Elastic properties of metastable crystalline and amorphous gasb-ge semiconductors synthesized under high pressure

We present the systematic study of the elastic shear G and bulk B moduli in amorphous and crystalline metastable ternary solid solutions (GaSb)_1?x Ge_2x . It is found that the moduli of crystalline phases initially decrease with Ge concentration, falling down to minimum values at 20-30% Ge. The minimal values of elastic moduli for amorphous samples are observed at 50-60% Ge. Elastic softness of crystalline solid solutions is assumed to be related to the increase of chemical disorder and, consequently, of static (non-thermal) geometrical disorder in positions of atoms. An additional topological disorder in amorphous solid solutions leads to additional elastic softening.     

The method of volumetric measurements under high pressure. II. Quantitative measurements

Abstract

Method of measurements of the jumps of volume during phase transitions at high pressures and temperatures is suggested. Theoretical ground of the method is given. Values of the jumps of volume during melting of Pb and In up to 8 GPa, transition in the melt of Sn (these are first quantitative measurements of the jump of volume in the melt under high pressure), are presented.     

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 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.     

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.     

Electro-and magnetotransport properties of disordered carbon phases synthesized from C60 fullerite at moderate pressures P syn < 1.5 GPa

Charge transport has been investigated in new disordered carbon phases synthesized from C₆₀ at “moderate” pressures P _syn < 1.5 GPa and high temperatures T _syn > 1000 K. Under these synthesis conditions, a structure transition from nanographite phases to new amorphous modifications is induced when pressure decreases. The transition from hopping transport to diffusive electron transport typical of “dirty” metals is observed in a number of samples synthesized at various pressures and temperatures in the indicated ranges. The magnetotransport properties of these phases under intermediate synthesis conditions can be described by diffusive electron transport.     

Elastic properties of crystalline and liquid gallium at high pressures

The elastic properties of gallium, such as the bulk modulus B, the shear modulus G, and the Poisson’s ratio σ, are investigated and the relative change in the volume is determined in the stability regions of the Ga I, Ga II, and liquid phases at pressures of up to 1.7 GPa. The observed lines of the Ga I-Ga II phase transition and the melting curves of the Ga I and Ga II phases are in good agreement with the known phase diagram of gallium; in this case, the coordinates of the Ga I-Ga II-melt triple point are determined to be 1.24 ± 0.40 GPa and 277 ± 2 K. It is shown that the Ga I-Ga II phase transition is accompanied by a considerable decrease in the moduli B (by 30%) and G (by 55%) and an increase in the density by 5.7%. The Poisson’s ratio exhibits a jump from typically covalent values of approximately 0.22–0.25 to values of approximately 0.32–0.33, which are characteristic of metals. The observed behavior of the elastic characteristics is described in the framework of the model of the phase transition from a “quasi-molecular” (partially covalent) metal state to a “normal” metal state. An increase in the Poisson’s ratio in the Ga I phase from 0.22 to 0.25 with an increase in the pressure can be interpreted as a decrease in the degree of covalence, i.e., the degree of spatial anisotropy of the electron density along the bonds, whereas the large value of the pressure derivative of the bulk modulus (equal to approximately 8) observed up to the transition to the Ga II phase or the melt is associated not only with the quasicovalent nature of the Ga I phase but also with the structural features. In view of the presence of seven neighbors for each gallium atom in the Ga I phase, the gallium lattice can be treated as a structure intermediate between typical open-packed and close-packed structures. Premelting effects, such as a flattening of the isothermal dependence of the shear modulus G(p) with increasing pressure and an increase in the slope of the isobaric dependences G(T) with increasing temperature, are revealed in the vicinity of the melting curve. The bulk modulus of liquid gallium near the melting curve proves to be rather close to the corresponding values for the normal metal Ga II.     

Electron transport in carbynes modified under high pressure

The studies of hopping conductivity in carbynes modified under high pressure are reviewed. Experimental data are presented on the dc and ac conductivities, thermopower, magnetoresistance, and Hall effect. The results obtained are discussed within the framework of a model that takes into account the substantially nonuniform distribution and clusterization of sp² bonds in the carbyne sp matrix.     

Structural transformations in liquid,crystalline, and glassy B<Subscript>2</Subscript>O<Subscript>3</Subscript> 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.