Laser shock XAFS studies at OMEGA facility

ABSTRACT

State-of-the-art laser facilities offer an excellent opportunity for studying materials at Mbar-Gbar pressures by dynamical compression. This paper summarizes recent experiments on EXAFS measurements of compressed solid iron up to 5?Mbar using OMEGA laser facility. The X-ray source is produced by a spherical implosion, providing enough brightness and spectral smoothness required for EXAFS measurements. The compression path is tuned by laser pulse shaping to achieve off-hugoniot states. With an anharmonic model, the density, temperature and upper limit of strength of the compressed iron are determined from EXAFS data. Prospects of XAFS study of other materials are also discussed.     

Laser-shock-driven laboratory measurements of the equation of state of hydrogen isotopes in the megabar regime

Abstract

A high intensity laser has been used to shock-compress liquid deuterium to pressures from 0.22 Mbar, near the upper end of gas-gun shock data, to 3.4Mbar. Using a variety of diagnostics, the density, pressure, and reflectivity of the shocked deuterium were determined, culminating in a knowledge of the behavior of hydrogen isotopes along the principal Hugoniot into the Mbar regime. It was found that hydrogen isotopes become highly compressible near 1 Mbar as a result of molecular dissociation and simultaneous ionization. Reflectivity measurements confirmed that deuterium transitions from an insulating state to a conducting phase at pressures near 0.5 Mbar on the Hugoniot and appears to do so in a continuous manner. At pressures above 0.5 Mbar, the measured reflectivity of the shocked deuterium is characteristic of a liquid metal.     

Siting and dynamics of Cu impurity in Ti lattice

EXAFS measurements on dilute Cu in Ti were done to clarify the mechanism of “amomalous” diffusion. Cu was substitutional, and the Cu-Ti and Ti-Ti force constants were about equal, and less than that of Cu-Ci. Implications for diffusion mechanisms are discussed.     

Shock-induced transformation of Al2O3 and LiF into semiconducting liquids

Shock compression of sapphire (Al2O3) and lithium fluoride (LiF) to pressures above 5 Mbar has been observed to transform these transparent, wide band-gap insulators into partially degenerate liquid semiconductors with optical reflectivities of several percent. Reflectivities rise steadily with shock pressure up to 45% in sapphire at 20 Mbar and 20% in LiF at 13 Mbar. Using a simple model, the electron scattering length was inferred to be approximately the interatomic distance. In addition, several equation-of-state points at these pressures were measured.     

Laser-driven plasma loader for shockless compression and acceleration of samples in the solid state

A new method for shockless compression and acceleration of solid materials is presented. A plasma reservoir pressurized by a laser-driven shock unloads across a vacuum gap and piles up against an Al sample thus providing the drive. The rear surface velocity of the Al was measured with a line VISAR, and used to infer load histories. These peaked between approximately 0.14 and 0.5 Mbar with strain rates approximately 10(6)-10(8) s(-1). Detailed simulations suggest that apart from surface layers the samples can remain close to the room temperature isentrope. The experiments, analysis, and future prospects are discussed.     

Accuracy evaluation in temperature‐dependent EXAFS measurements of CdTe

The evaluation of uncertainty in temperature‐dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K‐edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest‐neighbours parameters: bond thermal expansion, parallel and perpendicular mean‐square relative displacements and the third cumulant. Different causes of uncertainty, a comparison of experimental results with theoretical models, the difference between EXAFS and crystallographic thermal expansions and the meaning of the third cumulant are discussed.     

Temperature dependence of EXAFS in amorphous arsenic

EXAFS measurements at different temperatures on amorphous arsenic are reported. The low values of the EXAFS mean square relative displacement σ² for the first coordination shell are consistent with a distribution of As₄ pyramidal units throughout the amorphous network, giving a locally very ordered first shell, whose disorder is essentially dynamic in character. The temperature dependence of the vibrational contribution to the EXAFS Debye-Waller factor is well described by an Einstein oscillator model.     

