Nonequilibrium crystal-to-amorphcs phase transitions in “high pressure” phases SiOz and Ge(A1)

Abstract

The pressure dependences of the temperatures of the crystal-to-amorphous transformations for SiO₂ and Geo.7Alo.9 are presented. The field of the existence of amorphous phases of both substances vanishes at high pressure.

Les variations avec P de la temperature de transformations cristal-to-amorphous de SiO₂ et Ga. 7 Ala. 9 sont presentes. Le champ d'existence de phase amorphe de ces substances desparait a haute pression.     

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

Abstract

The kinetics of crystallization of eutectic alloys Al_100-xSi_x (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.     

Electron transport properties of lithium and phase transitions at high pressures

The electric resistivity and thermopower of lithium have been precisely measured at high pressures (up to 8 GPa) and temperatures from room temperature to 100°C. Transition to the fcc phase of lithium has been analyzed. The hysteresis of the direct and inverse transitions is 0.3 GPa at room temperature, decreases slightly with an increase in the temperature, and is almost independent of the prehistory of the sample. The phase transition line on the P-T diagram has a positive slope of dP/dT = 0.03 GPa/K. It is assumed that the fcc phase of lithium, which is stable at a high pressure, can appear for kinetic regions from the 9R phase, which is intermediate in energy between the bcc and fcc modifications.     

High-Precision Studies of the Compressibility and Relaxation of g-As2S3 Glasses at High Hydrostatic Pressures up to 8.6 GPa

Journal of Experimental and Theoretical Physics - The high-precision volume measurements of the g-As2S3 glass have been performed at high hydrostatic pressures up to 8.6 GPa, and room temperature....     

Hierarchy of Times for the Establishment of the Gibbs Distribution

Doklady Physics - It is shown that there are three characteristic time scales for the establishment of thermodynamic equilibrium: the time for the thermalization of the system; the time for...     

Thermodynamics of a Magnetic Transition in MnS<Subscript>2</Subscript> at High Pressures

The behavior of the specific heat of MnS₂ at high pressures has been studied. A significant increase in the transition temperature T_N to an antiferromagnetic state with the pressure from 48.2 K at atmospheric pressure to 76 K at a pressure of 5.3 GPa has been revealed. The initial pressure derivative is dT _N /dP = 4.83 K/GPa. It has been found that the parameter α = d(logT _N )/d(logV ) = ?6.6 ± 0.1 is significantly different from the value α = ?10/3 ≈ ?3.3 (Bloch relation), which is typical of numerous antiferromagnetic insulators—transition- metal oxides and fluorides. The volume jump at the magnetic transition point has been estimated. The necessity of direct dilatometric measurements of the volume has been justified.     

Phase Transitions in a Mixture of Amorphous C<Subscript>60</Subscript> and C<Subscript>70</Subscript> Fullerene Phases at High Temperatures and Pressures

Phase transitions in two types of amorphous fullerene phases (C₆₀–C₇₀ (50/50) mixtures and an amorpous C₇₀ fullerene phase) are studied via neutron diffraction at pressures of 2–8 GPa and temperatures of 200–1100°C. Fullerenes are amorphized by grinding in a ball mill and sintered under quasi-hydrostatic pressure in a toroidal-type chamber. Diffraction studies are performed ex situ. It is shown that the amorphous phase of fullerenes retains its structure at temperatures of 200–500°C, and amorphous graphite is formed at 800–1100°C with a subsequent transition to crystalline graphite. This process is slow in a mixture of fullerenes, compared to C₇₀ fullerene. According to neutron diffraction data, the amorphous graphite formed from amorphous fullerene phases has anisotropy that is much weaker in a fullerene mixture.     

AsS melt under pressure: one substance, three liquids

An in situ high-temperature--high-pressure study of liquid chalcogenide AsS by x-ray diffraction, resistivity measurements, and quenching from melt is presented. The obtained data provide direct evidence for the existence in the melt under compression of two transformations: one is from a moderate-viscosity molecular liquid to a high-viscosity nonmetallic polymerized liquid at P approximately 1.6-2.2 GPa; the other is from the latter to a low-viscosity metallic liquid at P approximately 4.6-4.8 GPa. Upon rapid cooling, molecular and metallic liquids crystallize to normal and high-pressure phases, respectively, while a polymerized liquid is easily quenched to a new AsS glass. General aspects of multiple phase transitions in liquid AsS, including relations to the phase diagram of the respective crystalline, are discussed.     

Thermodynamic and dilatometric properties of the dimerized phase of a C<Subscript>60</Subscript> fullerene

This paper reports on the results of complex investigations into the structural, thermodynamic, and dilatometric properties of the C₆₀ dimerized phase prepared under compression of a C₆₀ fullerite at a pressure up to 8 GPa and a temperature of 290 K. It is demonstrated that the dimerized phase has a face-centered cubic structure with a lattice parameter a=14.02±0.05 Å. The dimeric structure of the studied sample is confirmed by x-ray diffraction analysis. According to the dilatometric data, the volume jump observed in the vicinity of the orientational transition for the dimerized phase is estimated to be approximately 30 times less than that for the C₆₀ fullerite. The temperature dependence of the heat capacity of the (C₆₀)₂ crystalline dimer is examined using precision adiabatic vacuum calorimetry under normal pressure in the temperature range from T → 0 K to 340 K. The results obtained are used in the calculations of thermodynamic functions, namely, the heat capacity C _p ⁰ (T), the enthalpy H⁰(T)-H⁰(0), the entropy S⁰(T), and the Gibbs function G⁰(T)-H⁰(0). The fractal dimension D is determined as a function of the heat capacity. The standard entropy of the formation of the (C₆₀)₂ crystalline dimer from a simple compound (graphite) at T=298.15 K and normal pressure is calculated.     

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.