在石英界面处液态水的冲击结构相变

利用轻气炮加载技术和光透射测量技术, 观测了冲击加载过程中水/石英界面处的水结构变化.实验发现,冲击条件(0.5—2 GPa, 335—375 K)下水在液相区内能够发生结构改变且起始于水/石英界面,结构改变的速率和程度与石英界面的特性有关.证实在固/液相边界一定区域内的液态水,在经历高温高压状态的变化中表现出特殊的相转变现象.同时,研究表明液态水结构转变的过程区分为明显的四个动力学阶段.     

Phase transformations of the amorphous Zn-Sb alloy under high pressures

Abstract

Phase transformations occurring in initially amorphous Zn₄₁ Sb₅₉ semiconductor at pressures to 10 GPa and temperatures to 350C were studied using the measurement of electrical resistance, in situ energy dispersive X-ray diffraction and neutron diffraction on quenched high-pressure phases at ambient pressure. The studied T- P region involves the regions of reversible and irreversible crystallisation and phase transitions between the equilibrium crystalline low-pressure and high-pressure phases.     

Kinetics and systematics of structural phase transitions in the regular lanthanide metals under pressure

Abstract

Techniques and results of studies on the kinetics and hysteresis of the structural phase transitions in lanthanides under pressures up to 58 GPa and temperatures between 200 K and 520 K are presented. The transformation rates show the same time dependence as for diffusion controlled transitions, however, other interpretations of this time dependence are also possible. Estimates of the 0 K equilibrium transition pressures are derived from the pressure and temperature dependence of the activation free energies A, G. A comparison of critical radius ratios, R_ux/R_5p for all the regular lanthanides at the various phase transitions shows simple systematics in the high pressure behaviour of the lanthanides.     

On the possibility of the formation of a NaCl–KCl solid-solution crystal from an aqueous solution at room temperature in small-volume systems

Phase transitions in binary and ternary small-volume systems have been simulated by the methods of equilibrium chemical thermodynamics. Considerable dissimilarities of the equilibrium phase compositions of the systems of macroscopic and microscopic sizes have been revealed. A change in the system’s volume is accompanied by a change in the heterogeneity region in the phase diagram. This can increase considerably the solubility of small systems and lead to the emergence of phases that are thermodynamically unstable in macroscopic systems. Such size effects have been considered by the example of phase transformations in NaCl–KCl–H₂O and NaCl–KCl systems.     

Specific features of the electrical properties upon smeared phase transitions in multicomponent ferropiezoelectric ceramics based on lead zirconate titanate

The reverse dependences of the permittivity ? _r ^′ (E ₌) and the polarization and depolarization currents in multicomponent ferropiezoelectric ceramics based on lead zirconate titanate (PZT) are investigated over a wide range of temperatures. The results obtained make it possible to separate the effects associated with the phase transformation and the effects predominantly caused by the switching of the domain structure in the studied material. The assumption is made that two smeared phase transitions occur in the system under consideration.     

Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures

Abstract

X-ray diffraction has been measured for crystalline silicon, crystalline germanium, amorphous silicon and amorphous germanium at temperatures down to 100 K and pressures up to 20 GPa using a diamond anvil cell and synchrotron radiation. The structural phase transitions, including amorphization, take place in the pressure-temperature range. It has been found that the structures after the phase transitions strongly depend on the path in the pressure-temperature diagram through which the system undergoes the phase transitions. For any of the aforementioned four materials, the high-pressure phase with the p-Sn structure is quenched during a release of pressure at 100 K, and transforms into an amorphous state when heated up to around 2 GPa. The path dependence of the states is discussed in relation to the pressure dependence of the heights of the energy barriers which have to be overcome when phase transitions occur. The effect of a structural disorder on the phase transition is also discussed by comparing the experimental results for the crystalline and amorphous materials.     

Size effects under martensitic deformation of shape-memory alloys

The effect of the grain size and film thickness on the parameters of martensitic transitions and deformation behavior of shape-memory alloys is studied in terms of the theory of smeared martensitic transitions. Comparison of theoretical results with experimental data suggests that the transformation kinetics related to the growth mechanism and shrinkage of martensite lamellas when transformation steps (dislocations) move along their boundaries is most sensitive to the size factor.     

Nonlinear responses of electronic-excitation-induced phase transformations in GaSb nanoparticles

We studied electronic-excitation-induced phase transformations in nanoparticles using transmission electron microscopy. GaSb particles excited by 75 keV electrons transform to two phases consisting of an antimony core and a gallium shell or an amorphous phase, or remain in the original crystalline phase, depending on particle size and/or temperature. It is suggested that such nonlinear responses of the phase transformations may arise from synergistic effects of bond instability, localized excitations, enhanced diffusivity, or thermal equilibrium in reactions.     

