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.     

The influence of high pressure on the solidification of supercooled Se melt

Abstract

Under pressures between 1.75 and 8 GPa a supercooling of Se melt and an average grain size of the samples quenched under pressure with constant cooling rate 100 K/s were measured. In framework of the classic theory of nucleation and grain growth the numerical evaluation of surface tension and activation energy of crystal growth was performed. The comparison of properties of supercooled Se melt to that of Pb and In reveals anomalies on the pressure dependences of Se melt properties.

This circumstance is discussed in connection with the semiconductor-metal transition discovered earlier in Se melt.     

Pressure effect on emission spectra and crystal field for La OBr : Eu

Abstract

The emission spectra for LaOBr : Eu were measured at pressures up to 13 GPa and room temperature. The pressure dependences of levels of ⁷F_0,1,2,3,4 and ⁵D_0,1,2 are given. The crystal field parameters B^k _q were computed by fitting the experimental levels. The strength of crystal field decreases with increasing pressure. A brief discussion on the observed phenomena is presented.     

Discussion of the generalized phase diagram for ‘regular’ lanthanides under pressure (Abstract)

Abstract

X-ray diffraction studies on the kinetics and hysteresis of the structural phase transitions in lanthanides under pressures up to 58 GPa and temperatures between 200 and 520 K are presented. Estimates of the 0 K equilibrium transition pressures are derived from the pressure and temperature dependence of the activation free energies Δ_aG. A comparison of critical radius ratios, R_ws/R_5p, for all the regular lanthanides at the various phase transitions shows simple systematics in the high pressure behaviour of the lanthanides. The “volume collapse” transitions in lanthanides are compared with the behaviour of the actinides and discussed with respect to f-electron delocalization.     

The excitonic absorption of CdS under pressure

Abstract

We have investigated the pressure dependence of the absorption edge of tlie E_A-E_B- and the E_C-excitons of 1pi thick CtlS(hex) crystal slabs. We obtaiurd dE_Θ/dP =dE_c/dP = 47±3 meV/GPa for T=300K and dE_A/dP= dE_B/dP = 43±3 meV/GPa for T=78K. With tliese values we were able to determine the crystal-field splitting (68f4 nieV) aid the spin-orbit splitting (26±2 meV). The resulting pressure shifts of tlie rxcitonic energies were in agreement witli a κ · p-calculation of tlie transition energies using tlie perturbation niatrix of G.E. Pikus for Δ_so ? Δ_c. The refractive Index n‖c was iiieasured for T=300K up to 2.2 GPa and described by Marples Model.     

Pressure-induced phase changes of cubic monoolein - Cytochrome C mesophases

Incorporation of the protein cytochrome c (cyt c) into the hydrated bicontinuous Ia3d cubic mesophase of monoolein (MO) was investigated within a wide range of pressures by small-angle X-ray scattering (SAXS). We found that incorporation of cyt c into the cubic phase of MO has a drastic effect on the structure and pressure stability of the system: At high pressures, the lipid systems with less than 0.2 wt.% embedded protein undergo a transition to a fluid lamellar phase with smaller partial molar lipid volume. Incorporation of cyt c at levels above 0.2 wt.% promotes the formation of a new cubic phase, probably a cubic micellar phase of space group P4₃32 (Q²¹²) whose pressure stability rises with increasing protein content.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Hydrostatic pressure effect on the metal-insulator transition in LaO0.7Ca0.3Mn1−x(Fe/Ge)xO3 perovskites

Abstract

The temperature dependences of the resistivity of La_0.7Ca_0.3Mn_1?xFe_xO₃ and La_0.7Ca_0.3Mn_1?xFe_xO₃ (0 < × < 0.04) mixed crystals were studied under hydrostatic pressures up to 15kbar. The substitution of Fe for Mn results in an increase of the resistivity and a continuous decrease of the metal-insulator transition temperature T_mi while the substitution of Ge for Mn leads to a more complicated T_mi(x)-curve. In all cases T_mi shifts under pressure with a rate between 1.6 and 2.9K/kbar and a correlation between T_mi and its pressure derivative dT_mi/dP is observed which is in accordance with the general trend of dT_mi/dP versus T_mi as derived for other manganites and is discussed in terns of a competition between superexchange and double exchange.     

