The elastic properties of para-terphenyl C18H14 in the monoclinic and triclinic phases, as well as its positional isomer ortho-terphenyl in the crystalline and glassy states, were studied with the use of ultrasonic techniques over a wide P-T region (temperatures 77-300 K, pressures up to 1 GPa). We observed experimentally a second-order phase transition between the monoclinic and triclinic structures of para-terphenyl both under isothermal compression at 0.34 GPa (77 K) and under isobaric heating (182 K, 0.05 GPa). For ortho-terphenyl, we compared the elastic properties of the crystalline and glassy states at low temperatures (77 K) up to 1 GPa and also observed a glass-supercooled liquid-crystal transition with increasing temperature in the range of 77-300 K. Comparison of the elastic properties of these modifications showed that glassy and crystalline ortho-terphenyl have a similar bulk modulus, but the shear modulus of the glass is almost two times lower than that of crystalline ortho-terphenyl.

We consider the specific features of collective excitations in disordered media, mainly in liquids. We discuss the difference between proper excitations in disordered media and plane harmonic waves, and the related problem of effective excitation damping. A detailed comparative analysis of the spectra of collective excitations in normal liquids and in superfluid helium is carried out. The aspects discussed include the phonon and roton parts of the spectrum, the phenomenon of `fast sound,' shear waves, and the `mixing in' of longitudinal and transverse excitations in liquids. Excitations in liquids are shown to determine the thermodynamic properties in both normal and superfluid states. Despite the qualitative similarity of the spectra of superfluid helium and ordinary liquids, they show significant differences, which have not yet received a proper theoretical description in our opinion. We emphasize the major densates of ensembles of ultracold atoms. A hypothesis is put forward regarding the possible major role that the high zeroboth in understanding the features of the excitation spectra and in the superfluidity phenomenon itself.

Nano- and micron powders of cubic boron nitride were synthesized from ammonia borane and hexagonal boron nitride under high pressure and high temperature conditions without catalysts. A bright and narrow photoluminescence line at 683 nm was detected when 1 % silicon was added to the starting material. Calculations of the electronic structure of various lattice defects showed that the substitution of a nitrogen atom by silicon gave the best agreement with the experiment.

For a number of mixtures of rare gases at high pressures, sound speed minima are experimentally observed depending on the concentration. This behavior has not yet been explained. We have studied the behavior of a mixture of argon and helium using computer simulation. Sound speed minima have been observed at a certain molar fraction of the components, which is in good agreement with experimental data. It is shown that this behavior is due to the fact that the P and T parameters for gas mixtures are near the Frenkel line, separating the states of rigid and quasi-gas fluid.

The magnetic properties of Mn1–xRhxSi solid solutions with a noncentrosymmetric structure B20 synthesized at a pressure of 8 GPa and temperatures of 1500–1770 K have been studied in detail in a wide temperature range of 2–300 K in magnetic fields up to 9 T. An anomalous increase in the Curie temperature TC of compounds with 0.15 ≤ x ≤ 0.8 by a factor of 8.4–11.5 compared to the pure helical magnet MnSi has been found. It has been established that the Curie temperature TC increases with the rhodium content to TC(x = 0.15) = (244 ± 4) K, TC(x = 0.4) = (299 ± 5) K, and TC(x = 0.8) = (334 ± 6) K. Uniquely high magnetic transition temperatures up to room values occur in the disordered ferromagnetic Griffiths phase and may be due to the spin-fluctuation mechanism of enhancement of the magnetic interaction.

The effect of doping with two carbide-forming nonmetals - Si or B - (7 at. % each) on the phase transformations of crystalline C-60 fullerene at a pressure of 8 GPa in the temperature range of 400-950 C was studied using the X- ray diffraction method. Samples were synthesized from powders in a high pressure toroidal chamber, and X-ray examination was carried out at room temperature after rapid cooling. It has been shown that doping does not change the sequence of phase transformations of C-60 fullerene as the temperature increases, the following phases are still observed: fcc phase - rhombohedral phase - disordered (amorphous) graphite. Boron reduces the for-mation temperature of disordered graphite, but silicon does not. This influence of carbide-forming non-metallic elements is fundamentally different from the influence of carbide-forming metals - Al and Fe, the addition of which increases the stability of fullerene molecules and increases the temperature of its transformation into disordered (amorphous) graphite by hundreds of degrees (P. A. Borisova, M. S. Blanter, V. V. Brazhkin, and V. P. Filonenko, Phase Transformations in C60 Fullerene with Iron and Aluminum at High Pressures and Temperatures, Bulletin of the Russian Academy of Sciences: Physics, 84 (2020) 851-856).

