The method of volumetric measurements under high pressure I. Qualitative approach

Abstract

Method of detection of the first order phase transitions under high pressure is suggested. It based on the different pressure dependences of chromel-alumel and Pt+10%Rh thermocouples. Pressure-temperature diagrams of the melts of elements, obtained by this technique, are presented.     

Nonmetal-metal transitions in liquid substances under high pressure

Abstract

In the melts of Te, Se, S, I₂ and Mg₃Bi₂ the nonmetal-metal transitions were found under pressure. The transitions are accompanied by a decrease of the volume. The transitions seem to terminate at high temperature by “critical regions”. For S and Se the kinetics of the transitions and the pressure influence on the solidification of the melts were investigated.

The existence of the transitions of this kind gives an explanation of anomalies of melting curves of some substances.     

In situ characterization of liquid network structures at high pressure and temperature using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press

ABSTRACT

We review recent progress in studying structural properties of liquids using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press at third-generation synchrotron facilities. This experimental method allows for detecting subtle changes in atomic arrangements of melts over a wide pressure–temperature range. It has been also employed to monitor variations of the local coordination environment of diluted species contained in glasses, liquids and crystalline phases as a function of the pressure and temperature. Such information is of great importance for gaining deeper insights into the physico-chemical properties of liquids at extreme condition, including the understanding of such phenomena as liquid–liquid phase transitions, viscosity drops and various transport properties of geological melts. Here, we describe the experimental approach and discuss its potential in structural characterization on selected scientific highlights. Finally, the current ongoing instrumental developments and future scientific opportunities are discussed.     

Preparation of perfect glasses from zeolites

When glasses are traditionally formed from melts, due to slow crystallization dynamics, the configuration and free volume of the liquid state are frozen in a random structure with high entropy. The amorphous state can also be attained directly from a crystalline solid under pressure. In this case, at a certain rate of change in the pressure, some structural elements can remain ordered; i.e., the amorphous material can retain the crystalline topology. As a result, the amorphous material will have a very low entropy, close to that of the crystal, i.e., will be a “perfect” glass with new physical properties. The method for preparing such a perfect glass from zeolites and its properties are discussed.     

Pressure-induced structural transformations and the anomalous behavior of the viscosity in network chalcogenide and oxide melts

It is known that a number of compressed melts undergo structural phase transitions. Data on the structural changes at high pressures in chalcogenides (AsS, As₂S₃) and oxide (B₂O₃) melts with a network structure have been reviewed. Viscosity is one of the fundamental physical properties of a liquid. For various melts, it varies in a very wide range. Structural transformations in melts induce the corresponding changes in all physical properties, in particular viscosity. The measurements of the viscosity of a number of melts at high pressures and temperatures by the radiographic method have been reported. Changes in the viscosity by several orders of magnitude have been detected when the pressure is varied by several gigapascals. The diffusion mechanism in network-structure melts at various pressures has been analyzed. The prediction of the behavior of the viscosity of various melts at superhigh pressures is of high importance for the physics of glass transition, geophysics, and materials science.     

Density and structural changes of silicate glasses under high pressure

ABSTRACT

Density and structural changes of matter in their liquid or amorphous form, such as silicates melts, molecular fluids, or glasses, are of extreme importance to model the interior of planetary bodies. However, measuring the evolution of amorphous materials under extreme conditions of pressure and temperature remains challenging mainly because of the sample dimensions and the weak interaction with X-ray probes. This contribution highlights recent developments to measure the density of amorphous material, mainly silicate glasses made of light elements, to high pressure conditions. In particular, the X-ray absorption method at synchrotron sources is discussed as a new opportunity for high pressure experiments on glasses, fluids, and melts. Recent achievements using X-ray Raman scattering spectroscopy to obtain data on the local electronic environment of the main constituents of silicate glasses at high pressure are also presented. Finally, perspectives of these recent developments are discussed as well as their potential for high pressure research in the next years.     

Metallization of the melts of Se,S, I2 under high pressure

Abstract

In the melts of Se S and I₂ the nonmetal-metal transitions were found under pressure. For sulphur and selenium the transitions are accompanied by a decrease of the volume. For iodine two transitions take place. During the first one the volume changes very slightly, the second transition being accompanied by an essential decrease of the volume.

