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Ramp compression experiments characterize high-pressure states of matter at temperatures well below those present in shock compression. However, because temperature is typically not directly measured during ramp compression, it is uncertain how much heating occurs under these shock-free conditions. Here, we performed a series of ab initio simulations on carbon in order to match the density-stress measurements of Smith et al. [Smith, et al., Nature 511, 330 (2014)]. We considered isotropically as well as uniaxially compressed solid carbon in the diamond and BC8 phases, with and without defects, as well as liquid carbon. Our idealized model ascribes heating during ramp compression to an initially uniaxially compressed cell transforming isochorically into an isotropically (hydrostatic equivalent) compressed state having lower internal energy, hence higher temperature so as to conserve energy. Multiple such heating events can occur during a single ramp experiment, leading to higher temperatures than with isentropic compression. Comparison with experiments shows that heating alone does not explain the equation of state measurements on diamond, instead implying that a significant uniaxial stress component remains present at high compression. The temperature predictions of our ramp compression model remain to be verified with future laboratory measurements., 13 pages, 13 figures, https://link.aps.org/doi/10.1103/PhysRevB.104.134104

High-pressure x-ray diffraction on Ni3Al, non-hydrostatically compressed at room temperature to 30 GPa in radial geometry can be interpreted in terms of a hydrostatic pressure–volume equation of state. We found the yield strength of Ni3Al to increase from about 0.2 to 2 GPa as pressure increases from 0 to 30 GPa. Yield-strength values determined from x-ray diffraction in axial geometry by including a Poisson ratio effect and pressure gradient methods are in good accord with each other. Our results indicate that the strengths of both pure and Cr-doped (7 at. %) Ni3Al increase with pressure, with the yield strength of Ni3Al slightly higher than Ni-Al-Cr alloy.

In situ x-ray synchrotron diffraction experiments were carried out on nickel-based high-strength superalloys under pressure to understand their deformation mechanism using a diamond anvil cell (DAC). Radial x-ray diffraction determines the room-temperature equations of state and yield strengths of binary Ni3Al alloy and 500 ppm boron-doped Ni3Al to pressures of 20 and 46 GPa, respectively. Crystallographic preferred orientations observed in these superalloys due to anisotropic stress field in DAC indicate the onset of plastic deformation. Inverse pole figure analysis reveals that the underlying deformation mechanisms change from an octahedral slip to a simultaneous activation of octahedral and cube slips upon doping with boron. The yield-strength values were found to increase with pressure and are comparable to those determined from axial diffraction experiments. The results indicate that the yield strength of Ni3Al:B is about 0.5 GPa higher (at pressures below 20 GPa) due to grain boundary strengthening by boron. It is shown that due to high elastic anisotropy of Ni3Al alloy, the yield-strength estimations from diffraction experiments strongly depend on the micromechanical model used to convert the measured elastic strains into stresses.

Ni based superalloy Ni–Al–Cr with γ and γ′ phase was studied under high pressure up to 30 GPa using diamond anvil cell technique. In-situ X-ray diffraction data was collected on these alloys under hydrostatic and non-hydrostatic conditions. Cubic phase remains stable up to the highest pressure of about 30 GPa. Bulk modulus and its pressure derivative obtained from the volume compression of pressure data are K = 166.6 ± 5.8 GPa with K′ set to 4 under hydrostatic conditions and K = 211.3 ± 4.7 GPa with K′ set to 4 for non-hydrostatic conditions. Using lattice strain theory, maximum shear stress ‘t’ was determined from the difference between the axial and radial stress components in the sample. The magnitude of shear stress suggests that the lower limit of compressive strength increases with pressure and shows maximum yield strength of 1.8 ± 0.3 GPa at 20 GPa. Further, we have also determined yield strength using pressure gradient method. In both the methods, yield strength increases linearly with applied pressure. The results are found to be in good agreement with each other and the literature values at ambient conditions.