We perform a molecular dynamic study of collective excitations of carbon tetrachloride and compare the results with the experimental data from the literature. The data of simulations are in good agreement with the experimental ones. The results of the simulations confirm the presence of large positive sound dispersion (PSD) in carbon tetrachloride, which should be related to some relaxation processes which do not take place in atomic systems.

Experiments on the dynamic isentropic compression of solid CO2 samples by the megabar pressure induced by the superstrong magnetic field of an explosive magnetic generator have been performed with the X-ray diffraction detection of the state of the compressed samples. The generator operation is based on the fast compression of the initial magnetic flux in the cavity of the generator by a conducting cylindrical liner accelerated by the products of the explosion of a cylindrical explosive charge. Two points at pressures of 349 and 459 GPa on the compressibility diagram of CO2 have been determined in the experiments, where the degree of compression ρ/ρ0 of CO2 has reached currently highest values of 3.90 and 4.02, respectively. Comparison has shown that theoretically calculated equations of states for crystal phases of CO2 almost completely reproduce the experimental results, which confirms a high accuracy of theoretical predictions and the identity of the experimental and theoretical equations of states of CO2 modifications stable at high pressures.

A relation of the presence of metastable objects with various “lifetimes” in the Universe to both an increase in the entropy and “the arrow of time” is discussed. A finite lifetime of metastable objects makes it possible to assign many of them with their own “local clocks.” The decay of any metastable system in the Universe results in the appearance of more stable objects with the emission of photons and other particles. These photons and particles interact with particles of more stable subsystems in the Universe, leading to their ergodicity. The emission of photons from decaying metastable states in the expanding Universe makes these processes irreversible and specifies the arrow of time although the equations describing these physical processes are reversible.

Despite the 60-year history of research on band magnetism in MnSi, the field remains a vibrant area of study. This area is still of great interest although the physics of weak itinerant magnetism is complicated because of small magnetic moments and an uncertain role of local interactions. This work presents Rh-doped MnSi compounds in which a high-spin (HS) state of Mn magnetic moments has been detected in 55Mn nuclear magnetic resonance (NMR) measurements. The doping of MnSi with Rh results in a transition to the HS state for Mn1-xRhxSi at x = xc approximate to 0.025 with two Mn magnetic moments approximate to 1.3 and 2.2 mu B, which are ordered just below 200 K. This transition occurs only in part of the Mn atoms, while the other Mn atoms remain in a low-spin (LS) state. Concurrently, the Dzyaloshinsky-Moriya (DM) interaction for LS helical states of Mn moments is preserved up to x approximate to 0.13. Furthermore, variations in the Rh concentration result in discernible alterations in the magnetic field-temperature phase diagrams. In this case, it was observed that the temperature range of existence of the A phase, host skyrmion lattice, was markedly increased in presence of Rh doping, up to x = 0.025 at least. Small-angle neutron scattering has evidenced the existence of a skyrmion lattice in the helicoidal magnetic phase of Mn0.98Rh0.02Si. The Rh-doped MnSi compound thus demonstrates the coexistence of the HS and LS states of Mn. Our DFT calculations has indicated that this behavior can only be the case when Rh occupies not only Mn but also Si positions in the MnSi compound. Furthermore, our findings indicate that Ir doping of MnSi does not result in the formation of a high-temperature phase, but rather in the suppression of the DM interaction. Although Rh and Ir belong to the same column of Mendeleev's periodic table, they exhibit disparate behaviors upon MnSi doping.

Investigation of graphite defects has long and rich history. However, it turned out, that most point defects in graphene, such as vacancies or topological Stone-Wales defect, have high formation energies, preventing them from formation in large quantities. Recently, we have proposed a new type of extended defects in graphene, with formation energy per atom only slightly above the melting temperature. This means, that these planar defects may occur near the melting temperature in sufficient quantities to influence the thermodynamic properties of graphene. Still, there is discrepancy between thermodynamic properties of these defects and their topology, which can be successfully resolved by proposition of a new defects' type – irregular network of extended defects, which will be described in this paper. The study of network stability from these defects enables us to formulate a new criterion of crystal lattice melting, which supersedes the Lindemann/Born criteria.

