R. N. Voloshin

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...

Interplay between the structure and properties of new metastable carbon phases obtained under high pressures from fullerite C60 and carbyne

A brief review of structural, electrotransport, optical, elastic, and mechanical properties of carbon phases synthesized under pressure by heating fullerite C60 and carbynoid materials is given. A large variety of carbon modifications with a variable bonding type, a variable mean coordination number, a variable molecular or atomic structural type, a variable characteristic dimensionality (from zero-to three-dimensional structures), a variable degree of covalence, etc., were prepared. Emphasis in the review is given to the elucidation of the interplay between the structural and topological characteristics of carbon phases and their key electronic and mechanical properties. A version of the kinetic phase diagram of fullerite C60 transformations on heating under pressure is also suggested. This version is modified with respect to the interpretations known in the literature.

Metallization of liquid iodine under high pressure

Abstract In molten iodine two transitions accompanied by a large increase of conductivity (σ) were found under pressure between 3 and 4 GPa. During the first transition a increases by approximately 10′ times, the volume changing very slightly or remaining constant. During the second transition a increases by 2-10 times and then is accompanied by a decrease of volume.

Mechanism of the formation of a diamond nanocomposite during transformations of C60 fullerite at high pressure

The results of an investigation of the transformation of C60 fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C60 at pressures 12–14 GPa and temperatures ∼1400°C produces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700–1200°C) nanocomposites consisting of diamondlike (sp3) and graphitic (sp2) amorphous phases are formed. The nanocomposites obtained have extremely high mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diamond. Mechanisms leading to the transformation of C60 fullerite into diamond-based nanocomposites and the reasons for the high mechanical characteristics of these nanocomposites are discussed.

Anharmonicity of short-wavelength acoustic phonons in silicon at high temperatures

Second-order Raman spectra corresponding to transverse acoustic phonons are studied in detail for crystalline Si over the temperature range 20–620°C. The largest relative softening and anharmonicity at the boundaries of the Brillouin zone were observed for the TA(X) mode. Extrapolation of the TA(X) frequency to high temperatures suggests that the Si lattice should be dynamically unstable at temperatures on the order of a doubled melting temperature. It is found that the main contribution to the softening of the transverse acoustic phonons in silicon comes from the anharmonicity and not from the volume expansion.

The kinetics of solidification of Al-Si eutectic alloys under high pressure

Abstract The kinetics of crystallization of eutectic alloys Al100-xSix (χ=12, 18, 26 at.%) was investigated under pressures p=0.5, 2.5, 4.5 GPa. The values of supercooling and average grain size of silicon crystals were determined for alloys quenched from melt under different pressures with the cooling rate 103 K/s. The data obtained were used to evaluate quantitatively the pressure dependences of surface tension (between melt and crystal) and activation energy of crystal growth which, in turn, have made it possible to determine the relative change of nucleation frequency and of the rate of crystal growth with pressure. The possibility is shown, based on the investigation of mechanical properties of the samples obtained under high pressure, for improving the strength and the plasticity of A1-Si alloys by means of high pressure-high temperature treatment.

Structural transformations in liquid, crystalline, and glassy B2O3 under high pressure

We present in situ (x-ray diffraction) and ex situ (quenching) structural studies of crystalline, liquid, and glassy B2O3 up to 9 GPa and 1700 K, drawing equilibrium and nonequilibrium phase diagrams of B2O3. Particularly, we have determined the melting curve, the stability regions for crystalline B2O3 I and B2O3 II modifications, the regions of transformations, such as densification or crystallization, for both the liquid and glassy states, including the region of sharp first-order-like transition in liquid B2O3 to a high-density phase near 7 GPa. Quenching experiments also show that the transition to the high-density liquid can occur at much lower pressures in nonstoichiometric melts with an excess of boron. B2O3 is the first glassformer whose transformations in the disordered state have been comparatively studied for both liquid and glassy phases.

The influence of high pressure on the solidification of supercooled Se melt

Abstract Under pressures between 1.75 and 8 GPa a supercooling of Se melt and an average grain size of the samples quenched under pressure with constant cooling rate 100 K/s were measured. In framework of the classic theory of nucleation and grain growth the numerical evaluation of surface tension and activation energy of crystal growth was performed. The comparison of properties of supercooled Se melt to that of Pb and In reveals anomalies on the pressure dependences of Se melt properties. This circumstance is discussed in connection with the semiconductor-metal transition discovered earlier in Se melt.

Metallization of the melts of Se, S, I2 under high pressure

Abstract In the melts of Se S and I2 the nonmetal-metal transitions were found under pressure. For sulphur and selenium the transitions are accompanied by a decrease of the volume. For iodine two transitions take place. During the first one the volume changes very slightly, the second transition being accompanied by an essential decrease of the volume. The existence of transitions of this kind gives an explanation for anomalies in the melting curves of some substances.

High pressure influence on the kinetics of solidification of the supercooled melts Pb(In)

Abstract The measurements of supercooling AT and average grain size d of quenched f.c.c. solid solutions Pb1-x Inx (x=0.1, 0.2, 0.5) under normal and high (2.5 and 5.0 GPa) pressure and constant cooling rate 500 K/s were performed. Using the classical theory of nucleation and crystal growth the numerical evaluation of surface tension σ between a nucleus and a melt and activation energy ΔG was made of a crystal growth under different pressures. Different models for a description of pressure dependences σ(P), ΔG(P) and concentration dependences ΔT(x), d(x), §(x), AG(x) under different pressures are discussed.