In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to characterize the local structure of molten AuGa2 up to 16 GPa and 940 K. Our results document systematic changes in liquid structure due to a combination of bond compression and coordination increase. Empirical potential structure refinement shows the first-neighbor coordination of Ga around Au and of Au around Ga to increase from about 8 to 10 and 4 to 5, respectively between 0 and 16 GPa, and the inferred changes in liquid structure can explain the observed melting-point depression of AuGa2 up to 5 GPa. As intermetallic AuGa2 is an analogue for metallic SiO2 at much higher pressures, our results imply that structural changes documented for non-metallic silicate melts below 100 GPa are followed by additional coordination changes in the metallic state at pressures in the 0.2–1 TPa range achieved inside large planets.

X-ray diffraction from LaB 6 standards document a precision of 478 ppm in lattice-parameter determinations for beamline 12.2.2 at Lawrence Berkeley National Laboratory′s Advanced Light Source, a facility for characterizing materials at high pressures and temperatures using laser- and resistance-heated diamond cells. Melting of Ni, Mo, Pt and W, resistively heated at 1 atm pressure in Ar, provides a validation of the beamline spectroradiometric system that is used to determine sample temperatures. The known melting temperatures, which range from 1665 to 3860 K for these metals, are all reproduced to within ±80 K.

We have observed laser induced birefringence in optically isotropic glasses as well as in Nd:YAG crystal using polarized pulses of wavelength 1.06 µm and duration 100 ps with an intensity near 109 W cm−2. The laser used in the experiment was a Nd:YAG oscillator–amplifier system. The change in the material induced by the laser was not a permanent change but existed for as long as the laser pulse lasted. We observed conoscopic patterns when the laser was passed through glass or the Nd:YAG crystal. These patterns are typically seen when light is passed through birefringent materials kept between a polarizer and analyser. This observation immediately suggests that the glass or the crystal possesses optical birefringence. This birefringence is demonstrated in fused silica glass, Nd:glass and Nd:YAG crystal. The circular symmetric conoscopic patterns show that the optic axis is formed along the direction of laser propagation. The laser induced birefringence depended on the laser intensity and we characterized the magnitude of this change by measuring the output pulse at the end of the crossed polarizer.

A new polymorph of Al2(WO4)3 is observed from in situ high pressure powder X-ray diffraction (ADXRD) measurements at 3.4 GPa. The ADXRD pattern at 3.4 GPa could be explained based on a monoclinic lattice (space group P21) with unit cell parameters: a=9.5884(24), b=12.5204(38), c=7.8463(33) Å, and β=91.98(2)°.

We have carried out electronic structure calculations for iron under high pressure using pseudopotential plane wave methods. There is a controversy regarding the structure of iron at moderate pressures (30-100 GPa) and temperatures (1000-2400 K), with different experiments suggesting different structures, such as orthorhombic, double hexagonal close packed (dhcp), etc. Our earlier calculations using the linear muffin-tin orbital method within atomic-sphere-approximation had argued against the stability of the orthorhombic phase. The more accurate calculations presented here predict qualitatively the same results. We have additionally studied the stability of various phases of Fe at different compressions by calculating phonon frequencies. These rule out the stability of the orthorhombic phase. To validate our zero-temperature electronic-structure results at finite temperature, we have compared the shock Hugoniot and melting properties of iron with the results of our electronic structure calculations. Though we have used simplified models for these estimates, our predictions compare well with the experimental data. We thus propose that these models can be used to obtain the information about the high-pressure melting curves of planetary materials.

The impedance mismatch technique was used for shock pressure amplification in two layered planar foil targets. Numerical simulation results using one-dimensional (1D) radiation hydrocode MULTI in two layer target consisting of polyethylene (CH2)n-aluminium (Al) and polyethylene (CH2)n-gold (Au), show a pressure enhancement of 12 and 18 Mbar, respectively (or a pressure jump of 1.64 and 2.54, respectively), from initial pressure of 7 Mbar in the reference material (polyethylene) using laser intensity of 5 × 1013 Watts/cm2 at 1.064 μm. The simulation data was also corroborated by experiments in our laboratory. Results of laser driven shock wave experiments for pressure enhancement studies in CH2-Al and CH2-Au targets are also presented. A Nd:YAG laser chain (2 J, 1.064 μm wavelength, 200 ps pulse duration FWHM) is used for generating shocks in the planar CH2 foils of thickness varying from 4 to10 μm, and in two layered CH2-Al (or CH2-Au) targets with 8 μm CH2 and 1.5 μm Al or Au .

