V. V. Brazhkin

Harder than diamond: Dreams and reality

Abstract Analysis of correlations between various physical properties of solids provides a formulation of simple criteria applicable to the search for new superhard materials. The prospects for the synthesis of new substances with the key elastic moduli, whose values approach or even exceed those of diamond are also discussed. We introduce the concept of ideal hardness and strength, which relates the elastic properties of materials to the corresponding mechanical characteristics, the concepts that are less unambiguously determined and that depend on the conditions of measurements. The control of material nanostructure makes it possible to approach the ideal hardness. The formulation of trends interrelating physical properties is expected to allow an essential guidance in the synthesis of new classes of superhard materials.

High-pressure transformations in simple melts

Abstract Phase transitions in crystalline substances are a major field of study in high-pressure physics. The sequences of structural transformations and the boundaries separating different phases have been established for a majority of elements under high pressure and for different temperatures. Though both electron and structural transitions taking place in crystals should also be present in the liquid state, such questions as the metallization of the dielectric liquids, the nature of short-range order reconstruction, the presence of boundaries separating different liquid states, and the thermodynamic description of the transformations in melts, remain open. This review summarizes the experimental results on electrical, volumetric and structural properties of the melts of elements under high pressure. The data in hand suggest the possibility of liquid-liquid transformations which are very similar in many respects to first-order transitions in crystals. P-T diagrams of simple melts look like simplified a...

High-pressure phase transformations in liquids and amorphous solids

We outline the current state of experimental study and basic ideas for describing phase transitions in topologically disordered condensed matter, such as liquids and amorphous solids. Reviewing briefly the study of molten elementary substances under pressure, we pay primary attention to the results for liquid Se, S, and P and also to those substances that have not been represented in previous publications, mainly the liquid oxides B2O3 and GeO2. The experimental data reveal the possibility of rather sharp transformations in relatively simple liquids that are smoothed at high temperatures. Comparing the transitions in amorphous solids and in liquids, one should emphasize the metastable and non-ergodic nature of amorphous substances and the existence of static local atomic stresses fluctuating in thermally frozen amorphous networks. In particular, the kinetic study of amorphous–amorphous transformations (AATs) in SiO2 and GeO2 glasses and amorphous H2O ice under pressure highlights a number of anomalous features that distinguish the AATs from ordinary first-order transitions and from transformations in liquids. The recent in situ study of the volume changes in glassy silica a-SiO2 upon compression at high temperatures provides a new conclusion as regards the existence of two pressure-induced AATs in a-SiO2 with different microscopic mechanisms of structural rearrangements. We also perform the analysis of two possible kinetic scenarios for AATs, including sharp and diffuse transitions. The key relation determining the transformation scenario is the relationship between the radius of structural correlations in amorphous solid and the size of the critical nucleus of the growing disordered modification. The comparative analysis emphasizes the main difference between the transformations in liquids and amorphous solids that consists in the fact that the transitions in liquids are mainly determined by thermodynamic relationships, whereas the transitions in amorphous solids take place far away from equilibrium and are governed by the corresponding kinetics.

Widom line for the liquid-gas transition in Lennard-Jones system.

The locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, and density fluctuations for model particle systems with Lennard-Jones (LJ) potential in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into a single Widom line at T < 1.1T(c) and P < 1.5P(c) and become practically completely smeared at T < 2.5T(c) and P < 10P(c), where T(c) and P(c) are the critical temperature and pressure. The ridge for heat capacity approaches close to critical isochore, whereas the lines of extrema for other values correspond to density decrease. The lines corresponding to the supercritical maxima for argon and neon are in good agreement with the computer simulation data for LJ fluid. The behavior of the ridges for LJ fluid, in turn, is close to that for the supercritical van der Waals fluid, which is indicative of a fairly universal behavior of the Widom line for a liquid-gas transition.

Liquid-gas transition in the supercritical region: fundamental changes in the particle dynamics.

Recently, we have proposed a new dynamic line on the phase diagram in the supercritical region, the Frenkel line. Crossing the line corresponds to the radical changes of system properties. Here, we focus on the dynamics of model Lennard-Jones and soft-sphere fluids. We show that the location of the line can be rigorously and quantitatively established on the basis of the velocity autocorrelation function (VAF) and mean-square displacements. VAF is oscillatory below the line at low temperature, and is monotonically decreasing above the line at high temperature. Using this criterion, we show that the crossover of particle dynamics and key liquid properties occur on the same line. We also show that positive sound dispersion disappears in the vicinity of the line in both systems. We further demonstrate that the dynamic line bears no relationship to the existence of the critical point. Finally, we find that the region of existence of liquidlike dynamics narrows with the increase of the exponent of the repulsive part of interatomic potential.

