arXiv subjects

E. Bykova

Publications and source records attributed to E. Bykova.

5 recordsLinked to original sources

arXiv2020

Revealing the complex nature of bonding in binary high-pressure compound FeO$_2$

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides resulting in numerous compounds forming homologous series $n$FeO$\cdot m$Fe$_2$O$_3$ and the appearance of FeO$_2$. Here, based on the results of \emph{in situ} single-crystal X-ray diffraction, Mössbauer spectroscopy, X-ray absorption spectroscopy, and DFT+dynamical mean-field theory calculations we demonstrate that iron in high pressure cubic FeO$_2$ and isostructural FeO$_2$H$_{0.5}$ is ferric (Fe$^{3+}$), and oxygen has a formal valence less than two. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

physics.geo-ph↗

arXiv2019

FeOOH instability at the lower mantle conditions

Goethite, α-FeOOH, is a major component among oxidized iron species, called rust, which formed as a product of metabolism of anoxygenic prokaryotes (1, 2) inhabiting the Earth from about 3.8 billion years (Gy) ago until the Great Oxidation Event (GOE) of about 2.5 Gy ago. The rust was buried on the ocean floor (1, 2) and had to submerge into the Earth mantle with subducting slabs due to the plate tectonics started about 2.8 Gy ago (3). The fate and the geological role of the rust at the lower mantle high-pressure and high-temperature(HPHT) conditions is unknown. We studied the behavior of goethite up to 82(2) GPa and 2300(100) K using in situ synchrotron single-crystal X-ray diffraction. At these conditions, corresponding to the coldest slabs at the depth of about 1000 km, α-FeOOH decomposes to various iron oxides (Fe2O3, Fe5O7, Fe7O10, Fe6.32O9) and an oxygen-rich fluid. Our results suggest that recycling of the rust in the Earth mantle could contribute to oxygen release to the atmosphere and explain the sporadic increase of the oxygen level before the GOE linked to the formation of Large Igneous Provinces(4).

physics.geo-ph↗

arXiv2018

Breakdown of magnetic order in the pressurized Kitaev iridate $β$-Li$_2$IrO$_3$

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate $β$-Li$_2$IrO$_3$ is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation ($μ$SR) measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Neel temperature of $β$-Li$_2$IrO$_3$ increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field $H_c$ reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange $Γ$ under pressure.

cond-mat.str-el↗

arXiv2017

Pressure-induced spin pairing transition of Fe$^{3+}$ in oxygen octahedra

High pressure can provoke spin transitions in transition metal-bearing compounds. These transitions are of high interest not only for fundamental physics and chemistry, but also may have important implications for geochemistry and geophysics of the Earth and planetary interiors. Here we have carried out a comparative study of the pressure-induced spin transition in compounds with trivalent iron, octahedrally coordinated by oxygen. High-pressure single-crystal Mössbauer spectroscopy data for FeBO$_3$, Fe$_2$O$_3$ and Fe$_3$(Fe$_{1.766(2)}$Si$_{0.234(2)}$)(SiO$_4$)$_3$ are presented together with detailed analysis of hyperfine parameter behavior. We argue that $ζ$-Fe$_2$O$_3$ is an intermediate phase in the reconstructive phase transition between $ι$-Fe$_2$O$_3$ and $θ$-Fe$_2$O$_3$ and question the proposed perovskite-type structure for $ζ$-Fe$_2$O$_3$.The structural data show that the spin transition is closely related to the volume of the iron octahedron. The transition starts when volumes reach 8.9-9.3 Å$^3$, which corresponds to pressures of 45-60 GPa, depending on the compound. Based on phenomenological arguments we conclude that the spin transition can proceed only as a first-order phase transition in magnetically-ordered compounds. An empirical rule for prediction of cooperative behavior at the spin transition is proposed. The instability of iron octahedra, together with strong interactions between them in the vicinity of the critical volume, may trigger a phase transition in the metastable phase. We find that the isomer shift of high spin iron ions depends linearly on the octahedron volume with approximately the same coefficient, independent of the particular compounds and/or oxidation state. For eight-fold coordinated Fe$^{2+}$ we observe a significantly weaker nonlinear volume dependence.

cond-mat.str-el↗

arXiv2015

Thermal expansion and compressibility of monogermanides of 3d-metals

Synchrotron diffraction as a function of temperature and pressure, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. Similar but less pronounced behavior has also been observed in Mn$_{1-x}$Co$_x$Ge and Mn$_{1-x}$Fe$_x$Ge solid solutions. Spin density fluctuations and Mn spin state instability are discussed as possible candidates for the observed effects.

cond-mat.mtrl-sci↗