Theimpedancemismatchtechnique has been used for shock pressure amplification in two- and three-layer thin planar foil targets. Numerical simulation results using one-dimensional radiation hydrocode MULTI in two-layer targets consisting of Al–Au and Al–Cu and three-layer target consisting of plastic–Al–Au and foam–Al–Au, respectively, are presented. These results show a pressure enhancement up to 25 and 29 Mbar for plastic–Al–Au and foam– Al–Au targets, respectively, from an initial pressure of 7 Mbar in the reference material using laser intensity of 5 × 10 13 Wc m −2 at 1.064 µm. This enhancement is more as compared with 18 and 22 Mbar found in plastic–Au and foam–Au two-layer targets, respectively. Results of laser driven shock wave experiments for equation of state (EOS) studies of Au and Cu in two-layer target are also presented. A Nd : YAG laser chain (2 J, 1.06 µm wavelength, 200 ps pulse FWHM) is used for generating shocks in the planar Al foils and Al–Au (or Al–Cu) layered targets. The EOS of Au and Cu in the pressure range of 9–14 Mbar obtained shows remarkable agreement with the simulation results and with experimental data of other laboratories and SESAME data.

An easily assembled setup employing diamond anvil cell, stainless steel gasket and leads, and mylar embedded Al2O3 (alumina) pressure medium for the measurement of electrical resistance of materials under pressure is described. The use of a mylar sheet prevents the alumina layer from sticking to the anvil in the precompacting stage of Al2O3 and also reduces the pressure gradients in the final assembly. The technique is used to reproduce the known transition in Si, Ge, and Fe. The results of measurements of electrical resistance of ytterbium up to 40 GPa are reported. In the hcp phase of ytterbium the electrical resistance increases with pressure. Efforts are made to explain the variation of resistance with pressure from known band structure calculations.

X-ray holography is a new method of structure determination based on measurement of interference of a known reference wave with an unknown object wave (containing information on atomic sites scattering the reference wave) so that phase information is preserved. Unlike X-ray diffraction, it does not demand for translational periodicity in the material. It is based on the idea similar to that of optical holography and has been tested on crystals, quasicrystals, thin films and doped semiconductors for their structure determination. In order to analyse potentials and limitations of this technique, we have carried out theoretical simulation studies on simple structures. In this paper we describe the basic algorithm of hologram generation and reconstruction of atomic positions from generated data. We illustrate this technique using Fe (bcc) single crystal as sample case to demonstrate its capabilities and limitations. Simulations were carried out on the Cu (fcc) structure and on complex structure such as the Al-Pd-Mn quasicrystal. Technical issues such as low signal to noise ratio, twin image problem etc have been discussed briefly to emphasize the need for high intensity X-ray source such as synchrotron for experiments and proper reconstruction algorithm. Finally the scope and potential of this technique have been discussed.

Transition metal dichalcogenides, because of their layered structure, are well suited for extreme pressure lubrication. These materials being semiconducting and of layered structure may undergo structural and electronic transitions under pressure. Here we report the details of the preparation and characterization of single crystals of NbTe2 and the results of electrical resistance measurements under pressure carried out on it to investigate this possibility. Single crystals were grown by the chemical vapor transport technique, using iodine as a transporting agent. The composition of the grown crystals was confirmed on the basis of Energy Dispersive Analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray Diffraction (XRD) studies. Electrical resistance was measured employing a Bridgman anvil set up to 10 GPa and diamond anvil cell (DAC) assembly up to 25 GPa. A technique slightly modified from that described in the literature for carrying out electrical resistivity...