Pressure and temperature effects on the Γ, N and L-phonons in zirconium

Abstract

The effective potentials for the E_2g,-phonon at the T point of the Brillouin zone of hep Zr, the transverse N- and longitudinal L-phonons of bcc Zr are calculated for different pressures by the “frozen phonon” method. The temperature and pressure dependences of the phonon frequencies are studied within the framework of a modified pseudo-harmonic approximation. The results obtained are in good agreement with the experimental ones. The stability of the hep, bec and ω phases of zirconium at different temperatures and pressures is discussed.     

First-principles study of structural,elastic, electronic and optical properties of RDX under pressure

ABSTRACT

The influences of pressure on structural, elastic, electronic and optical properties of α-RDX under pressure from 0 to 40?GPa have been investigated by performing first-principles calculations. The obtained structural parameters based on the GGA-PBE+G calculations are consistent with previous experimental values. The results of B/G, C₁₂-C₄₄ and Poisson's ratio show that α-RDX has changed to ductility under pressure between 0 and 5?GPa. The obvious rotation of NO₂ group in the equatorial position appears, especially in the range of pressure from 10 to 15?GPa, which influences the elastic and mechanical properties of α-RDX. Moreover, we find that the electrons of α-RDX become more active under higher pressure by comparing the curves of DOS under different pressure. Furthermore, the anisotropy of optical properties under different pressures has been shown.     

Probing the local,electronic and magnetic structure of matter under extreme conditions of temperature and pressure

ABSTRACT

In this paper we present recent achievements in the field of investigation of the local, electronic and magnetic structure of the matter under extreme conditions of pressure and temperature. These results were obtained thanks to the coupling of a compact laser heating system to the energy-dispersive XAS technique available on the ID24 beamline at the ESRF synchrotron. The examples chosen concern the melting and the liquid structure of 3d metals and alloys under high pressures (HPs) and the observation of temperature-induced spin crossover in FeCO₃ at HP.     

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.     

Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

ABSTRACT

The potential-energy surfaces of the amino radical (NH₂) with IO reaction have been studied at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level. Two kinds of pathways are revealed, namely H-abstraction and addition/elimination. Rice–Ramsperger–Kassel–Marcus theory and transition state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that, at atmospheric pressure with N2 as bath gas, the formation of P1 (HI?+?HNO) is the dominant pathways at 200–700?K, while the direct H-abstraction leading to P3 (³NH?+?HOI) takes over the reaction at a temperature above 700?K. At the high-pressure limit, IM1 [IONH₂] formed by collisional stabilisation is dominant at 200–700?K; the direct H-abstraction resulting in P3 (³NH?+?HOI) plays an important role at higher temperatures. However, the total rate constants are independence on the pressure; however, the individual rate constants are sensitive to pressure. The atmospheric lifetime of NH₂ in IO is around one week. TD-DFT computations imply that IM1 [IONH₂], IM1A [IONH₂′], IM2 [IN(H₂)O], IM3 [OINH₂], IM4 [HOINH], tra-IM5 [tra-HON(H)I] and cis-IM5 [cis-HON(H)I] will photolyze under the sunlight.     

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.     

New trends in baro-enzymology

Abstract

The recent interest on high pressure biotechnology (mainly in food industry) requires fundamental studies on the pressure behavior of biochemical constituants¹ In this laboratory, we use pressure as a thermodynamic parameter, such as temperature, to determine the energetic quantities of enzyme reactions². Two essential requirements for the study of the mechanism of enzyme action are that, first, a simple rate constant has to be measured (determination of a composite rate constant, such as kcat, can lead to ambiguous results) and, second, a maximum number of physico-chemical ways must be used to perturb the system under study. To assess this simple usually very rapid rate constant, cryoenzymology was used. It was then possible to probe, at constant pressure, the thermodynamics of the interconversion of two successive intermediates, thereby obtaining the classical ΔG? ΔS? and ΔH? parameters. Ify however, we can also vary another intensive parameter, namely the pressure, it is possible to determine the activation volume (ΔV?) of the reaction. In addition to pressure and temperature, a third variable has to be considered also : the nature of the cryosolvent which allows experiments to be extended to subzero temperatures in the first place (with the medium being kept fluid) but which can also act as a perturbing agent thereby inducing controlled and reversible changes in equilibrium and me processes. The interdependence of the two variables, namely temperature and pressure, which is predicted by the general equation for the standard variation of free energy ΔG = f(T, P), is presented. This approach will be illustrated using different model reactions where data are analysed according to the classical transition state theory.     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