The existence of transitions of this kind gives an explanation for anomalies in the melting curves of some substances.     

Specific features of T-C-P diagrams for binary systems of B-elements

Abstract

The possibility of reaching high pressures gave rise to intense studies of the phase state of substances, their structures, and properties depending on a new thermodynamic parameter-pressure. Characterizing the extensive development of these studies, one should distinguish three stages: (i) the study of polymorphism for elements under high pressure, and the construction of T-P diagrams; (ii) the study of polymorphism for compounds; and (iii) the construction of three-dimensional T- C-P diagrams for binary and multicomponent systems. At present, the first stage is almost completed in the pressure range from ～100 kbar (see Ref. 1) to 1 Mbar. A series of empirical laws have been established that determine the general direction of structural changes for chemical elements with a pressure rise. Thus, for the elements of the B-subgroup of the Periodic Table, the “coordination rule” has been established according to which an increase in pressure produces structural changes in B elements characterized by a higher packing density and coordination number. According to another rule, the so-called homology rule, B-elements under pressure acquire structures typical for their heavier homologues at normal pressure. These rules manifest themselves most clearly for the elements of the IVB subgroup. In a C-Si-Ge-Sn row structure types change with an increase of the coordination number as follows: graphite (3)-diamond (4)-white tin (6). The same sequence is also observed for the elements of the M3 subgroup with a pressure rise: graphite transforms into diamond, and the diamond-like structure of germanium and silicon transforms into the white-tin structure.     

High-pressure DTA-investigations on liquid crystals

Abstract

High pressure studies on the phase behaviour of liquid crystals up to a maximum pressure of 7 kbar were performed using a computer-assisted high-pressure differential thermal analysis apparatus that has been developed for the present experiments. T (p) - phase diagrams were determined for selected liquid crystals differing systematically in structure.

Data are presented for some phenylcyclohexanes, some bicyclo-hexanes and cyclohexylcarboxylic acid phenyl ester being substituted with different functional groups e.g. alkyl, alkoxy and/or cyano. Pressure-induced nematic and smectic phases were found, and for one substance even an “interdigitated” smectic B phase could be stabilized under pressure.     

Phase transformation of quasicrystals in aluminium-transition metal alloys

Quasicrystals in Al–Mn, Al–Cu–TM (TM = Fe, Cr, Mn and Ru) and Al–Cu–Fe–Cr alloys can undergo two different modes of phase transformation. Discontinuous transformations of quasicrystals are characterized by the existence of a definite reaction front separating the quasicrystalline phase from the resulting crystalline one; the kinetics are controlled by the migration of the reaction front. Continuous transitions, on the other hand, proceed by structural evolution such as modulation or chemical ordering inside the quasicrystalline phase without creating any high-energy interfaces. Both types of transformations are thermally activated and need atomic diffusion.     

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.     

Electronic level diagram and structure of metastable centers in CdF2:Ga and CdF2:In semiconducting crystals

A study is reported of the optical properties of wide-gap, predominantly ionic cadmium fluoride crystals in photo-and thermally stimulated transformations of metastable indium and gallium centers. An analysis of these properties leads one to a conclusion of gallium having two metastable states (two types of deep centers). The deep-center binding energies and the barriers separating the shallow (hydrogenic) and deep centers have been determined for both impurities. Configuration-coordinate diagrams are developed, and microscopic models for the deep centers are proposed. It is concluded that these centers are identical with the metastable DX centers in typical semiconducting crystals. Thus cadmium fluoride is the most ionic among the crystals where DX centers have thus far been found. The potential of using such crystals for optical information recording is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2130–2136 (December 1997)     

A theory of fusion of molecular crystals—II Phase diagrams and relations with solid state transitions

The theory of melting of molecular crystals with orientational degrees of freedom developed in a previous paper⁽¹⁾ is extended to cover disordering solid-solid transitions and complete phase diagrams for transitions under pressure. It is predicted that solid-solid and melting transitions should become second-order under certain conditions and also that a solid transition may appear at high pressures in some materials which show only a melting transition at zero pressure. The theory is compared with available experimental data.     

