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Gold is believed to retain its ambient crystal structure at very high pressures under static and shock compression, enabling its wide use as a pressure marker. Our in situ x-ray diffraction measurements on shock-compressed gold show that it transforms to the body-centered-cubic (bcc) phase, with an onset pressure between 150 and 176 GPa. A liquid-bcc coexistence was observed between 220 and 302 GPa and complete melting occurs by 355 GPa. Our observation of the lower coordination bcc structure in shocked gold is in marked contrast to theoretical predictions and the reported observation of the hexagonal-close-packed structure under static compression. [ABSTRACT FROM AUTHOR]

Determining the temporal evolution of twinning and/or dislocation slip, in real-time (nanoseconds), in single crystals subjected to plane shock wave loading is a long-standing scientific need. Noncubic crystals pose special challenges because they have many competing slip and twinning systems. Here, we report on time-resolved, in situ, synchrotron Laue x-ray diffraction measurements during shock compression and release of magnesium single crystals that are subjected to compression along the c axis. Significant twinning was observed directly during stress release following shock compression; during compression, only dislocation slip was observed. Our measurements unambiguously distinguish between twinning and dislocation slip on nanosecond timescales in a shocked hexagonal-close-packed metal. [ABSTRACT FROM AUTHOR]

The thermodynamic response of liquid nitrogen has been studied extensively, in part, due to the long-standing interest in the high pressure and high temperature dissociation of shocked molecular nitrogen. Previous equation of state (EOS) developments regarding shocked liquid nitrogen have focused mainly on the use of intermolecular pair potentials in atomistic calculations. Here, we present EOS developments for liquid nitrogen, incorporating analytical models, for use in continuum calculations of the shock compression response. The analytical models, together with available Hugoniot data, were used to extrapolate a low pressure reference EOS for molecular nitrogen [R. Span et al., J. Phys. Chem. Ref. Data 29, 1361 (2000)] to high pressures and high temperatures. Using the EOS presented here, the calculated pressures and temperatures for single shock, double shock, and multiple shock compression of liquid nitrogen provide a good match to the measured results over a broad range of P-T space. These calculations provide the first comparison of EOS developments with recently measured P-T states under multiple shock compression. The present EOS developments are general and are expected to be useful for other liquids that have low pressure reference EOS information available.

The experimental determination of atomistic mechanisms linking crystal structures during a compression-driven solid-solid phase transformation is a long-standing and challenging scientific objective. Using new capabilities at the Dynamic Compression Sector at the Advanced Photon Source, the structure of shocked Si at 19 GPa was identified as simple hexagonal, and the lattice orientations between ambient cubic diamond and simple hexagonal structures were related. The approach is general and provides a powerful new method for examining atomistic mechanisms during stress-induced structural changes.

Liquid nitrogen was subjected to multiple shock compression to examine its response to pressures (15-40 GPa) and temperatures (1800-4000 K) previously unexplored in static and shock compression studies. Raman spectroscopy measurements were used to characterize the molecular bond response and to determine temperatures in the peak state. By extending our analysis to include other Raman spectroscopy measurements, an empirical relation was developed that describes the pressure and temperature dependence of the Raman shift (of the 2330 cm(-1) mode) for both shock and static compression. Based on the P-T dependence of the Raman shifts, the liquid nitrogen molecular response is best understood by considering three temperature regimes: below 1500 K, 1500-4000 K, and above 4000 K. For the pressures and temperatures accessed in the present work, liquid nitrogen remains a molecular fluid, and becomes a grey-body emitter at the highest pressures.

Time-resolved, Raman spectroscopy measurements were used to determine temperatures in multiply shocked liquid benzene to peak pressures of ~19 GPa. The previously reported equation of state (EOS) for benzene provides temperatures that are lower than the experimental measurements. To achieve improved temperature calculations, key thermodynamic parameters in the existing EOS were varied to examine and understand the sensitivity of temperatures to those parameters. Using the sensitivity studies, the benzene EOS parameters were refined to provide a good agreement between the calculated temperatures and the measured temperatures in multiply shocked benzene. Use of the improved EOS also leads to an increase in the decomposition temperature from 1315 K to 1485 K, for singly shocked liquid benzene. The present work underscores the importance of temperature measurements for developing accurate EOS for shocked liquids.