In situ study of the mechanism of formation of pressure-densified Sio2 glasses

The volume of glassy a-SiO2 upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO2 glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO2. A nonequilibrium phase diagram is suggested for glassy SiO2. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.

Elastic constants of α-GeO2

The elastic constants of α-GeO2, the quartzlike polymorph of germanium dioxide, have been determined using Brillouin scattering. The data were obtained using a high quality single crystal of α-GeO2. When compared with existing data on α-SiO2, our results show that the shear constants are considerably softened in GeO2. This could be an indication that the observed pressure induced phase change is related to a shear instability.

Collective modes and thermodynamics of the liquid state.

Strongly interacting, dynamically disordered and with no small parameter, liquids took a theoretical status between gases and solids with the historical tradition of hydrodynamic description as the starting point. We review different approaches to liquids as well as recent experimental and theoretical work, and propose that liquids do not need classifying in terms of their proximity to gases and solids or any categorizing for that matter. Instead, they are a unique system in their own class with a notably mixed dynamical state in contrast to pure dynamical states of solids and gases. We start with explaining how the first-principles approach to liquids is an intractable, exponentially complex problem of coupled non-linear oscillators with bifurcations. This is followed by a reduction of the problem based on liquid relaxation time τ representing non-perturbative treatment of strong interactions. On the basis of τ, solid-like high-frequency modes are predicted and we review related recent experiments. We demonstrate how the propagation of these modes can be derived by generalizing either hydrodynamic or elasticity equations. We comment on the historical trend to approach liquids using hydrodynamics and compare it to an alternative solid-like approach. We subsequently discuss how collective modes evolve with temperature and how this evolution affects liquid energy and heat capacity as well as other properties such as fast sound. Here, our emphasis is on understanding experimental data in real, rather than model, liquids. Highlighting the dominant role of solid-like high-frequency modes for liquid energy and heat capacity, we review a wide range of liquids: subcritical low-viscous liquids, supercritical state with two different dynamical and thermodynamic regimes separated by the Frenkel line, highly-viscous liquids in the glass transformation range and liquid-glass transition. We subsequently discuss the fairly recent area of liquid-liquid phase transitions, the area where the solid-like properties of liquids have become further apparent. We then discuss gas-like and solid-like approaches to quantum liquids and theoretical issues that are similar to the classical case. Finally, we summarize the emergent view of liquids as a unique system with a mixed dynamical state, and list several areas where interesting insights may appear and continue the extraordinary liquid story.

Hardening of fullerite C60 during temperature-induced polymerization and amorphization under pressure

We report a detailed study of Vicker’s hardness and ultrasonic elastic moduli for carbon phases prepared by heating fullerite C60 at pressures of 3.5, 5, and 8 GPa. It is shown that the transformation of two-dimensional C60 polymers into graphite-like amorphous carbon is accompanied by an increase in hardness of 100–200 times, as well as an increase in bulk and shear moduli by 4–5 and 2–3 times, respectively, with no density jump taking place. It is proposed that the high hardness (up to 40 GPa) of the disordered phases synthesized is caused by the three-dimensional ordering of sp2-based network. It was found that, in the 3.5–8 GPa interval, the mechanical properties of the phases obtained depend basically on the temperature rather than on the pressure of synthesis.

Non-Traditional Carbon Semiconductors Prepared from Fullerite C60 and Carbyne under High Pressure

We present the detailed study of X-ray diffraction, Raman and absorption edge spectra, mechanical, and transport properties of new metastable carbon phases prepared from fullerite C60 and cumulene carbyne by high-pressure–temperature treatment and also review some recent relevant results. The sequence of phases obtained gives the picture of temperature-induced transformations under pressure, which are described in terms of covalent bonding of C60 molecules or cumulene chains in carbyne. Special attention is paid to the three-dimensional polymerization of fullerite C60. Experimental data suggest certain relations between the physical properties of prepared carbon phases, the majority of which are semiconductors, and the bonding nature of materials, i.e., the number of atoms in differently hybridized carbon states, structure topology, contribution of van der Waals interaction, etc.