A commercial mode locked cavity dumped Nd:YAG dye laser operating at 10 Hz repetition rate is modified to produce a high contrast (>5000:1) single laser pulse while maintaining the energy stability and high beam quality. A trigger generator biases the cavity dumping photodiode, which is triggered externally by a pulse from the microprocessor-based control unit controlling a ∼2 J/200 ps laser chain. In the laser chain, the high contrast (>5000:1) is achieved by an external pulse selector based on single Pockel's cell to select a single laser pulse of high contrast, which is a prerequisite for experimental study of the equation of state. Laser-induced shock velocity measurement in thin aluminum, gold on aluminum, and copper on aluminum foil targets using this modified laser system are also presented. The equation of state of Al, Au, and Cu obtained using an impedance matching technique are in agreement with the reported results of SESAME and simulation results.

We have simulated the shock Hugoniot of copper and uranium based on the results of first principles electronic structure calculations. The room temperature isotherm has been obtained by evaluating the accurate ground state total energies at various compressions, and the thermal and electronic excitation contributions were obtained by adopting isotropic models using the results obtained by the band structure calculations. Our calculations ensure smooth consideration of pressure ionization effects as the relevant core states are treated in the semi-core form at the ambient pressure. The pressure variation of the electronic Grüneisen parameter was estimated for copper using the band structure results, which leads to good agreement of the simulated shock Hugoniot with the measured shock data. The simulation results obtained for U are also compared with the experimental data available in literature and with our own data.

The results of electrical resistance measurements under pressure on single crystals of WSe2 are reported. Measurements up to 8.5 GPa are carried out using Bridgman anvil set up and beyond it using diamond anvil cell (DAC) up to a pressure of 27 GPa. There is no clear indication of any phase transition till the highest pressure is reached in these measurements.

We discuss the high pressure behaviour of zinc as an interesting example of controversy, and of extensive interplay between theory and experiment. We present its room temperature electronic structure calculations to study the temperature effect on the occurrence of its controversial axial ratio (c/a) anomaly under pressure, and the related electronic topological transition (ETT). We have employed a dense 63 X 63 X 29 k-point sampling of the Brillouin zone and find that the small (c/a) anomaly near 10 GPa pressure persists at room temperature. A weak signature of the anomaly can be seen in the pressure-volume curve, which gets enhanced in the universal equation of state, along with that of K-point ETTs. We attribute the change of slope in the universal equation of state near 10 GPa pressure, mainly to hybridization effects. The temperature effect in fact enhances the possibility of L-point ETT. We find that the L-point ETT is very sensitive to exchange correlation terms, and hence we suggest that further refinements in the theoretical techniques are needed to resolve the controversies on the ETT in Zn.

We have carried out electronic structure total energy calculations on rhenium in the hexagonal close packed (hcp) and body centred cubic (bcc) phases, by the full potential linear muffin-tin orbital method, in order to verify the stability of the ambient pressure hep phase against transition to the bcc structure at high pressures. As per our results, no hcp to bcc structural transition can occur up to 1 TPa pressures. Moreover, our Bain path calculations show that face centred cubic and body centred tetragonal structures are also not energetically preferred over hcp in this pressure range. The axial ratio (c/a) of Re changes by less than 0–33% in the pressure range studied.

Results of ab initio electronic structure calculations on the compound, MgB2, using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated, D(EF) and the experimental specific heat data. The Tc is found to be 37 K. We use a parametrized description of the cal- culated band structure to obtain the T = 0 K values of the London penetration depth and the superconducting coherence length. The penetration depth calculated by us is too small and the coherence length too large as compared to the experimentally determined values of these quantities. This indicates the limitations of a the- ory that relies only on electronic structure calculations in describing the superconducting state in this mate - rial and implies that impurity effects as well as mass renormalization effects need to be included.

Results of ab initio electronic structure calculations on the compound MgB2 using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E F) and the experimental specific heat data. The T c is found to be 24.7 K.

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn2, AuGa2 and low Z scatterer adamantane (C10Hl6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In0.25Sn0.75 and AuGa2. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.