The Advanced Photon Source (APS) at Argonne National Laboratory was used to obtain real-time, high-resolution x-ray diffraction measurements to determine the microscopic response of shock-compressed single crystals. Disk shaped samples were subjected to plane shock wave compression by impacting them with half-inch diameter, flat-faced projectiles. The projectiles were accelerated to velocities ranging between 300 and 1200 m/s using a compact powder gun designed specifically for use at a synchrotron facility. The experiments were designed to keep the sample probed volume under uniaxial strain and constant stress for a duration longer than the 153.4 ns spacing between x-ray bunches. X-rays from a single pulse (<100 ps duration) out of the periodic x-ray pulses emitted by the synchrotron were used for the diffraction measurements. A synchronization and x-ray detection technique was developed to ensure that the measured signal was obtained unambiguously from the desired x-ray pulse incident on the sample while the sample was in a constant uniaxial strain state. The synchronization and x-ray detection techniques described can be used for a variety of x-ray measurements on shock compressed solids and liquids at the APS. Detailed procedures for applying the Bragg-Brentano parafocusing approach to single crystals at the APS are presented. Analytic developments to determine the effects of crystal substructure and non-ideal geometry on the diffraction pattern position and shape are presented. Representative real-time x-ray diffraction data, indicating shock-induced microstructural changes, are presented for a shock-compressed Al(111) sample. The experimental developments presented here provided, in part, the impetus for the Dynamic Compression Sector (DCS) currently under development at the APS. Both the synchronization∕x-ray detection methods and the analysis equations for high-resolution single crystal x-ray diffraction can be used at the DCS.

The use of Raman measurements to examine molecular changes associated with shock-induced structural and chemical changes in condensed materials often poses two challenging requirements: high spectral resolution and significantly reduced background light. Here, we describe an experimental method that addresses these requirements and provides better quality data than the time resolved approach used previously. Representative measurements are presented for shock compression of two energetic crystals: pentaerythritol tetranitrate and cyclotrimethylene trinitramine. The high spectral resolution data have provided insight into molecular changes that could not be obtained from lower-resolution, time-resolved methods.

To determine the nonlinear elastic response of diamond, single crystals were shock compressed along the [100], [110], and [111] orientations to 120 GPa peak elastic stresses. Particle velocity histories and elastic wave velocities were measured by using laser interferometry. The measured elastic wave profiles were used, in combination with published acoustic measurements, to determine the complete set of third-order elastic constants. These constants represent the first experimental determination, and several differ significantly from those calculated by using theoretical models.

Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electronic structure of anthracene single crystals compressed statically to 9 GPa. Two pressure-transmitting media, nitrogen (hydrostatic) and water (nonhydrostatic above approximately 5.5 GPa), were utilized. It was found that nonhydrostatic compression generates several new features both in the absorption and fluorescence spectra: (i) formation of new absorption and fluorescence bands, (ii) deviations in pressure shift of fluorescence peaks, (iii) extensive broadening of vibrational peaks, and (iv) irreversible changes in the spectra shape upon pressure unloading. Furthermore, the time-resolved fluorescence decay curves measured at the wavelength corresponding to the new fluorescence band show clear initial increase. These new features are accompanied by inhomogeneous color changes and macroscopic lines on the (001) plane of the crystal. All of the changes are discussed and correlated with microscopic transformations in the crystal. It is demonstrated that nonhydrostatic compression in anthracene crystal introduces inelastic changes in the form of dislocations along [110] and [110] directions. These dislocations lead to the development of dimeric structures and, consequently, to various changes in the electronic response of the compressed anthracene crystal.

Shock wave experiments utilizing stepwise-loading, with peak stresses ranging between 4 and 25 GPa, were performed to examine the dynamic high pressure response of liquid benzene at thermodynamic conditions not attainable in single shock experiments. Time-resolved Raman spectroscopy was used to monitor the molecular and chemical changes on sub-mus time scales. Up to 20 GPa, the Raman modes showed pressure-induced shifting and broadening but no indication of a chemical change. At 24.5 GPa, however, the Raman modes become indistinguishable from an increasing background within 40 ns after the sample attained peak pressure, indicating a chemical change. A thermodynamically consistent equation of state (EOS) was developed to calculate the relevant thermodynamic variables in multiply shock compressed liquid benzene. Idealized molecular configurations were used in combination with the thermodynamic quantities in the shocked state to calculate the intermolecular separation between benzene molecules and to ascertain the likelihood of pi-orbital overlap. These idealized calculations show that sufficient energy and pi-orbital overlap exist in multiply shock compressed liquid benzene to permit intermolecular bonding at 24.5 GPa. Analysis of the Raman spectra, using the thermodynamic and intermolecular separation calculations, suggests that benzene undergoes polymerization through cycloaddition reactions. The rapid rate of polymerization is attributed to the benzene remaining in a liquid state on the sub-mus experimental time scale. The results from the present work demonstrate the importance of time, pressure, temperature, and phase in chemical changes associated with pi-bonded molecules.

