Single-crystal X-ray diffraction at extreme conditions in mineral physics and material sciences

Single crystal X-ray diffraction (XRD) is a powerful non-destructive method which allows unambiguously identify crystalline phases, determine a crystal structure (unit cell parameters, a space group, atomic coordinates and atomic occupancies) and, if required, a phase composition. This thesis deals with applications of single-crystal XRD in high pressure and high temperature (HPHT) research using laser-heated diamond anvil cells (DACs). The thesis describes methodological aspects of our single-crystal XRD experiments which involve crystals selection, DACs preparation, maintaining experiments, data processing, and structure solutions and/or refinements. We demonstrate a great potential and novel opportunities provided by high-pressure crystallography in materials- and geo-sciences on the examples of studies of transition metal borides, a metal-doped boron phase, silicates, and oxides. Particularly, we solved and refined crystal structures of Co5B16, MnB4, Al-doped β-boron, knorringite, and Fe3+-bearing bridgmanite, investigated the high-pressure behaviour of FeB4, Fe2B7, FexB50, and FeOOH. We also undertook detailed structural studies of a number of high-pressure iron oxides, which allowed us to resolve some of decade-long controversies. This work has led to discovery of new phases including a mixed iron oxide Fe5O7 with an unusual stoichiometry. One of the focuses of my research was investigation of the crystal structures of Al-doped rhombohedral β-boron (AlB44.8(5) or AlB37.8(5)) and FexB50. For the first compound we determined positions of interstitial Al- and B-atoms and their occupancies. We found that the disordering model of Al-doped β-boron is similar to one observed in SiB30.17C0.35. In FexB50 the metal fills the tetrahedral positions with the occupancy varying from 50 to 65%. We also studied crystal structures of novel transition metal borides, namely MnB4, FeB4, Co5B16, and Fe2B7. Tetraborides are composed of edge-shared columns of MB12 polyhedra, inside which the metal atoms form one-dimensional chains. While in the orthorhombic FeB4 the metal-metal distances are uniform, in MnB4 Peierls distortion leads to a pairing of Mn atoms accompanied with a lowering of the crystal structure symmetry to monoclinic. If a metal to boron ratio is higher than 1:4, the borides contain not only 12-coordinated metal atoms MB12, but also 10- and 9- coordinated ones (CoB9 in Co5B16, FeB10 in Fe2B7). Due to boron deficiency “metals” packing becomes denser that is reflected in a sharing of common faces between the coordination polyhedra. We found that studied transition metal borides possess short B-B bonds which influence their mechanical properties. In the FeB4 and Fe2B7 we found certain crystallographic directions in which the borides are as stiff as a diamond. The analysis of high-pressure single-crystal XRD data suggests that such incompressibility originates from the stiffness of the oriented short B-B bonds. A unique atomic arrangement in the FeB4 brings it to a class of superhard materials with a nanoindentation hardness of 62(5) GPa. We found that the structure of FexB50 composed of B12 icosahedra has large cavities, so it can contract more effectively than boron polymorphs (α-, β- and γ-boron), also containing chemically bonded B12 icosahedra. Our data confirm previous experimental observations on compression of boron phases that intraicosahedral bonds are stiffer than intericosahedral ones. Detailed structural studies of garnet knorringite synthesized at 26 GPa and 1800 °C in multianvil apparatus reveal that it has Mg3(Cr1.58Mg0.21Si0.21)Si3O12 composition and thus contains 21 mol % of a majorite MgSiO3 end-member. The distribution of iron in Al-free, Fe3+-bearing Mg-perovskite (bridgmanite) was derived from single-crystal XRD combined with Mössbauer spectroscopy. We found that the compound has the composition (Mg0.946(17)Fe2+0.045(4)Fe3+0.011(1))Si0.997(16)O3. The important result from single-crystal XRD was that iron does not occupy Si-position (so called B-site). We applied methods of high-pressure single-crystal XRD to study the behaviour of FeOOH at pressures over 70 GPa. At ambient conditions the compound has a hydrogen bond located in channels created by irregular FeO3(OH)3 octahedra. Below 16 GPa the channels (and consequently hydrogen bonds) contract more effectively than individual Fe-O bonds in octahedra; above 16 GPa both kinds of bonds contract uniformly. At ~45 GPa a spin crossover in Fe3+ drastically decreases the unit cell volume (by ~ 11%) and provokes symmetrization of the hydrogen bonds that was deduced from the analysis of the interatomic distances in the Fe(O…H)3(OH)3 moiety. The hydrogen bond symmetrization linked with the high-spin to low-spin crossover in Fe3+ was detected for the first time from high-pressure single-crystal XRD. High-pressure and high-temperature (HPHT) single-crystal XRD was used to search for HPHT polymorphs of Fe2O3 and Fe3O4 in a megabar pressure range and to uncover the fate of the iron oxide in subducted banded iron formations (BIFs) in the Earth’s lower mantle. We confirmed that above 29 GPa Fe3O4 adopts the crystal structure of CaTi2O4 which is stable to at least 70(1) GPa and 2400(100) K. We have resolved the over 50-year old controversy regarding the structure of the Fe2O3 polymorph stable above ~50 GPa. Particularly, we demonstrate that the phase has a double perovskite-type structure and triclinic symmetry. Moreover we found that the compression above 67 GPa provokes the transition to another high-pressure phase with the orthorhombic unit cell (space group Aba2). This phase does not sustain heating and transforms to a HPHT Fe2O3 polymorph with a CaIrO3 (post-perovskite, η-Fe2O3) structure at ~67 GPa. Under decompression to 41 GPa and heating to 1800 K this phase transforms to a polymorph with a Rh2O3-II structure. Our most intriguing finding is the observation that at the conditions of the Earth’s lower mantle, the η-Fe2O3 releases oxygen and can even decompose to form a novel Fe5O7 compound. Fe2O3 and Fe3O4 constitute up to 85 wt.% of BIFs that can be transported into the lower mantle due to subduction with lithospheric plates. Thus, the Fe2O3 from subducted BIFs may be a source of an oxygen-rich fluid to the deep Earth’s interior with significant amount of oxygen (up to 8 times the amount of oxygen in the modern atmosphere), leading to significant heterogeneity in oxygen fugacity in different parts of the mantle.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Bykova, Elena

Contributors dc:contributor

• Dubrovinsky, Leonid

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2124/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2124