Pressure-induced crossover between diffusive and displacive mechanisms of phase transitions in single-crystalline alpha-GeO2

We present the first example of the phase transition occurring via the different kinetic mechanisms, displacive or diffusive, competing with each other in quartz-like alpha-GeO2 single crystals. Upon room-pressure heating, alpha-GeO2 transforms to the rutile-type phase (the alpha-->r transition) via the diffusive mechanism. With increase of the treatment pressure the diffusive mode of the temperature-induced alpha-->r transition is substituted at approximately 4 GPa by a displacive-like mode, and then at approximately 6 GPa the transition type changes from the alpha-->r sequence to a displacive martensitic-like transition to a distorted rutile-like phase (alpha-->r'. A crossover between diffusive and displacive transition modes suggests a new way to control the meso- and nanometer-scale morphology of high-pressure phases.     

Network rigidity and properties of SiO2 and GeO2 glasses under pressure

We report in situ studies of SiO2 glass under pressure and find that temperature-induced densification takes place in a pressure window. To explain this effect, we study how rigidity of glasses changes under pressure, with rigidity percolation affecting the dynamics of local relaxation events. We link rigidity percolation in glasses to other effects, including a large increase of crystallization temperature and logarithmic relaxation under pressure.     

Characteristic features of elastic wave propagation in a one-dimensional model of an unconsolidated medium

The characteristic features of elastic wave propagation in a one-dimensional model of a discrete inhomogeneous unconsolidated medium are investigated. The model is represented by a linear chain of 80 uncoupled steel spheres with a diameter of 6.5 mm. Nonlinear effects that may arise in such systems are reviewed. The experimental setup is described. Results of studying the dispersion of elastic waves in the system and the dependence of the elastic wave velocity on the wave amplitude under increasing compression are presented. The results are analyzed using the Hertz contact theory.     

Experimental researches of propagation of acoustic compression pulses in copper wires

The results of experimental research related to the propagation of elastic-plastic compression pulses in polycrystalline copper wires are presented. We have developed a technique of generating compression pulses with amplitudes of pressure exceeding 100 MPa; this essentially exceeds the elasticity limit of copper. This allows us to carry out the research for the propagation of compression pulses in copper wire (as simples). Significant influence of the pulse amplitude on its spectrum, velocity and absorption will be discussed. The experiments showed that as the amplitude of the pulse increase the pulses velocity is decreased and absorption will be increased. The pulse spectrum also changes correspondingly. The analysis and discussion of experimental results will be presented through the context.     

Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures P ～ 5–15 GPa. Data on the thermopower of calcium in the pressure range up to 9 GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at 5–6 GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower (5–12 μV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of 5–7.5 GPa is fairly noticeable (～10%). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.     

Van der Waals supercritical fluid: exact formulas for special lines

In the framework of the van der Waals model, analytical expressions for the locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, density fluctuation, and sound velocity in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into single Widom line only at T < 1.07T(c), P < 1.25P(c) and become smeared at T < 2T(c), P < 5P(c), where T(c) and P(c) are the critical temperature and pressure. The behavior of the Batschinski lines and the pseudo-Gruneisen parameter γ of a van der Waals fluid were analyzed. In the critical point, the van der Waals fluid has γ = 8/3, corresponding to a soft sphere particle system with exponent n = 14.     

Peculiarity of the amorphous state obtained by rapid quenching from melt under high pressure

Abstract

The influence of the high pressure - high temperature treatment on the relaxation properties of the metallic glass Fe-18% B was investigated. To explain the experimental data we proposed the phenomenological two-parametric model of stresses in the amorphous state: tensile and compressing, similar to the two types of the defects.     

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 sp ³ 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. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 755–759 (10 December 1996)     

Anomalies of the baric and temperature dependences of the elastic characteristics of ice during solid-phase amorphization and the phase transition in the amorphous state

The elastic characteristics of ice up to pressures of 1.7 GPa are determined for the first time at a temperature of 77 K, along with features of their variation associated with the phase transformation of hexagonal ice Ih into high-density amorphous ice hda. The elastic instability of the ice lattice before solid-phase amorphization is experimentally confirmed. Elastic instability during a transition from one amorphous state to another amorphous state was also observed for the first time; this took place when hda ice was warmed at p=0.05 GPa from T=77 K. Zh. éksp. Teor. Fiz. 112, 200–208 (July 1997)     

Pressure-driven "molecular metal" to "atomic metal" transition in crystalline Ga

We present the ultrasonic study of gallium (Ga I) under high pressure up to 1.7 GPa, including the measurements of the density and elastic properties during phase transitions to Ga II and to a liquid state. The observed large drop of both bulk and shear moduli (by 30% and 55%, correspondingly) during the phase transition to Ga II, as well as the increase of the Poisson's ratio from typically "covalent" ( approximately 0.22) to "metallic" ( approximately 0.32) values, experimentally testifies to the coexistence of a molecular and metallic behavior in Ga I and to the disappearance of the "covalency" during the transition to Ga II. A high value of the pressure derivative of the bulk modulus for Ga I and the increase in the Poisson's ratio can be associated with the weakening of the covalency in compressed Ga I and considered as a precursor of the transition to normal metal.