S. V. Popova

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...

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.

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.

Pressure-temperature diagram of liquid bismuth

The transitions in the Bi melt were found under high pressures. These transitions were accompanied by anomalies of electrical resistivity and volume. The (P, T) phase diagram of liquid Bi was investigated up to 7.7 GPa and 1020 K.

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.

The kinetics of the transition of the metastable phases of SiO2, stishovite and coesite to the amorphous state

Abstract The non-equilibrium phase transitions of the metastable silica phases, stishovite and coesite, were investigated at room pressure by the isochronal annealing method. The measured heat release, Q, activation energy, ΔG, and Avrami index, n, of the transitions stishovite-glass were Q = 41 ± 3 kJ/mol, Δ = 220−280 kJ/mol, n = 1; glass-cristabolite: Q = 5 ± 1.5 kJ/molΔG = 400 ± 50 kJ/mol, n = 3 ± 0.5. Annealing of coesite leads to a solid state transition to an amorphous state and crystallization of the stable crystalline phase in the same temperature interval, 1380–1420 K, releases Q = 8 ± 2 kJ/mol. It was shown by annealing under high pressure that the temperature of the transition of stishovite to an amorphous state increases with pressure, and the temperature of crystallization of the amorphous state produced decreases. At P > 3.5 GPa. the direct stishovite-coesite transition takes place during heating. From the experimental results, it is assumed that the solid state transition of stishovite to an amorphous state is connected with the small value of the activation energy of seeding at the transition temperature. It is believed that the known examples of the solid state transition to an amorphous state of high pressure phases are also connected with large rate of nucleation without significant crystal growth.

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.

Ultrasonic study of the phase diagram of methanol

The phase diagram of methanol is studied by an ultrasonic technique over the temperature range 90–290 K at pressures up to 1.2 GPa. The pressure and temperature dependence of the velocity of longitudinal ultrasonic waves and the density of crystalline and liquid phases has been determined. Weak anomalies in the velocity of ultrasound in the liquid phase of methanol and the corresponding anomalous additional compression of the liquid at 230–250 K and 0.2–0.6 GPa have been found, and they are likely attributable to structural changes in the liquid phase.

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.