High pressure study of phase transitions inα-FePO4

High pressure behaviour of FePO₄ in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO₄ tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.     

Effect of correlated temperature fluctuations on the phase dynamics in an ultrathin lubricant film

The melting of an ultrathin lubricant film at friction between atomically smooth surfaces is studied with allowance for fluctuations of its temperature, which are described by the Ornstein-Uhlenbeck process. The behavior of the most probable types of shear stresses arising in the lubricant is considered, and phase diagrams for second-and first-order phase transformations (the melting of an amorphous lubricant and that of a crystalline lubricant, respectively) are constructed. It is shown that, in the former case, lubricant temperature fluctuations lead to the formation of a stick-slip friction domain separating the domains of dry and sliding friction, which is typical of first-order transitions. In the latter case, three domains of stick-slip friction arise, which mark the transitions between dry friction and metastable and stable sliding friction. As the time of correlation of lubricant temperature fluctuations gets longer, the temperature of rubbing surfaces rises to the point where sliding friction sets in.     

Structural studies of phase transitions in crystalline and liquid halides (ZnCl<Subscript>2</Subscript>, AlCl<Subscript>3</Subscript>) under pressure

The results of investigating the phase diagrams of ZnCl₂ and AlCl₃ halides, as well as the structure of the shortrange order of the corresponding melts under pressures up to 6.5 GPa, by the method of energy-dispersive x-ray diffraction are reported. When a ZnCl₂ crystal is compressed, a phase transition occurs from the γ phase (HgI₂ structure type) to the δ phase (distorted CdI₂ structure, WTe₂ type). The structural studies of the liquid state of ZnCl₂ and AlCl₃ indicate that the intermediate-range order decreases rapidly in the tetrahedral network of both melts as the pressure increases to 1.8 and 2.3 GPa for ZnCl₂ and AlCl₃, respectively. With further compression, the transitions in both melts occur with a change in the structure of the short-range order and with an increase in the coordination number. In this case, the transition in AlCl₃ occurs at ≈4 GPa and is a sharp first order transition, whereas the transition in ZnCl₂ occurs more smoothly in a pressure range of 2–4 GPa with a maximum intensity near 3 GPa. Thus, the AlCl₃ and ZnCl₂ compounds exemplify the existence of two phenomena—gradual decay of intermediate-range structural correlations and a sharper liquid-liquid coordination transition.     

Application of X-ray resonant diffraction to structural studies of liquid crystals

Liquid crystals are soft materials that combine the fluidity of disordered liquids and the long range orientational or positional order of crystalline solids along one or two directions of space. X-ray scattering is widely and generally successfully used to investigate and characterize the microscopic structure of most liquid crystals. In many cases however, the Bragg reflections are forbidden by special symmetries of the unit cell and the low dimensional structure of the liquid crystalline phases are out of reach of conventional X-ray experiments. We show in this paper that this problem can be overcome by resonant scattering of X-rays as it reveals the anisotropy of the tensor structure factor. We review various examples in which the restored forbidden reflections reveal unambiguously the hidden structure of liquid crystalline phases. Moreover, we show that in some cases, a fine analysis of the polarization of the Bragg reflections enables one to discriminate between different structural models. These studies solved long standing questions about biaxial liquid crystal structures and provided new insights into physical phenomena such as supercritical behaviour or commensurate-incommensurate transitions.     

High pressure studies of sodium and silver halides

Abstract

We have investigated the structural phase transitions in sodium and silver halides theoretically under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. Our results confirm the recent high pressure experimental observations of crystallographic phase transformations in sodium halides (Leger et al. (1998) J. Phys.: Condens. Matter, 10, 4201) and silver halides (Hull and Keen (1999) Phys. Rev., B59, 750. The calculated transition pressures agree with the experimental data.     

Phase Diagrams of Polytype Transformations in Close-Packed Crystals Including Metastable States

The paper offers a method for calculating phase diagrams of polytype transformations in close-packed crystals based on the generalized axial Ising's model of finite dimensions at non-zero temperatures. Using the Metropolis algorithm, the behavior of systems with polytype transitions is studied under the conditions of changing external shear stress and temperature. The longest polytype period considered here is taken to be 30 close-packed planes.