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Carbonates in the system Na2CO3-CaCO3 are nowadays suggested as having a wide stability field at conditions of the mantle transition zone. Our structural analysis of nyerereite, which has limited stability fields at ambient conditions, and its similarities with already known carbonates that are stable at high-pressure conditions, allowed us to propose that nyerereite likely undergoes phase transitions at both high-pressure and high-temperature conditions. This supports the hypothesis that nyerereite takes part in carbon transportation from the mantle/deep crust toward the surface, with important implications for the deep carbon cycle associated with carbonatites. K-free nyerereite [Na2Ca(CO3)2] was synthesized both at hydrothermal conditions and from the melt. The structure of nyerereite was refined as a three-component twinned structure in the centrosymmetric Pbca space group with ratios of the three twinning components 0.221(3):0.287(3):0.492(3). Twinning at micro- and nano-level can introduce some minor structural deformations that influence the likely occurrence of the inversion center as one of the symmetry elements in the nyerereite structure. Based on the automated topological algorithms, we show that nyerereite has a unique crystal structure, not having analogs among the known structures, except for the structure with a similar composition K2Ca(CO3)2 fairchildite. A comparison between the centrosymmetric Pbca nyerereite structure and that of aragonite (CaCO3, Pmcn space group) reveals two main scenarios for the high-pressure form of Na2Ca(CO3)2: (1) poly-somatic relations as the interlayering of the high-pressure polymorph Na2CO3 and CaCO3-aragonite, and (2) high-pressure structure with ninefold-coordinated Na and Ca sites resembling that of aragonite. Our discussion heightens the interest in the high-pressure behavior of the nyerereite structure and strengthens the hypothesis about the possibility for nyerereite to be stable at high-pressure/high-temperature conditions.

Using molecular dynamics simulation and evolutionary metadynamic calculations, a series of structures were revealed that possessed enthalpies and Gibbs energies lower than those of aragonite but hi...

—Based on first-principle calculations in the framework of the density functional theory and structure prediction algorithms, we have determined iron carbide phases stable at the Earth’s core pressures and temperatures. It is shown that Fe7C3 is unstable and decomposes into the mixture Fe2C + Fe3C over the entire range of pressures and temperatures specific to the Earth’s inner core. Subsequent decomposition of Fe3C into the mixture Fe + Fe2C is unfavorable. We also predict a new low-temperature modification Fe3C-C2/m-II dynamically and thermodynamically stable over the pressure range 290–305 GPa.

The combinatorial-topological analysis and simulation of self-assembly of the Na96Hg36-hR132 (space group R-3c, a = b = 9.228 A, c = 52.6380 A, V = 3881.91 A3) and Na12Hg8-tP20 (space group P42/mnm, a = b = 8.520, c = 7.800 A, V = 566.2 A3) crystal structures are conducted by computer-based methods (ToposPro program package). Polyhedral cluster precursors K11 = 0@11(Na8Hg3) and K9 = Hg@Na8 are first determined for the intermetallic compound Na96Hg36-hR132, while polyhedral cluster precursors K5 = 0@Na3Hg2 are determined for the intermetallic compound Na12Hg8-tP20. The symmetry and topology code of the self-assembly processes of the 3D structure Na96Hg36-hR132 from the nanocluster precursors K11 and K9, and of the 3D structure Na12Hg8-tP20 from the K5 clusters are reconstructed as the primary chain $$S_{{\text{3}}}^{1}$$ → layer $$S_{{\text{3}}}^{2}$$ → frame $$S_{{\text{3}}}^{3}.$$ The structural analysis of all known intermetallic compounds is conducted, and numerous examples of the assembly of their structures from the K5, K9, and K11 clusters are found.

The stability of Fe4H, Fe2H, FeH, Fe3H5, FeH2, FeH3, FeH4, Fe3H13, FeH5, and FeH6 iron hydrides at temperatures of 0–5000 K and pressures of 100–400 GPa has been analyzed for the first time in the density functional theory using the lattice dynamics method in the quasiharmonic approximation, and the corresponding PT phase diagrams have been obtained. It has been found that heating expands a set of stable stoichiometric compounds, so that a number of structures metastable at room temperature are stabilized at temperatures above 1000 K. The topological analysis of structures of iron hydrides indicates that most of them belong to rare or unique topological types. An increase in the amount of hydrogen in a structure is accompanied by the reduction of the length of an H-H bond, which results in the formation of dumbbell-like hydrogen molecules H2 in FeHx structures with x > 6. However, these structures are thermodynamically unstable and decay into a mixture of FeH6 and solid H.

We present a novel approach to the generation of new crystalline phases, which is based on a combination of the topological description of crystal structures as a periodic net and the extended Zintl-Klemm concept, which establishes the structural relations between chemically and structurally simpler and more complex inorganic compounds. With this approach, we have explored the structural similarities between all known binary sulfides, selenides, and the corresponding simple sulfates and selenates and have theoretically revealed seven new high-pressure phases in the last two groups of compounds. Using density functional theory methods, we have studied the thermodynamic and mechanical stability of the new phases, have adjusted the transition pathways in the sulfate and selenate systems, and have revealed new structural correlations of the baric polymorphism in these systems. The advantages of the topological approach compared to conventional methods of modeling crystal structures are discussed and illustrated.