d) that alternate with (Fe,Mg) polyhedra perpendicular to the b-axis. We can distinguish three crystallographic cation positions (Figure 1c): 1) M1 site occupied by Fe and Mg in a 0.11 Fe/Mg ratio surrounded by eight oxygen atoms forming distorted square antiprisms (dark blue), 2) M3 with 0.66 Fe/Mg ratio and coordination number 10 (blue; can be described as half cuboctahedra merged through hexagonal based faces with hexagonal pyramids), and 3) M2 fully occupied by Mg in MgO6 octahedra (magenta). The maximum and minimum bond distances of each cation from its neighboring oxygen atoms are shown in Table 3. At 98 GPa the C-O bond varies from 1.28-1.41 Å and the C-O-C inter-tetrahedral angle is ~112o(Figure 1d, Table 3).
From all proposed structural models for magnesite-II over the last two decades, only one appears to be comparable to the structure solution model that we report here. On the basis of PXRD experiments and variable-cell simulations, Oganov et al. (2008) suggested several energetically favorable structural models for MgCO3-II, one of which with space group 𝐶2/𝑚. This model has many similarities to the one proposed here using the single-crystal X-ray diffraction (SCXRD) method (Figure 1a-b). Both models suggest similar lattice parameters and the same space group. Further comparisons revealed that both models explain equally well our SCXRD patterns, resulting in R1 ~ 8.4% (this study) or R1 = 8.7% (Oganov et al. 2008) for the same Fe distribution in M1 and M3 sites. In order to test if one model is more preferable than the other, we performed additional DFT-based model calculations using the plane wave/pseudopotential CASTEP package (Clark et al., 2005). Pseudopotentials were generated “on the fly"
using the parameters provided with the CASTEP distribution. These pseudopotentials have extensively been tested for accuracy and transferability (Lejaeghere et al., 2016). The pseudopotentials were employed in conjunction with plane waves up to a kinetic-energy cutoff of 1020 eV. The calculations were carried out with the PBE exchange-correlation function (Perdew et al., 1996). For simplicity we assumed that all three M1, M2 and M3 positions are fully occupied by Mg2+. The calculations revealed that the energies of our structural model and that of Oganov et al. (2008) are identical. Indeed, shifting the origin of the unit cell by (0 , ½ , ½ ) for all (x, y, z) atomic coordinates of our model (Figure 1a) reproduces the
192 Oganov et al. (2008) model (Figure 1b) and vice versa. The DFT calculations gave C-O distances in good agreement with experimental data. Each carbon atom is coordinated by two oxygen atoms that are each shared with another tetrahedrally coordinated carbon, and two that are not shared. The C-O distances for the latter are significantly shorter (1.29 Å < d(C-O) < 1.32 Å) than the former (1.33 Å < d(C-O) < 1.41 Å).
Figure 2. Unrolled X-ray diffraction images collected at room temperature (λ=0.411 Å). a) Sharp and intense reflections of (Mg2.6Fe0.4)C3O9 appear after laser heating of the starting material at 98 GPa and 2500 K. b) The crystal phase gradually deteriorates during decompression and c) nearly disappears at ~74 GPa. d) Consequent laser heating treatment results in the formation of the initial carbonate structure.
Green circles mark a few of the characteristic reflections of (Mg2.6Fe0.4)C3O9, the position of Ne reflections and in some cases Re reflections are marked with blue and orange arrows, respectively. The 2theta positions of three characteristic carbonate (𝑅3̅𝑐) reflections are indicated with white arrows. Diamond reflections are marked in red.
193 A Mulliken bond population analysis shows that for the long C-O bonds there is a significant bond population of ~0.5 e/Å3. This is less than the value for the short bonds, where the bond population is ~0.9 e/Å3, but this still is a predominantly covalent bond, and justifies the description as a tetrahedrally coordinated carbon atom. The formation of (C3O9)6- carbonate rings was previously observed in Ca(Fe,Mg)C3O9 (dolomite-IV) after laser heating of Ca(Fe,Mg)CO3 at 115 GPa (Merlini et al. 2017).
However, dolomite-IV is topologically different from the magnesite-II structure that we report here.
Unlike (Fe0.4Mg2.6)C3O9, Ca(Fe,Mg)C3O9 crystallizes in the orthorhombic system (space group Pnma), thus highlighting the significance of the metal cations that are involved in the carbonate.
Upon decompression at ambient temperature, (Fe0.4Mg2.6)C3O9 reflections become broad and weak, and almost disappear at about 74 GPa (Figure 2a-c). This may be an indication of either amorphization or sluggish back-transformation to a carbonate with trigonal symmetry. Anticipating that further heating would aid re-crystallization, we laser-heated the sample at 74 GPa and 2000(150) K for a few seconds. Powder X-ray diffraction patterns collected on the temperature-quenched sample indicated the formation of the calcite structure-type carbonate (Figure 2d).