A recent report on obtaining the n-type conductivity in diamonds doped with boron-oxygen complexes in a metal solvent (X. Liu et al., PNAS 2019) stimulates interest in the synthesis of diamonds in heterohydrocarbon systems with oxygen and boron. The simultaneous effect of boron and oxygen heteroatoms on phase transitions in a hydrocarbon system is examined in phenylboronic acid C6H5B(OH)2 at pressures of 7-8.5 GPa and temperatures up to 1600 degrees C. At pressures of about 7 GPa and temperatures up to 1100 degrees C, the transformation of the precursor occurs through the stage of polymerization into a graphane-like phase with the subsequent formation of nanographite. Micro- and nanodiamonds are synthesized at 8.5 GPa and 1600 degrees C from the initial precursor and nanographite, which is a product of preliminary carbonization, respectively. Despite the presence of oxygen and boron in the growth system, the n-type conductivity in diamonds and nanographite is not detected. It is found that the degree of boron doping of diamond in hydrocarbon systems decreases in the presence of oxygen with a high chemical affinity to boron and that nanodiamonds in the carbonized product can be obtained when volatile components leave the system.

We carried out an experimental ultrasonic study of polyhydric alcohols with the general chemical formula CnHn+2(OH)n with an increasing number of OH groups: glycerol (n = 3), erythritol (n = 4), xylitol (n = 5), sorbitol (n = 6). The baric and temperature dependences of the elastic characteristics of these substances in the crystalline and glassy states were studied both under isothermal compression up to 1 GPa and during the isobaric heating of 77-295 K. For glycerol, glasses were obtained at different cooling rates, glass-liquid transitions were studied at different pressures. All the studied glasses have lower elastic moduli than the same substances in the crystalline state at the same pressure-temperature conditions. We obtained a cascade of glass-supercooled liquid-crystal transitions during heating of glassy erythritol. In the series of substances with n = 3, 4, 5 the bulk moduli show a tendency to decrease with increasing n. However, sorbitol (n = 6) unexpectedly has the highest elastic moduli among the studied substances in both the glassy and crystalline states. The studied glassformers show a general tendency to increase the glass transition temperature Tg and the fragility coefficient m with increasing n.

An efficient one-pot procedure has been developed for the synthesis of bridged 1,3,5-triazinanes, unexplored classes of heterocyclic polydentate scaffolds. By means of a pseudo five-component reaction between primary diamines (ethylenediamine, 1,3-propylenediamine, 1,4-butylenediamine), sulfonamides, and three equivalents of paraformaldehyde, a broad series of 1,3,5-triazabicyco[3.2.1]octanes, 1,3,5-triazabicyco[3.3.1]nonanes and 1,6,8-triazobicyclo[4.3.1]decanes were prepared. Boiling chloroform and an additive of Mg(ClO4)2 were shown to be the most efficient conditions for the condensation. Putrescine is probably the longest diamine which can be involved in this protocol. It was demonstrated that сadaverine was unsuitable for preparation of corresponding 1,7,9-triazabicyclo[5.3.1]undecanes. The structure of all classes of the obtained azabicycles was elucidated by XRD analysis.

The stability of CO2 phases at pressures up to 1600 GPa is confirmed using evolution methods for predicting crystal structures. Stable CO2 phases are as follows: I4̅2d (to 279 GPa), P4_2/nmc (279–952 GPa), Pbcn (952–1018 GPa), and Pa3̅ (above 1018 GPa). The equations of state for stable CO2 phases up to pressures of about 1600 GPa are calculated for the first time using ab initio methods and high-temperature calculations within the quasi-harmonic approximation. It is shown that high-pressure P4_2/nmc , Pbcn , and Pa3̅ phases have rather high bulk moduli (290–415 GPa). Phases with sixfold coordination of carbon atoms (Pbcn and Pa3̅ ) have higher coefficients of thermal expansion in comparison with the P4_2/nmc phase.

The diverging relaxation time in approaching hypothetical ideal glass transition is a subject of hot debate. In the current paper we demonstrate, how diverging relaxation time and turning excess entropy to zero (which is an essence of Kauzmann's paradox) can be avoided, using as an example the model molecular glassformer, propylene carbonate. For this purpose we compare its thermodynamic and dielectric relaxation properties, both known from the literature. The agreement between two sets of data can be achieved, if we suppose, that enthalpy of supercooled liquid propylene carbonate is governed by activation law, and relaxation time follows double exponential law. We propose the generalized Adam-Gibbs law to reconcile this two dependencies, and qualitatively discuss its implications.