Translation. Region: Russian Federal
Source: Novosibirsk State University –
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The discovery of a new class of chemical compounds, the study of general trends in the change of the atomic structures of carbonates was the result of a ten-year study conducted by Associate Professor Faculty of Geology and Geophysics, Novosibirsk State University Pavel Gavryushkin.
Back in November, the scientist successfully defended his doctoral dissertation "Crystal chemistry of carbonates under extreme pressures and temperatures based on the results of a comprehensive theoretical and experimental study." The defense took place at the Academic Council of Lomonosov Moscow State University and was the result of a comprehensive study that combined the efforts of both Russian and foreign theorists and experimenters. In June, the diploma conferring the academic degree of Doctor of Chemical Sciences to Pavel Gavryushkin was personally presented by the rector of Moscow State University, Academician Viktor Antonovich Sadovnichy.
Subduction is a geological process in which one oceanic plate moves (geologists say "dives") under a continental plate, thus sinking into the depths of the Earth's mantle. This process occurs in deep-sea trenches that run along the boundaries of continents.
It is difficult to establish exactly how deep this submersion occurs; available geophysical and geochemical data indicate that subducting oceanic plates can reach the lower mantle and even the mantle-core boundary. During this submersion, carbonates deposited on the surfaces of oceanic plates experience enormous pressures of up to 125 GPa and temperatures of up to 300 K. At such pressures, minerals undergo a variety of phase transformations, including melting and decomposition; the resulting carbon dioxide can return to the atmosphere during the eruption of volcanoes in island-arc complexes that form along the boundaries of subduction zones. Due to the extremely high pressures and temperatures, the study of these transitions requires special equipment or special modeling methods, which are usually performed on supercomputers.
— As part of my research, I set a goal — to determine how the atomic structures of carbonates will change under high pressures and temperatures. The structures they have in near-surface conditions are well known and have been studied for over 100 years, but what happens to it in the Earth's mantle, especially in its lower horizons, has only recently begun to be studied, and many questions remained. But to establish this, it is necessary to either compress and heat a carbonate sample to high pressures and temperatures, or simulate the process on a computer. The first option is expensive, labor-intensive and requires the use of synchrotrons, presses, diamond cells, etc. The second method also has its limitations, but it is still more flexible and less expensive. As part of my dissertation, I both performed calculations and conducted experiments. Several times I managed to implement an ideal scheme, when the calculation yields a very interesting result and it is precisely confirmed in the experiment. This does not always happen, but it cannot be said that it is rare. In general, the theory and calculations have now reached a high level of reliability, and if everything is done correctly, they can be trusted when planning an experiment. In particular, with the help of calculations, we were able to consider a wide range of carbonates, including Li, Na, K, Mg, Ca, Sr, Ba, Pb, which allowed us to generalize the general patterns of structural changes that occur at high pressures to them, – said Pavel Gavryushkin.
The research was conducted over a period of 10 years, some experiments, especially those involving synchrotron radiation, were conducted jointly with foreign colleagues from Japan, Slovenia, Sweden, the USA and Germany. Breakthrough results in the synthesis of orthocarbonates predicted by the scientist were obtained jointly with colleagues from Goethe University (Germany, Frankfurt am Main) and the GFZ center (Potsdam). As part of this study, Pavel Gavryushkin and his colleagues showed that in the high-pressure region, carbonates can react with oxides, yielding orthocarbonates. This was predicted by theoretical methods and subsequently confirmed in numerous experiments.
— It was possible to establish that in carbonates, under high pressure and high temperature, a rearrangement of atoms occurs, somewhat similar to that which occurs when graphite is transformed into diamond. In carbonate, as in graphite, carbon has a triangular coordination, and in orthocarbonate, as in diamond, it has a tetrahedral coordination. We assume that the reaction of formation of orthocarbonates, in particular magnesium orthocarbonate, can not only be carried out in idealized laboratory conditions, but can also take place in the deep shells of the Earth, fundamentally influencing the global carbon cycle.
The use of computational methods allowed us to move purposefully in setting up the experiment and concentrate on promising results. New phases stable at high pressures were discovered for CaCO3, SrCO3, BaCO3, PbCO3, Na2CO3, K2CO3 and FeCO3, the possibility of forming orthocarbonates as a result of the reaction of carbonates and oxides in the region of high pressures and temperatures was shown, and the existence of pyrocarbonate structures of CaC2O5 and BaC2O5 and orthooxalate for FeC2O5 was revealed. Pavel Gavryushkin's colleagues noted the novelty of this study for high-pressure crystallography and its significant contribution to modern crystal chemistry of inorganic compounds, which was enriched with examples of new types of structures containing orthooxalate [O3C–CO3] groups, pyrocarbonate [C2O5] groups, and tetrahedral [CO4] groups.
— In the theoretical part of the study, first-principles methods of structure prediction based on evolutionary approaches and random structure generation were used to determine the structure of high-pressure phases and construct their phase diagrams. Energy optimization in all cases was carried out within the framework of the density functional theory. The lattice dynamics method within the quasi-harmonic approximation was used to calculate the Gibbs free energies. The dynamic stability of the phases was estimated by calculating the phonon dispersion curves. In some cases, molecular dynamics modeling was also carried out. The VASP, USPEX, Phonopy, and ToposPro software packages were used to conduct the research. All this together allowed for a reliable prediction of new structures stable at high pressures, — the scientist said.
The main part of the experiments on the synthesis of the predicted structures was carried out at high pressures using the synchrotron radiation sources Spring8 (Japan), APS (USA), DESY (Germany) and the Siberian Center for Synchrotron and Terahertz Radiation (Russia). The experiments were carried out in diamond cells and in multi-punch apparatuses.
X-ray diffraction analysis of powder and single-crystal samples, Raman spectroscopy, and transmission electron microscopy were used to diagnose phases in the high-pressure region. Sample compositions were determined using microprobe analysis and scanning electron microscopy.
— The data we have obtained expands the existing knowledge about the global processes that occur in the depths of our planet. Now we know more about what happens to carbonates when they are immersed to depth. Man has so far managed to drill only to a depth of 12.2 km. This is the depth of the Kola Superdeep Borehole. In our calculations and experiments, we went to a depth of 3,000 km and made a forecast of what might happen to carbonates there. It is possible to study the structure of the Earth using seismic methods, shining sound waves through the thickness of the earth. These methods say very little about the properties of the substance and say nothing about its composition and structure. This information must be obtained from an experiment, from a calculation, or from natural samples. Each of these sources of information has both serious advantages and serious disadvantages, and only their combined use can truly expand our knowledge of the structure of the Earth. I hope that the theoretical and experimental data I have obtained will allow us to at least make a little progress on this path, — concluded Pavel Gavryushkin.
Material prepared by: Elena Panfilo, NSU press service
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