arXiv subjects

Sergey S. Lobanov

Publications and source records attributed to Sergey S. Lobanov.

14 recordsLinked to original sources

arXiv2026

High-Pressure Refractive Indices of NaCl, KCl, CaO, SrO, and MgO Reveal the Dependence of Anion Polarizability on Coordination Number and Bond Length

Several alkali chlorides and alkaline-earth oxides adopt the B1 structure at ambient conditions and transform to B2 under compression, allowing to change coordination number and cation-anion distance independently across chemically distinct hosts. The change in anion polarizability under high pressure dominates the optical response of these prototypical ionic solids, yet its dependence on local structure and cation chemistry remains experimentally unconstrained at high pressure. Here we report refractive indices of NaCl, KCl, CaO, and SrO to ~100, ~100, ~120, and ~60 GPa, each in both the B1 and B2 structures, complemented by published MgO-B1 indices to 140 GPa and DFT calculations. Lorentz-Lorenz analysis yields pressure-independent strain polarizability parameters $Λ$ ranging from 0.34$\pm$0.05 for KCl-B1 to 1.57$\pm$0.02 for MgO-B2. Our results establish coordination number and cation-anion distance as separable predictors of anion polarizability in both B1 and B2 structures. Increasing anion coordination from six to eight at fixed cation-anion distance lowers the polarizability of both $Cl^{-}$ and $O^{2-}$ by ~0.34-0.40 $\mathring{A}^3$ and ~0.38-0.47 $\mathring{A}^3$ respectively. Our results suggest that the polarizability of $O^{2-}$ is transferable to within ~0.06 $\mathring{A}^3$ between MgO-B1 and CaO-B1 at fixed cation-anion distance, and that this transferability possibly extends to SrO-B1 and the B2 oxides. In contrast, the polarizability of $Cl^{-}$ is not transferable between NaCl-B2 and KCl-B2: the difference between them varies with cation-anion distance, from ~0.2 $\mathring{A}^3$ at 2.65 $\mathring{A}$ to 0 at ~2.5 $\mathring{A}$. The results show that the change of the strain polarizability parameters across the B1-B2 transition is governed by the decreasing and increasing sensitivities of $O^{2-}$ and $Cl^{-}$ polarizabilities to densification, respectively.

cond-mat.mtrl-sci↗

arXiv2020

Polymorphism of superionic ice

Water is abundant in natural environments but the form it resides in planetary interiors remains uncertain. We report combined synchrotron X-ray diffraction and optical spectroscopy measurements of H2O in the laser-heated diamond anvil cell up to 150 gigapascals (GPa) and 6500 kelvin (K) that reveal first-order transitions to ices with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattices above 900 (1300) K and 20 (29) GPa, respectively. We assigned these structures to theoretically predicted superionic phases based on the distinct density, increased optical conductivity, and greatly decreased enthalpies of fusion. Our measurements address current discrepancies between theoretical predictions and various static/dynamic experiments on the existence and location of melting curve and superionic phase(s) in the pressure-temperature phase diagram indicating a possible presence of the conducting fcc-superionic phase in water-rich giant planets, such as Neptune and Uranus.

cond-mat.mtrl-sci↗

arXiv2020

Low thermal conductivity of iron-silicon alloys at Earth core conditions with implications for the geodynamo

Earth core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth history. Here we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m-1 K-1, lower than pure Fe at similar pressure-temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.

cond-mat.mtrl-sci↗

arXiv2019

Blocked radiative heat transport in the hot pyrolitic lower mantle

The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.

physics.geo-ph↗

arXiv2019

Opaque Lowermost Mantle

Earth/s lowermost mantle displays complex geological structures that likely result from heterogeneous thermal and electromagnetic interaction with the core. Geophysical models of the core-mantle boundary (CMB) region rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase, which is linked to both their radiative and electrical conductivity, measured in dynamically- and statically-heated diamond anvil cells as well as computed from first-principles at CMB conditions. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements suggest a very low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle, at odds with previous studies. This implies that the radiative mechanism has not contributed significantly to cooling the Earth/s core throughout the geologic time and points to a present-day CMB heat flow of 9-11 TW. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.