Measurement of Seebeck effect (thermoelectric power) at high pressure up to 40 GPa

The paper reports details of a high-pressure thermoelectric power (Seebeck effect) technique up to 40 GPa. Several different types of high-pressure cells with anvil insets are presented. The technique was applied for measurements of pressure dependence of the thermopower of several substances including elemental metals (lead, Pb; indium, In), cerium-nickel alloy, Ce-Ni and sulphur, S. Two peculiarities in the pressure dependences of the thermopower of CeNi were found and attributed to structural transformations, near ∼5 and ∼10 GPa. These transitions were confirmed in direct X-ray diffraction studies. Sulphur compressed to 40 GPa exhibited a hole type conductivity and the thermopower value was about ∼+1 mV/K. Additionally, as an example of pressure calibration, the data on the electrical resistivity of zinc selenide, ZnSe, are given in a range of 0-23 GPa. These data suggest three possible scenarios of phase transitions from a rock salt (RS) high-pressure phase of ZnSe under decompression: RS→zinc blende (ZB), RS→cinnabar→ZB, and RS→wurtzite.     

Martensitic transformation and electrical properties of a Ni<Subscript>2.14</Subscript>Mn<Subscript>0.81</Subscript>Fe<Subscript>0.05</Subscript>Ga alloy in its different structural states

Phase transformations in a Ni_2.14Mn_0.81Fe_0.05Ga alloy in different structural states are studied from the temperature dependences of its electrical resistivity. The dependences obtained indicate that, in the coarse-grained state, this alloy undergoes two structural phase transformations: intermartensitic modulation transformation and martensite-austenite transformation. In the nanocrystalline state, these transformations are absent. The recrystallization of a nanocrystalline sample at 773 K for 30 min results in the martensite-austenite transformation; however, the phase transformation related to a change in the martensite modulation period does not occur in this state. The resistivity is shown to depend on the structural state of the alloy.     

Phase transformations in films deposited by laser ablation of Hf in an oxygen Atmosphere

The structure of the hafnium dioxide films grown during pulsed laser sputtering of an Hf target in an oxygen atmosphere is examined, and the phase transformations that occur in them at annealing are studied by transmission electron microscopy and electron diffraction. Amorphous, tetragonal, orthorhombic, and monoclinic HfO₂ phases are detected in the films. The tetragonal modification of HfO₂ exhibits an epitaxy effect on a KCl(001) substrate. When the amorphous film is annealed in vacuum or air, it crystallizes to form the monoclinic modification of HfO₂. The action of an electron beam on the amorphous film in vacuum leads to the formation of the orthorhombic and monoclinic modifications of HfO₂. The transformation from the orthorhombic to the monoclinic modification of HfO₂ is accompanied by a phase size effect. At the final stage of crystallization, the monoclinic modification represented by HfO₂ dendrite crystals is the predominant structural constituent.     

Analysis of the parameters of a smeared orientational transition at 250–260 K in C60 crystals

The orientational structural transformation in C₆₀ crystals at temperatures of 250–260 K is investigated within the theory of smeared first-order phase transitions. The parameters of this transformation are analyzed using the experimental temperature dependences of the heat capacity and the inelastic (ferroelastic) strain rate in the phase transition range. The elementary volume of the transformation (11–83 nm³) in a correlated motion of C₆₀ molecules and the spontaneous shear strain of the lattice (2.4×10^?2) upon its transformation from the simple cubic to the face-centered cubic structure are determined.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Phase transitions in ferromagnetic Ni2+x Mn1−x Ga alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni_2+x Mn_1?x Ga is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter.     

Polymorphic transitions in molecular crystals—III.: Transitions exhibiting unusual behavior

Some polymorphic transitions in molecular crystals, among the few classified earlier as displacive or martensitic on the basis of the existence of a definite orientation relationship between the lattices of the phases and the “instantaneous” transformation rate, have been experimentally investigated. The investigation showed no evidence of a high rate of transition. All occurred through nucleation at the lattice imperfections and continuous growth of the new phase at a rate depending on ΔT = T_tr ? T_a, where T_tr is the actual transition temperature and T_o is the temperature of the thermodynamic equilibrium of the phases. The interface can be observed; its motion can be controlled, be done so slow as it is desirable, or be stopped. The “instantaneous transformations” reported in the literature are caused by temperature delays of nucleation in the absence of appropriate lattice defects. In reality these are polymorphic transitions which appear to be instantaneous only when observed through an optical microscope; actually they occur at the rate corresponding to ordinary temperature dependence. It was established that lamination or cleavage of the crystals in a definite direction is a common feature of the substances exhibiting “atypical” behavior. (Typical behavior has been described in [1,2]). The observed characteristics of the polymorphic transitions (uniform direction of the interfaces, definite orientation relationship of the lattices, small temperature hysteresis of the transition, etc.) can be readily explained in terms of epitaxial growth of the new phase on the cleavage planes of the original phase. At the same time the role of the proximity of crystal structural parameters is elucidated. Simple model calculations showed that a coherent interface may occur when the structural differences in the junction plane of the two phases are small, while in the other cases the contact interface described in [2] is energetically advantageous. However, a cooperative rearrangement at the coherent interface is rejected. There is not any evidence that polymorphic transitions in molecular crystals may occur by means of a “shift”, a “displacement”, a “deformation”, an “overturn”, etc. It is concluded that they always occur as a result of growth. The molecular mechanism of the growth in solids has been described in [2].     

