A. G. Lyapin

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 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.

Mechanical properties of the 3D polymerized, sp2–sp3 amorphous, and diamond-plus-graphite nanocomposite carbon phases prepared from C60 under high pressure

Mechanical properties (Vicker’s hardness, Young’s modulus, and fracture toughness coefficient) have been studied for the three-dimensionally polymerized, amorphous, and nanocrystalline diamond-plus-graphite composite carbon phases prepared from fullerite C60 by temperature treatment under pressure. The hardness was found to increase gradually with the synthesis temperature. The experimental values of hardness are well correlated with the density of samples regardless of the phase structural nature, displaying the same dependence as amorphous carbon films. It has been shown that the hardness and Young’s modulus of both polymerized crystalline and disordered phases, though not as high as those of diamond, are comparable to the properties of cubic BN, while fracture toughness coefficient can be higher than that for diamond. An unusual combination of high hardness and high plasticity has been established for strongly polymerized C60.

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.

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.

Glassy dynamics under superhigh pressure.

Nearly all glass-forming liquids feature, along with the structural alpha-relaxation process, a faster secondary process (beta relaxation), whose nature belongs to the great mysteries of glass physics. However, for some of these liquids, no well-pronounced secondary relaxation is observed. A prominent example is the archetypical glass-forming liquid glycerol. In the present work, by performing dielectric spectroscopy under superhigh pressures up to 6 GPa, we show that in glycerol a significant secondary relaxation peak appears in the dielectric loss at P>3 GPa. We identify this beta relaxation to be of Johari-Goldstein type and discuss its relation to the excess wing. We provide evidence for a smooth but significant increase in glass-transition temperature and fragility on increasing pressure.

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.

Mechanism of three-dimensional polymerization of fullerite C60 at high pressures

A series of polycrystalline phases corresponding to different stages of three-dimensional polymerization and destruction of C60 molecules has been synthesized by heating fullerite C60 under a pressure P=12.5 GPa. The structure of the phases can be identified as fcc, and the lattice period decreases with increasing heating temperature. A model of three-dimensional polymerization in which the lattice parameter is a continuous function of the fraction of covalently bonded molecules is proposed. The model makes it possible to estimate the number of atoms in the sp3 state. The hardness of the polymerized fcc phases is studied on the basis of percolation of rigidity. It is shown experimentally that the period a≈13.8 Å is the threshold for the formation of a three-dimensionally rigid C60 polymer. It is found that the thermal stability of the strongly and weakly polymerized phases is qualitatively different.