EXAFS measurements of bond-stretching force constants in arsenic and arsenic compounds

The Einstein frequency ν_E obtained from temperature-dependent EXAFS measurements for the bond between an arsenic atom and a nearest neighbor in arsenic and certain arsenic halides and chalcogenides is considered. It is shown theoretically that in these materials ν_E should be approximately equal to the “natural” frequency of the bond, and hence that ν_E² is proportional to the bond-stretching force constant ⨍_r. Experimental EXAFS measurements on crystalline (c-) AsBr₃ and AsI₃, glassy (g-) As₂S₃ and As₂Se₃, and amorphous (a-) As yield force constants in reasonable agreement with those obtained from Raman and infrared studies.     

EXAFS measurements of lattice vibrations of Ti and Cu in Ti

Lattice vibrations of Cu, Ti, and a dilute solution of Cu in Ti were studied by means of the EXAFS Debye-Waller factor. The pure metals had the same first-shell mean-squared relative displacement (MSRD). This was expected since they have similar moduli, atomic masses, and X-ray diffraction Debye-Waller factors. The MSRD was linear in the $\text{3}\text{2}$ power of absolute temperature, and was about twice as great as for Cu in Ti as for Ti or Cu. This difference suggests a difference in interatomic force constants between TiCu and CuCu or TiTi.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

High pressure insulator-metal transition in molecular fluid oxygen

We report the first experimental evidence for a metallic phase in fluid molecular oxygen. Our electrical conductivity measurements of fluid oxygen under dynamic quasi-isentropic compression show that a nonmetal-metal transition occurs at 3.4 fold compression, 4500 K, and 1.2 Mbar. We discuss the main features of the electrical conductivity dependence on density and temperature and give an interpretation of the nature of the electrical transport mechanisms in fluid oxygen at these extreme conditions.     

First-principles equations of state for Al,Cu, Mo,and Pb to ultrahigh pressures

Abstract

Using accurate electronic-structure, interatomic-potential, and statistical methods, first-principles theoretical equations of state (EOSs) have been developed for the prototype metals Al, Cu, Mo, and Pb over wide ranges of volume and temperature. Emphasis is given to both the cold compressed solid, relevant to diamond-anvil-cell (DAC) experiments, and to the hot compressed liquid, relevant to multimegabar shock-wave experiments. The A1 EOS has recently been used to analyze nuclear-impedance-match (NIM) shock data on AI-Cu, AI-Mo, and A1-Pb sample pairs in the 5–30 Mbar regime [J. Appl. Phys. 69, 2981 (1991)]. Below 10 Mbar, calculated theoretical Hugoniots are in very good agreement with both absolute and NIM shock data for all four metals. Above 10 Mbar, some quantitative uncertainties remain, especially for Mo. Useful analytic representations of the calculated 300-K isotherms below 10 Mbar have also been developed for fcc Al, Cu, and Pb and for bcc Mo. The theoretical isotherms are in excellent agreement with a variety of ultrasonic, DAC, and reduced-shock data obtained in the assumed phases below 3 Mbar. At higher pressures complicating solid-solid phase transitions are expected and/or observed in Al, Mo, and Pb, but not in Cu, which is consequently a good candidate as a static ultrahigh-pressure EOS standard.     

Opposed type double stage cell for Mbar pressure experiment with large sample volume

ABSTRACT

A new opposed type double-stage large volume cell has been developed to compress large volume samples to more than 100?GPa (Mbar) pressure. A pair of second-stage diamond anvils is introduced into the first-stage Paris–Edinburgh press. The double-stage large volume cell allows the generation of ultrahigh pressures using a large culet diameter of the second-stage diamond anvils (diameters of 0.5–1.2?mm). Pressure generation up to 131?GPa has been achieved by using the culet diameter of 0.5?mm. Sample volume of the double-stage large volume cell can be more than ～100 times larger than that of conventional Mbar experiment using a diamond anvil cell. The double-stage large volume cell has a large opening in the horizontal plane for X-ray measurements, which is particularly suited for the multi-angle energy dispersive X-ray diffraction measurement, thus opening a new way of in situ structural determinations of amorphous materials at Mbar pressures.     