Pressure effects on the vibrational structure of alpha-RDX were examined using density functional theory (DFT) up to 4 GPa. The calculated vibrational frequencies at ambient conditions are in better agreement with experimental data than are previous single molecule calculations. The calculations showed the following pressure-induced changes: (i) larger shifts for lattice modes and for internal modes associated with the CH(2) and NO(2) groups as compared to the pressure shifts for modes associated with the triazine ring, (ii) enhancement of mixing between different vibrations, for example, between NN stretching and CH(2) scissor, wagging, twisting vibrations, and (iii) increase in mixing between translational lattice vibrations and the NO(2) wagging vibrations, reducing the distinction between internal and lattice modes. The calculated volume and lattice constants at ambient pressure are larger than the experimental values, due to the inability of the present density functional approach to correctly account for van der Waals forces. Consequently, the pressure-induced frequency shifts of many modes deviate substantially from experimental data for pressures below 1 GPa. With increasing pressure, both the lattice constants and the frequency shifts agree more closely with experimental values.

The response of pyrene crystals to high pressure was examined using Raman and FTIR spectroscopies. Raman spectra of external and internal modes were measured up to 11 GPa. Changes in the external modes were observed at approximately 0.3 GPa, indicating the onset of a phase transition. We demonstrated that at this pressure pyrene I (P2(1)/a, 4 mol/unit cell) transforms to pyrene III (P2(1)/a, 2 mol/unit cell). Further increase of pressure produced a gradual broadening of the internal modes and an increase of fluorescence background, indicating the formation of another phase above 2.0 GPa. Irreversible chemical changes were observed upon gradual compression to 40 GPa. FTIR spectroscopy of the recovered product indicated a transformation of pyrene into an amorphous hydrogenated carbon (a-C:H) structure.

Time-resolved Raman scattering measurements were performed on ammonium perchlorate (AP) single crystals under stepwise shock loading. For particular temperature and pressure conditions, the intensity of the Raman spectra in shocked AP decayed exponentially with time. This decay is attributed to shock-induced chemical decomposition in AP. A series of shock experiments, reaching peak stresses from 10-18 GPa, demonstrated that higher stresses inhibit decomposition while higher temperatures promote it. No orientation dependence was found when AP crystals were shocked normal to the (210) and (001) crystallographic planes. VISAR (velocity interferometer system for any reflector) particle velocity measurements and time-resolved optical extinction measurements carried out to verify these observations are consistent with the Raman data. The combined kinetic and spectroscopic results are consistent with a proton-transfer reaction as the first decomposition step in shocked AP.

To gain insight into the anisotropic sensitivity of shocked pentaerythritol tetranitrate (PETN) single crystals, single-pulse Raman spectroscopy was used to examine the response of crystals shocked along the [100] (insensitive) and [110] (sensitive) orientations. High-resolution Raman spectra revealed several orientation-dependent features under shock compression: (i) substantially different stress dependence of the Raman shift for the CH(2) and NO(2) stretching modes for the two orientations, (ii) discontinuity in the stress dependence of the Raman shift for the CH(2) stretching modes above 4 GPa for the [110] orientation, and (iii) large broadening for the CH(2) and NO(2) asymmetric stretching modes for stresses above 4 GPa for the [110] orientation. The present data in combination with previous static pressure results provide support for conformational changes in PETN molecules for shock compression along the [110] (sensitive) orientation. Implications of the present results for the anisotropic sensitivity of shocked PETN are discussed.

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I --> II), 5.0 GPa (II --> III), and 8.0 GPa (III --> IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(4))) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.

Water samples were subjected to multiple shock wave compressions, generating peak pressures of 1-5 GPa on nanosecond time scales. This loading process approximates isentropic compression and leads to temperatures where the ice VII phase is more stable than the liquid phase above 2 GPa. Time resolved optical transmission and imaging measurements were performed to determine the solidification rate under such conditions. Freezing occurred faster at higher pressures as water was compressed further into the ice VII phase, in agreement with classical micleation theory. Water consistently froze when in contact with a silica window, whereas no solidification occurred in the presence of sapphire windows. The transition was determined to be a surface initiated process--freezing began via heterogeneous nucleation at the water/window interface and propagated over thicknesses greater than 0.01 mm. The first optical images of freezing on nanosecond time scales were obtained. These images demonstrate heterogeneous nucleation and irregular solid growth over 0.01-0.10 mm lateral length scales and are consistent with latent heat emission during the transformation. The combination of optical transmission and imaging measurements presented here provide the first consistent evidence for freezing on short time scales., (Copyright 2004 American Institute of Physics.)