<Emphasis Type="Italic">In situ</Emphasis> study of the mechanism of formation of pressure-densified Sio<Subscript>2</Subscript> glasses

The volume of glassy a-SiO₂ upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO₂ glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO₂. A nonequilibrium phase diagram is suggested for glassy SiO₂. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.     

Electron and lattice stages in the valence transition in SmTe under a high hydrostatic pressure

The results of precision measurements of the resistivity, thermopower, volume, and thermal conductivity of the compound SmTe under truly hydrostatic pressure conditions at room temperature are reported. High quality stoichiometric and doped (n-type, n ≈ 8 × 10¹⁸ cm^?3) single crystals are studied. It is found that the valence transition occurs as consecutive stages of rearrangement of the electron subsystem and the crystal lattice, which take place under different pressures. At the initial stage of the transition, metallization is observed, which is accompanied by anomalies in kinetic coefficients; the curve describing the pressure dependence of the volume deviates from the curve corresponding to the initial semiconductor phase only slightly. The next stage is accompanied by a substantial change in the sample volume (lattice collapse); in this pressure range, however, the resistivity and thermopower become independent of pressure. At the final stage of the transition, the sample compressibility decreases; the resistivity and thermopower become again functions of pressure; and a state emerging in the sample in this case corresponds to the “golden” phase of SmS in all the properties being measured.     

Precise in situ study of the kinetics of pressure-induced phase transition in CaF<Subscript>2</Subscript> including initial transformation stages

The kinetics of room-temperature phase transition in fluorite (CaF₂) single crystals under hydrostatic pressure up to 9 GPa was studied in situ by means of strain gauge compressibility measurements. Initial stages of the pressure-induced first-order phase transition kinetics (corresponding to less than 1% content of the new phase) were studied for the first time. In a broad range of concentrations of the new phase (5–20%), the transformation kinetics is well described within the framework of the classical Kolmogorov-Avrami-Mehl-Johnson model. The laws governing the initial and late stages of the transformation are more complicated and do not conform to the classical model. The initial stages involve avalanche growth in the nucleation rate corresponding to giant values of the Avrami exponent (n ≈ 20). At large concentrations of the new phase (above 30%), the transformation rate significantly decreases (saturation) as a result of the formation of a rigid cellular structure of the new phase.     

Effect of high pressure on the thermopower in gallium

The absolute thermopower and relative electrical resistance of gallium have been measured at room temperature and at a pressure increased up to 7 GPa. As the initial phase, all of the Ga phases retain the positive value of the thermopower. At 4 GPa, a phase transition has been revealed.     

Pressure effects on the kinetic properties and phase transitions in lithium

The pressure dependences of the electrical resistance and thermal electromotive force of lithium were measured at room temperature. The results substantiated the occurrence of a phase transition caused by increasing pressure (6.7 GPa). A phase transition was detected when pressure was decreased (6.4 GPa). Temperature effects on the pressures of these transitions were studied near room temperature. At pressures above 4 GPa, the pressure dependences of thermal electromotive force and of the velocity of ultrasonic shear waves in BCC lithium exhibited anomalies. The suggestion was made that applying pressure increased the role played by electron-phonon and phonon-phonon interactions in lithium.     

Measurements of compressibility of solids and powder compacts by a strain gauge technique at hydrostatic pressure up to 9GPa

Abstract

An experimental technique is described which enables one to measure the pressure-volume (P-V) relationship of solids and powder compacts and the linear compressibility of anisotropic single crystals by means of the resistive strain gauges at hydrostatic pressure up to 9 GPa. The potential of this technique is demostrated for solids possessing pressure induced phase transitions (PbTe, SmSe) and anisotropic crystals (Sb). For the first time P-V relationship is measured for highly compressible powder compact at increase and decrease of pressure.     

An Elementary Method on a Minimization Problem

一、ＴｈｅＰｒｏｂｌｅｍ .Ｌｅｔｆ(ｘ) =ａｘ2 ｂｘ ｃ ａｘ2 ｐｘ ｑ,ｗｈｅｒｅａ >0 ,ｂ2 - 4ａｃ≤ 0ａｎｄｐ2 - 4ａｑ≤ 0 .ＯｕｒＰｒｏｂｌｅｍｉｓ“Ｗｈｅｔｈｅｒｄｏｅｓｍｉｎｘ∈Ｒｆ(ｘ)ｅｘｉｓ?Ｍｏｒｅｏｖｅｒ ,,ｉｆｍｉｎｘ∈Ｒｆ(ｘ)ｅｘｉｓｔｓ,ｔｈｅｎｍｉｎｘ∈Ｒｆ(ｘ)=?ａｎｄｉｎｔｈｉｓｃａｓｅｘ =?” .Ｎａｔｕｒａｌｌｙ ,ｗｅｋｎｏｗｔｈａｔｍｉｎｘ∈Ｒｆ(ｘ)ｅｘｉｓｔｓｆｒｏｍｔｈｅｋｎｏｗｌｅｄｇｅｏｆｍａｔｈｅｍａｔｉｃａｌａｎａｌｙｓｉｓ.Ａｌｓｏ,ｗｅｃａｎｇｉｖｅｔｈｅ…     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.