Shock compression of deuterium near 100 GPa pressures

The shock-compression curve (Hugoniot) of D2 near 100 GPa pressures (1 Mbar) has been contro-versial because the two published measurements have limiting compressions of fourfold and sixfold. Our purpose is to examine published experimental results to decide which, if either, is probably correct. The published Hugoniot data of low-Z diatomic molecules have a universal behavior. The deuterium data of Knudson et al. (fourfold limiting compression) have this universal behavior, which suggests that Knudson et al. are correct and shows that deuterium behaves as other low-Z elements at high tem-peratures. In D2, H2, N2, CO, and O2, dissociation completes and average kinetic energy dominates average potential energy above approximately 60 GPa. Below approximately 30 GPa, D2, H2, N2, CO, and O2 are diatomic. D2 dissociation is accompanied by a temperature-driven nonmetal-metal transition at approximately 50 GPa.     

EXAFS measurement of iron bcc-to-hcp phase transformation in nanosecond-laser shocks

Extended x-ray absorption fine structure (EXAFS) measurements have demonstrated the phase transformation from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometry. This is a direct, atomic-level, and in situ proof of shock-induced transformation in iron, as opposed to the previous indirect proof based on shock-wave splitting.     

Correlation between I-Ag distance and ionic conductivity in AgI fast-ion-conducting glasses

A large number of AgI-based fast-ion-conducting glasses have been investigated by K-iodine extended x-ray absorption fine structure spectroscopy (EXAFS) measurements at liquid nitrogen temperature. A general correlation between the I-Ag distance measured by EXAFS and the glass activation energy for dc ionic conductivity has been found out: glasses with longer I-Ag distances display higher ionic conductivity, independently from the chemical composition of their host glassy matrix. This behavior can be related to the progressive increase of the "pathway volume" for ionic conduction.     

Structural Fluctuations in the spin-liquid state of Tb2Ti2O7

High-resolution x-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin-liquid or cooperative paramagnetic state at temperatures below approximately 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd3[over ]m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at approximately 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures.     

用赝势方法计算五种简单金属的Hugoniot曲线

基于本文作者在文献[1]中所导出的冷压P_x,利用德拜固体模型并考虑电子的热压,得到了金属的高温高压状态方程。在与Hugoniot方程联立后,就能算出Hugoniot曲线。本文计算了五种简单金属的Hugoniot曲线。结果表明,在现有实验数据所能达到的范围内,理论计算与实验符合都比较好。 关键词：     

Surface EXAFS via differential electron yield

Surface‐sensitive analysis via extended X‐ray absorption fine‐structure (EXAFS) spectroscopy is demonstrated using a thickness‐defined SiO₂ (12.4 nm)/Si sample. The proposed method exploits the differential electron yield (DEY) method wherein Auger electrons escaping from a sample surface are detected by an electron analyzer. The DEY method removes local intensity changes in the EXAFS spectra caused by photoelectrons crossing the Auger peak during X‐ray energy sweeps, enabling EXAFS analysis through Fourier transformation of wide‐energy‐range spectral oscillations. The Si K‐edge DEY X‐ray absorption near‐edge structure (XANES) spectrum appears to comprise high amounts of SiO₂ and low Si content, suggesting an analysis depth, as expressed using the inelastic mean free path of electrons in general electron spectroscopy, of approximately 4.2 nm. The first nearest neighbor (Si—O) distance derived from the Fourier transform of the Si K‐edge DEY‐EXAFS oscillation is 1.63 Å. This value is within the reported values of bulk SiO₂, showing that DEY can be used to detect a surface layer of 12.4 nm thickness with an analysis depth of approximately 4.2 nm and enable `surface EXAFS' analysis using Fourier transformation.     

Quantum-classical reentrant relaxation crossover in Dy2Ti2O7 spin ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.