Single-crystal structure determination of (Mg,Fe)SiO₃postperovskite

Abstract

Knowledge of the structural properties of mantle phases is critical for understanding the enigmatic seismic features observed in the Earth's lower mantle down to the core-mantle boundary. However, our knowledge of lower mantle phase equilibria at high pressure (P) and temperature (T) conditions has been based on limited information provided by powder X-ray diffraction technique and theoretical calculations. Here, we report the in situ single-crystal structure determination of (Mg,Fe)SiO 3 postperovskite (ppv) at high P and after temperature quenching in a diamond anvil cell. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, crystallographic orientations of over 100 crystallites were simultaneously determined at high P in a coarse-grained polycrystalline sample containing submicron ppv grains. Conventional single-crystal structural analysis and refinement methods were applied for a few selected ppv crystallites, which demonstrate the feasibility of the in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device. The similarity of structural models for single-crystal Fe-bearing ppv (∼10 mol% Fe) and Fe-free ppv from previous theoretical calculations suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase. T he D″ layer, which represents the lowermost several hundred kilometers of the Earth's mantle, plays a critical role in the dynamics and evolution of our planet's mantle and core. Compositional models suggest that the lower mantle consists mainly of (Mg,Fe)SiO 3 in the perovskite (pv) structure with about 10 mol% Fe (1). MgSiO 3 pv transforms to postperovskite (ppv, CaIrO 3 -type) with the space group Cmcm at the high pressure-temperature (P-T) conditions corresponding to the D″ layer (2-4), and the changes associated with the pv-ppv transition may explain several of the intriguing seismic observations of this region (5-7). The in situ determination of the crystal structure of (Mg,Fe)SiO 3 ppv in its stability field will provide key information for understanding the mechanism of the pv-ppv transition and the processes occurring within the D″ layer. Furthermore, compositional changes in (Mg, Fe)SiO 3 may result in subtle changes in the crystal structure of the ppv phase. Recent powder X-ray diffraction (XRD) experiments on ppv with different Fe content suggest that (Mg 0.6 Fe 0.4 )SiO 3 ppv adopts a structure with the Pmcm space group, indicating a structural change in ppv due to . Precise structure analysis for the ppv phase is required to understand subtle effects of pressure and/or composition on its crystal structure.Single-crystal XRD provides the most precise structural information that can be used to uniquely determine the space group and structural model. However, preparation of suitable singlecrystal ppv samples for conventional single-crystal XRD technique has not been possible. The ppv phase crystallizes into a polyc...

High-pressure studies with x-rays using diamond anvil cells

“…Apparently, the multigrain approach is an effective way to increase redundancy and completeness in data collection, thus significantly improving the HP singlecrystal data quality. HP multigrain XRD possesses great potential for the crystallographic studies of materials under HP, as demonstrated recently in studies of crystal structural determination of (Mg,Fe)SiO3 bridgmanite [194], post-bridgmanite [166], SiO2 [167], and -Ge [102].…”

“…HP multigrain crystallography has been recently developed for single-crystal structure determination [151], which is particularly useful for studying the structural properties of phases that are not recoverable at ambient condition. For instance, structure refinement has been made for (Mg,Fe)SiO3 post-bridgmanite at pressures above one megabar [166]; electron density distribution of -Ge has been determined at 12 GPa [102,370]. With the precise structural determination enabled by the multigrain approach, it is anticipated that many previously determined HP structures will be revisited with the technique.…”

“…Single-crystal XRD was first applied to HP using the EDXRD technique [79,186]. Recently, synchrotron monochromatic beams have been widely used in HP single-crystal XRD by rotating samples coupled with either a point 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 detector [187] or an area detector [101,166,188]. Many HP beamlines in the world have established routine procedures for conducting HP single-crystal XRD experiments.…”

“…Several HP XRD techniques have been developed accordingly, including single-crystal XRD, polycrystalline (powder) XRD, and amorphous XRD. Recently, the multigrain crystallography method which identifies multiple single crystals in a polycrystalline sample, has been proposed [165] and applied successfully to HP studies (e.g., [166,167]). …”

Earth science: Crystallography's journey to the deep Earth

“…A further challenge in post-perovskite research is that singlecrystal samples cannot yet be synthesized by heating at high pressures. So, a combination of polycrystalline and single-crystal methods is being tried: individual grains in a polycrystalline sample are isolated and then subjected to single-crystal X-ray techniques 6,7 .…”