physics.geo-ph↗

arXiv2018

Insulator-metal transition in liquid hydrogen and deuterium

The insulator-to-metal transition in dense fluid hydrogen is an essential phenomenon to understand gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, we observe an onset of metal-like reflectivity in the visible spectral range at P>150 GPa and T>3000 K. The reflectance increases rapidly with decreasing photon energy indicating free-electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectivity spectra also suggest much longer electronic collision time (>1 fs) than previously inferred, implying that metallic hydrogens at the conditions studied are not in the regime of saturated conductivity (Mott-Ioffe-Regel limit). Combined with previously reported data, our results suggest the existence of a semiconducting intermediate fluid hydrogen state en route to metallization.

cond-mat.mtrl-sci↗

arXiv2018

Metallization and molecular dissociation of dense fluid nitrogen

Diatomic nitrogen is an archetypal molecular system known for its exceptional stability and complex behavior at high pressures and temperatures, including rich solid polymorphism, formation of energetic states, and an insulator-to-metal transformation coupled to a change in chemical bonding. However, the thermobaric conditions of the fluid molecular-polymer phase boundary and associated metallization have not been experimentally established. Here, by applying dynamic laser heating of compressed nitrogen and using fast optical spectroscopy to study electronic properties, we observe a transformation from insulating (molecular) to conducting dense fluid nitrogen at temperatures that decrease with pressure, and establish that metallization, and presumably fluid polymerization, occurs above 125 GPa at 2500 K. Our observations create a better understanding of the interplay between molecular dissociation, melting, and metallization revealing features that are common in simple molecular systems.

cond-mat.mtrl-sci↗

arXiv2017

Raman spectroscopy and X-ray diffraction of sp3-CaCO3 at lower mantle pressures

The exceptional ability of carbon to form sp2 and sp3 bonding states leads to a great structural and chemical diversity of carbon-bearing phases at non-ambient conditions. Here we use laser-heated diamond anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that post-aragonite CaCO3 transforms to the previously predicted P21/c-CaCO3 with sp3-hybridized carbon at 105 GPa (~30 GPa higher than the theoretically predicted crossover pressure). The lowest enthalpy transition path to P21/c-CaCO3 includes reoccurring sp2- and sp3-CaCO3 intermediate phases and transition states, as reveled by our variable-cell nudged elastic band simulation. Raman spectra of P21/c-CaCO3 show an intense band at 1025 cm-1, which we assign to the symmetric C-O stretching vibration based on empirical and first principles calculations. This Raman band has a frequency that is ~20 % lower than the symmetric C-O stretching in sp2-CaCO3, due to the C-O bond length increase across the sp2-sp3 transition, and can be used as a fingerprint of tetrahedrally-coordinated carbon in other carbonates.

physics.geo-ph↗

arXiv2017

Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur

Synchrotron X-ray diffraction and Raman spectroscopy have been used to study chemical reactions of molecular hydrogen (H2) with sulfur (S) at high pressures. We find theoretically predicted Cccm and Im-3m H3S to be the reaction products at 50 and 140 GPa, respectively. Im-3m H3S is a stable crystalline phase above 140 GPa and it transforms to R3m H3S on pressure release below 140 GPa. The latter phase is (meta)stable down to at least 70 GPa where it transforms to Cccm H3S upon annealing (T<1300 K) to overcome the kinetic hindrance. Cccm H3S has an extended structure with symmetric hydrogen bonds at 50 GPa and upon decompression it experiences a transformation to a molecular mixed H2S-H2 structure below 40 GPa without any apparent change in the crystal symmetry.

cond-mat.supr-con↗

arXiv2016

Optical signatures of low spin Fe3+: a new probe for the spin state of bridgmanite and post-perovskite

Iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been vigorously established at P ~ 60 GPa and 300 K, experimental evidence for spin transitions in other rock-forming minerals, such as bridgmanite and post-perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post-perovskite are difficult to resolve with conventional spin-probing techniques. Optical spectroscopy, on the other hand, is sensitive to high/low spin ferrous/ferric iron at different sites; thus, it can be a powerful probe for spin transitions. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post-perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in NAL (new aluminous phase) is very sensitive to the iron spin state and represents a model behavior of bridgmanite and post-perovskite in the deep lower mantle across a spin transition. Specifically, an absorption band centered at ~ 19000 cm-1 is characteristic of the 2T2g to 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa. This new spectroscopic information constrains the crystal field splitting energy of low spin Fe3+ to ~ 22200 cm-1 which we also independently confirm by our first-principles calculations. Together with available information on the electronic structure of Fe3+O6-compounds, we constrain the spin-pairing energy of Fe3+ in an octahedral field to ~ 20000-23000 cm-1. This implies that octahedrally-coordinated Fe3+ in bridgmanite is low spin at P > ~ 40 GPa.

physics.geo-ph↗

arXiv2016

Radiative conductivity and abundance of post-perovskite in the lowermost mantle

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (< 1.2 W/m/K) is ~ 40% smaller than bridgmanite at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.

physics.geo-ph↗

arXiv2016

Reduced radiative conductivity of high and low spin FeO6 octahedra in the Earth's lower mantle

The ability of Earths mantle to conduct heat by radiation is determined by optical properties of mantle phases. Optical properties of mantle minerals at high pressure are accessible through diamond anvil cell experiments, but because of the extensive thermal radiation at T above 1000 K such studies are limited to lower temperatures. Particularly uncertain is the temperature-dependence of optical properties of lower mantle minerals across the spin transition as the spin state itself is a strong function of temperature. Here we use laser-heated DACs combined with a pulsed bright supercontinuum laser probe and a synchronized time-gated detector to examine optical properties of high and low spin ferrous iron at 45-73 GPa and to 1600 K in FeO6, one of the most abundant building blocks in the mantle. Siderite (FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron and exhibits a sharp optically apparent spin transition at 44 GPa, simplifying data interpretation. We find that the optical absorbance of low spin FeO6 is substantially increased at 1000-1200 K due to the partially lifted Laporte selection rule. The temperature-induced low to high spin transition, however, results in a dramatic drop in absorbance of the FeO6-unit. The absorption edge (Fe-O charge transfer) red-shifts (~ 1 cm-1/K) with increasing temperature and at T above 1600 K becomes the dominant absorption mechanism in the visible range, suppressing radiative conductivity. This implies that the radiative conductivity of analogous FeO6-bearing minerals such as ferropericlase, the second most abundant mineral in the Earths lower mantle, is substantially reduced approaching the core-mantle boundary conditions. Finally, our results emphasize that optical properties of mantle minerals probed at room temperature are insufficient to model radiative thermal conductivity of planetary interiors.

physics.geo-ph↗

arXiv2015

Pressure, stress, and strain distribution in the double-stage diamond anvil cell

Double stage diamond anvil cells (DAC) of two designs have been assembled and tested. We used a standard symmetric DAC as a primary stage and CVD microanvils machined by a focused ion beam - as a second. We evaluated pressure, stress, and strain distributions in Au and Fe-Au samples as well as in secondary anvils using synchrotron x-ray diffraction with a micro-focused beam. A maximum pressure of 240 GPa was reached independent of the first stage anvil culet size. We found that the stress field generated by the second stage anvils is typical of conventional DAC experiments. The maximum pressures reached are limited by strains developing in the secondary anvil and by cupping of the first stage diamond anvil in the presented experimental designs. Also, our experiments show that pressures of several megabars may be reached without sacrificing the first stage diamond anvils.

cond-mat.mtrl-sci↗

arXiv2015

Stable magnesium peroxide at high pressure

Rocky planets are thought to comprise compounds of Mg and O as these are among the most abundant elements, but knowledge of their stable phases may be incomplete. MgO is known to be remarkably stable to very high pressure and chemically inert under reduced condition of the Earth's lower mantle. However, in 'icy' gas giants as well as in exoplanets oxygen may be a more abundant constituent (Ref. 1,2). Here, using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we show that MgO and oxygen react at pressures above 94 GPa and T = 2150 K with the formation of the theoretically predicted I4/mcm MgO2 (Ref.3). Raman spectroscopy detects the presence of a peroxide ion (O22-) in the synthesized material as well as in the recovered specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO. Our finding suggests that MgO2 may substitute MgO in rocky mantles and rocky planetary cores under highly oxidizing conditions.

cond-mat.mtrl-sci↗