Phase transformations in crystalline Ti-Ni-Cu alloy upon severe plastic deformation

X-ray diffraction and transmission electron microscopy were used to study features of the structural and phase transformations of initially crystalline T1₅₀Ni₂₅Cu₂₅ alloy upon severe plastic deformation in a Bridgman cell. Three cycles of successive phase transitions of the crystal ⟹ amorphous state and amorphous state ⟹ crystal type were revealed under continually increasing deformation. The results are explained within the superposition of different channels of elastic energy dissipation at severe plastic deformation.     

Amorphization under shock loading

Abstract

The problem of crystalline to amorphous (c → a) phase transformation, in the context of shock loading, is analyzed. The studies under static pressures show that beyond the stability field of the ambient phase, an amorphous phase may be realized if the kinetic conditions are not favorable to the accessibility of equilibrium phase of higher coordination. The examination of shock results of some of these substances reveals that the so-called “mixed phase” region begins at the pressureswhich are same as that oc c → a transformation in the static experiments. Further the Hugoniot temperatures at these pressures are far lower than required for the realization of the high pressure crystalline phase. From these observations as well as from an understanding gained through static experiments, we infer that these materials amorphize under shock loading. The analysis indicates that the existence of hot-spots is not essential to the formation of diaplectic glass. Instead, it seems to emerge directly from the solid-solid phase transformation, adequately aided by favourable kinetic conditions.     

Thermodynamic analysis of ageing-induced multiple-stage transformation behaviour of NiTi

Near-equiatomic NiTi shape memory alloys normally exhibit three martensitic transformations among three phases: the B2 phase, the monoclinic (M) phase and the rhombohedral (R) phase. Some recent work, however, has revealed complex transformation behaviour involving multiple-stage martensitic transformations and multiple-stage R-phase transformations. This paper presents an analysis of these complex transformation behaviours based on thermodynamic concepts of reversible and irreversible energies associated with the transformations. The analysis is successful in identifying all observed transformations and in defining relative positions of various stages of transformations on a temperature scale. It also defines positions of thermodynamically prohibited transformations as well as permitted transformations that have not been experimentally measured. Such identifications enable the determination of actual transformation hystereses that are not directly measured experimentally. Based on the thermodynamic principles adopted, the analysis also renders it possible to identify the possible causes that contribute to the complex multiple-stage transformation behaviour.     

Crossover between the thermodynamic and nonequilibrium scenarios of structural transformations of H<Subscript>2</Subscript>O I<Emphasis Type="Italic">h</Emphasis> ice during compression

A detailed investigation of different scenarios of structural transformations of H₂O Ih ice during compression to a pressure of 2 GPa in the temperature range from 77 to 200 K is performed. In the range of temperatures and pressures being treated, detailed data are obtained for the density and the shear modulus for different phases of ice including the hda, IX, and XII phases. The experimentally obtained correlations for the density and ultrasonic velocities, with due regard for the available data of structural investigations, are used to identify the transformation sequences Ih→hda (below 135 K), Ih→II→VI (above 165 K), and Ih→IX→VI (from 155 to 180 K). In the vicinity of the crystallization temperature of amorphous ice, i.e., at about 140 K, an anomalous transformation pattern is observed, which is interpreted as predominantly the Ih→XII phase transition. The temperature crossover is discussed between the mode of solid-phase amorphization (Ih→hda) and crystal-crystal transitions, as well as the metastable nature of IX ice and the mechanism of solid-phase amorphization.     

Atomic-level simulations of nanoindentation-induced phase transformation in mono-crystalline silicon

Molecular dynamics (MD) simulations of nanoindentation are carried out to investigate the phase transformations in Si with a spherical indenter. Since the phase transformation induced by deformation in micro-scale is closely related to the carrier mobility of the material, it has become a key issue to be investigated for the chips especially with smaller feature size. Up to now, however, it is not possible to carry out the nanoindentation experimentally in such a small feature. Consequently, molecular dynamic simulation on nanoindentation is resorted to and becomes a powerful tool to understand the detailed mechanisms of stress-induced phase transformation in nano-scale. In this study, the inter-atomic interaction of Si atoms is modeled by Tersoff's potential, while the interaction between Si atoms and diamond indenter atoms is modeled by Morse potential. It is found that the diamond cubic structure of Si in the indentation zone transforms into a phase with body-centred tetragonal structure (β-Si) just underneath the indenter during loading stage and then changes to amorphous after unloading. By using the technique of coordinate number the results reveal that indentation on the (0 0 1) surface exhibits significant phase transformation along the <1 1 0> direction. In addition, indentation on the (1 1 0) surface shows more significant internal slipping and spreading of phase transformation than on the (0 0 1) surface. Furthermore, during the indentation process phase transformations of Si are somewhat reversible. Parts of transformed phases that are distributed over the region of elastic deformation can be gradually recovered to original mono-crystal structure after unloading.