Phonon Spectroscopy of Oriented hcp Iron

High-pressure phonon spectroscopy was performed on iron in the bcc and hcp phase up to 40 GPa using the nuclear inelastic scattering (NIS) of synchrotron radiation (SR). In hcp iron we observe differences in the density of phonon states for spectra measured with different orientations of the diamond anvil cell (DAC) with respect to the SR beam. These differences are attributed to a preferred orientation of the hexagonal c -axis along the load axis of the DAC. These texture effects are used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the c -axis of hcp iron.     

Pressure calibration to 304 GPa on the basis of X-ray diffraction measurements of Pt,Fe and CsI

Abstract

X-ray diffraction measurements have been carried out above 300 GPa for the first time using Pt as a primary pressure standard. The equations of state of iron up to 304 GPa and of CsI up to 302 GPa have been obtained. These materials can therefore be used as secondary pressure calibrants.     

The investigation of cold sintering mechanism of iron powder compact

Abstract

In this paper the mechanism of cold sintering of iron powder compacts has been investigated. In the wide range of pressure between 4…5 and 12…12, 5 GPa, a considerable changes of properties, such as electrical resistivity, thermo-electric force, coercive force, changes in X-ray diffraction structures as well as changes of thermal properities (obtained by DTA analysis), have been determined.     

Shock adiabat features,phase transition macrokinetics,and spall fracture of iron in different phase states

Abstract

Results are presented of systematic experimental investigations of shock adiabat and evolution of multiwave configurations for a number of steels in the range up to 100GPa; of stress relaxation kinetics in the front of the elastic and phase precursor from different initial states: uσ_x = 15, 25, 49, and 59GPa; and of data on spa11 fracture of iron in α —, ε —, γ —, and liquid phases under conditions of uniaxial deformation as well as under volumetric omnidirectional tension. Changes in material microstructure and microhardness of samples subjected to shock wave loads of 15, 25, 200, and 1000 GPa are analyzed.

Investigations of steels with the initial state in α and γ phases of iron and with different original heat treatment show that, in stress waves of low intensity (σ_x? 10–25 GPa), as well as under quasi-static tension-compression tests and impact tests, differences in their behavior and properties are clearly displayed. As the amplitude of longitudinal stresses increases up to 200–250 GPa, differences in their compressibility and in the character of spal1 fracture begin to smooth down.     

High-Pressure Phase Relationships for FeS

Phase relationships for FeS polymorph have been studied at the pressure of 16-20 GPa and temperature of 300-1350 K by in situ X-ray observation using a large volume high-pressure apparatus and a synchrotron radiation. Contrary to Fei's prediction [1], we found the stability field of NiAs-type phase of FeS extending at least to 18 GPa. Results of in situ X-ray observation correspond with the NiAs-hexagonal phase boundary determined by Kusaba et al . [2, 3]. Assuming the straight NiAs-hexagonal phase boundary, we estimate that the NiAs-hexagonal-liquid triple junction is located at 39.5 GPa and 2300 K.     

Determination of Phonon Dispersion Curves at Gigapascal Pressures by Inelastic X-ray Scattering

The study of phonon dispersion curves of materials under hydrostatic pressure provides important information such as the evolution of sound velocities, elastic constants, interatomic potentials, phase transition mechanisms, etc. Until very recently, coherent inelastic neutron scattering was the only spectroscopic technique, which allowed performing these types of studies up to typically 10 GPa. Today, inelastic X-ray scattering with meV energy resolution provides a complementary spectroscopic technique, where, using diamond anvil cell techniques, pressures beyond 100 GPa can be reached.     

Experience in the design of free-cylinder pressure transducers to about 1 GPa

Abstract

Free-cylinder pressure transducers for pressure measurements to about 1.0 GPa have been developed; they use at present foil strain gauges bonded on the cylindrical surface in order to measure strains as a function of the applied hydrostatic pressure. All such transducers can be used with thin-or thick-film gauges; distortions of the active cylinder can be measured also by optical techniques. Pressure transducers of three types (different design as to pressure full scale and cylinder materials) are described and their metrological characteristics are given as well.     

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 diagram of GaAs

Abstract

Optical measurements in the diamond anvil cell (DAC) as well as thermodynamics, show cubic GaAs I to be unstable at 300 K, at 13 GPa. The thermodynamic phase line from GaAs I to the high pressure (H.P.) form(s) is at 11 ± 2 GPa. Large hyteresis makes the actual I→II transition observable only at 17.5 ± 1 GPa.     

Ab initio study of the structural,electronic, elastic and thermal conductivity properties of SrClF with pressure effects

Abstract

SrClF is an important optical crystal and can be used as pressure gauge in diamond anvil cell at high pressure. In this work, we performed a systematic study on the structural, electronic and elastic properties of SrClF under pressure, as well as its thermal conductivity, by first-principles calculation. Different exchange-correlation functionals were tested and PBESOL was finally chosen to study these properties of SrClF. Studies reveal that SrClF has a bulk modulus of about 56.2 GPa (by fitting equation of states) or 54.3 GPa (derived from elastic constants), which agree well with the experimental result. SrClF is mechanically and dynamically stable up to 50 GPa. Its elastic constants increase with the applied pressure, but its mechanical anisotropy deteriorates as the pressure increases. Investigation of its electronic properties reveals that SrClF is a direct band-gap insulator with a gap value of 5.73 eV at 0 GPa, which decreases with the increasing pressure and the reason is found by analysing the partial density of states. Based on the calculated phonon dispersion curves, thermal conductivity of SrClF is predicated. At ambient conditions, the predicted thermal conductivity is about 3.74 Wm^?1 K^?1, while that obtained using the simplified Slack model give a slightly larger value of 4.62 Wm^?1 K^?1.     