High pressure,high resolution synchrotron x-ray powder diffraction with a position-sensitive detector

Abstract

We report results of high-pressure experiments with a new diamond-anvil cell in a monochromatic, high-resolution x-ray scattering geometry with alinear position-sensitive detector. The experiments make possible the study of factors controlling line widths of diffraction profiles at pressures in the 100 GPa range, and demonstrate the potential for the use of line profile analysis and Rietveld refinement techniques with high-pressure powder diffraction data. Combined data for various materials indicate that relative contributions to linewidths due to particle size, intrinsic material strength, pressure and state of stress in the sample can be resolved. With light rare-gas solids as pressure-transmitting media, measured FWHMs of the order 0.03? 2 θ corresponding to resolution Δd/d of 2.5 × 10^?3 for 2θ～10-15? are reported. Formation of a high pressure phase appears to involve growth of submicron domains, judging from substantially broadened diffraction peaks under quasihydrostatic conditions. Detailed analysis of complex, non-quenchable high-pressure phases will likely require annealing techniques such as thermal cycling at pressure.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Phonon-assisted luminescence anomalies in Eu2+ doped Ca1-xSrxF2 mixed crystals

Abstract

With increasing X on Ca_1-xSr_xF₂ mixed crystals doped with Eu²⁺ (4f⁶5d) ions, the absorption peak shifted linearly to that of SrF₂:Eu, while the luminescence one moved parabolically with a minimum at around x=0.5. The linewidth broadened nearly twice as large as that of CaF₂ in the vicinity of x=0.5 and subsequently renarrowed for x > 0.5. The temperature dependences of the luminescence intensity and life time revealed that the nonradiative activation energy has a minimum of 0.40 eV around x=0.5. From the calculated result of the lineshape, these non-linear behaviors against x were explained by adjusting the electron-phonon coupling strength.     

Anisotropy of Linear Thermal Expansion and Compressibility of Y 2 Fe 17 Under Pressure and its Correlation to Magnetic Structure

A complex temperature dependence of a.c. susceptibility of Y 2 Fe 17 under high pressures together with recent neutron diffraction studies under pressure proved the instability of the collinear ferromagnetic structure and the development of a non-collinear magnetic arrangement of Fe moments in Y 2 Fe 17 . To study the correlation between magnetic structure and volume in more detail we performed compressibility and linear thermal expansion studies under high pressures up to 100 kbar and 10 kbar, respectively. The compressibility in the paramagnetic state s P (above 10 kbar at room temperature) was determined from the Murnaghan equation of state using the X-ray data, \kappa_{P}=0.80\ {\rm Mbar}^{-1} . The linear thermal expansion and compressibility in the ferromagnetic state at low temperatures are highly anisotropic. As a consequence, the c/a ratio decreases with increasing pressure. The magnetic phase diagram of Y 2 Fe 17 compound was compiled up to 20 kbar.     

Phase relations in the Fe-P system at high pressures and temperatures from ab initio computations

ABSTRACT

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth’s core and temperatures up to 4000?K were determined. A new low-temperature modification FeP-P2₁/c stable above ～75?GPa was predicted. Fe₂P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750?K it transforms into the barringerite structure. The transition from Fe₃P with schreibersite structure to Fe₃P-Cmcm was observed at 27?GPa, and the phase transition boundary is nearly isobaric. Fe₂P and FeP are thermodynamically stable at the Earth’s inner core pressures and 0?K according to the obtained results, whereas Fe₃P stabilizes with respect to decomposition to Fe?+?Fe₂P at high temperatures above ～3200?K.