Coordination changes in crystalline and vitreous GeO2

Abstract

High pressure x ray absorption spectroscopy (XAS) has been performed up to 29 GPa on crystalline and amorphous GeO₂. The modification of the x ray absorption near edge structure (XANES) as well as the variation of the Ge-O distance, indicate that the coordination of Ge changes from 4 to 6 above 6.5 GPa. The transition is confirmed by Raman spectroscopy.     

High-pressure Raman study of SnGeS3

Abstract

We report Raman-scattering studies of SnGeS₃ under hydrostatic pressures up to 19.5 GPa. An assignment to internal-external modes is proposed, based on the pressure slopes obtained. Our data show evidence for two critical pressures, one around 7 GPa and a second one around 12 GPa. The material renders itself Raman inactive at 19.5 GPa. The observed changes are reversible upon pressure release.     

Synthesis experiments on in-Sb and B-Sb systems in a laser heated diamond-anvil cell

Abstract

High pressure and high temperature synthesis experiments were carried out on In-Sb and B-Sb systems with a laser heated diamond-anvil cell. InSb was synthesized starting from In and Sb at various pressures ranging from 0.2 to 10 GPa. The cubic as well as the high pressure phases were successfully synthesized. Experiments above 8 GPa, wherein antimony exists in the tetragonal phase, have revealed no new phases of InSb. Trials of synthesizing a compound from B and Sb gave negative result to at least 30 GPa.     

Phase transitions in poly(ethylene oxide) and polystyrene at high pressure

Abstract

The temperature and enthalpy of melting for poly(ethy1ene oxide) have, for the first time, been studied as a fuction of pressure up to 1 GPa by means of differential scanning calorimetry. The initial increase of the temperature of melting with increasing pressure is 64 K/GPa, whereas the enthalpy decreases by 40% in the 1 GPa pressure range. Using Clausius-Clapeyrons equation the volume change on melting is estimated to be 1.5 cm³/mol. The glass transition temperature T_g for polystyrene has also been studied by the same technique for pressures up to 0.1 GPa. The measurements show that T_g increases with increasing pressure by 250 K/GPa.     

Thermal equation of state of goethite (α-FeOOH)

ABSTRACT

The pressure–volume–temperature (P–V–T) equation of state (EoS) of natural goethite (α-FeOOH) has been determined by an X-ray diffraction study using synchrotron radiation. Fitting the volume data to the third-order Birch–Murnaghan EoS yielded an isothermal bulk modulus, B₀ of 85.9(15)?GPa, and a pressure derivative of the bulk modulus, B′, of 12.6(8). The temperature derivative of the bulk modulus, (?B/?T)_P, was –0.022(9)?GPa?K^?1. The thermal expansion coefficient α₀ was determined to be 4.0(5)?×?10^?5?K^?1.     

Pressure dependence of zone-boundary phonons in AlSb

Abstract

We have investigated the effect of hydrostatic pressure on zone-boundary and other critical-point phonon frequencies of AlSb by second-order Raman scattering. A softening of the TA(X), TA(L) and (L/T)A([Sgrave]) modes has been observed for pressures up to the first phase transition at 7.7 GPa. The LA(L) as well as the optical TO at X-, L-, and LO at [Sgrave]-, X-points harden with increasing pressure. Mode Griineisen parameters of all the resolved modes were calculated. Reflectivity measurements indicate that the high pressure phase above 7.7 GPa is metallic.     

Metrology of high pressures to approximately 3.0 Gpa

Abstract

The pressure scale up to approximately 3. 0 GPa is considered in order to improve pressure measurement accuracy.

The main subjects presented are related to absolute and gage pressure measurements with gases to about few megapascal and gage pressure measurements in liquids to about 1. 5 GPa by means of piston gauges.

The use of a pressure scale based on fixed points is mentioned.     

Dependence of the annihilation pressure of ruby R-line fluorescence on the laser exciting wavelength

Abstract

Using solid argon as pressure medium, quasi-hydrostatic pressure was obtained at room temperature in the diamond cell up to 90 GPa. The mechanism of the disappearance of ruby R lines and the applicability of ruby pressure scale under quasi-hydrostatic pressure above 100 GPa was discussed. The deviation of every pressure measured at nine positions in the cell per mean pressure was less than 1.5% at pressure below 80 GPa.     

Comparative Study on Pressure-Induced Structural Changes between C 6 O 2 I 4 and C 6 I 6

We have investigated the pressure effects on the structural properties of C 6 O 2 I 4 up to 39 GPa by powder x-ray diffraction measurements, which were compared with those of C 6 I 6 . The diffraction patterns of C 6 O 2 I 4 indicated a phase transition starting at 26.8 GPa. The mixed state of the low- and high-pressure phases continued up to 39 GPa well above an insulator-to-metal transition pressure of 33 GPa. The C 6 O 2 I 4 molecule remains planar structure in the low-pressure phase below 26.8 GPa in contrast to the non-planar molecular structure of C 6 I 6 at ambient and high pressures.