Details of a two-dimensional X-ray area detector developed using a charge coupled device, a image intensifier and a fibre optic taper are given. The detector system is especially optimized for angle dispersive X-ray diffraction set up using rotating anode generator as X-ray source. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on various materials such as intermetallics AuIn2, AuGa2, high Z material Pd and low Z scatterer adamantane (C10H16) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell is demonstrated by reproducing the known pressure induced structural transitions in RbI, KI and a new structural phase transition in AuGa2 above 10 GPa. Various softwares have also been developed to analyze data from this detector.

In the high pressure laboratory at BARC, we are pursuing a program to study the behaviour of materials under static and dynamic pressures. Theoretical component has been an integral part for guiding and interpreting new experiments. The initial phase of such efforts was devoted to the development of equation of state models at arbitrary temperatures and matter densities. With the advent of diamond anvil cell device and the simultaneous provision for laser heating of the compressed samples for static high pressure studies, and with the improvements of the diagnostic techniques in dynamic shock methods, the focus of our studies switched over to the predictions and interpretations of phase transitions. Often these efforts have led to intense experimental studies and sometimes helped in resolving the controversies in data. We adopted the first principles electronic structure calculations for high pressure studies. Our work on the electronic topological transition in zinc led to many experimental and theoretical investigations. The results of electronic structure changes in similar metal cadmium shall be compared with existing understanding in Zn under pressure. Our studies on Nb and other compounds like intermetallics and borocarbides have revealed interesting electronic structure changes under pressure. However, the electronic structure based investigations of structural stabilities at high pressures involve tedious trial and error effort, which is avoided in theab initio molecular dynamics simulations. The current status of our efforts in the use of this technique is illustrated with the example of quasicrystal based clusters.

The electronic and structural behaviour of YNi2B2C and LuNi2B2C at high pressures were investigated by electrical resistivity, thermoelectric power (TEP), and angle dispersive X-ray diffraction measurements and by electronic band structure calculations. The pressure variation of TEP shows a peak around 2 GPa in YNi2B2C. However, the X-ray powder diffraction does not show any structural transition up to 16.4 GPa. The equation of state of YNi2B2C yielded a relatively high bulk modulus of 200 GPa. The observed peak in TEP and the pressure variation of the superconducting transition temperature can be correlated with different components of the electronic density of states. The universal equation of state shows deviation from linearity around 1.5 GPa pressure and correlates well with the TEP peak.

The electronic and lattice structure, and equation of state behavior of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}$C has been investigated by electrical resistivity, thermoelectric power, and angle-dispersive x-ray-diffraction measurements. The electrical resistance under pressure has also been measured for other lanthanide-nickel borocarbides $(R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ $R=\mathrm{Ho},$ Er, and Tm). The high pressure behavior is analyzed by electronic band-structure [tight-binding linear muffin-tin orbital (LMTO) and full-potential LMTO] calculations on ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ The measured thermoelectric power shows a peak around 2 GPa pressure. X-ray powder diffraction measurements of the lattice parameter under pressure reveal that the carbon-filled variant of ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type tetragonal structure prevailing under ambient conditions is preserved in ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ up to 6 GPa. Pressure-volume data on ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ yielded a bulk modulus of 200 GPa. This is in agreement with the value computed by the first-principles electronic-structure calculations. It is also shown that the observed peak in the thermoelectric power in ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ and the weak pressure dependence of the superconducting-transition temperature can be correlated with the details of the electronic density of states.

Electrical resistivity, thermoelectric power, and high-pressure x-ray-diffraction measurements are carried out to investigate the anomaly observed earlier in fusion data around 3 GPa in the intermetallic compound ${\mathrm{AuIn}}_{2}.$ While the imaging plate high-pressure angle-dispersive data indicate a structural phase transition beyond 8 GPa, the thermoelectric power shows a peak around 2 GPa, indicating the occurrence of an electronically driven isostructural transition. The first-principles linearized muffin-tin orbital calculations reveal that this transition is brought about by interception of the Fermi level by the energy-band maximum. The Lifshitz nature of this transition is responsible for the anomaly in the high-pressure electrical and fusion data.

A three-term equation of state is used for the simulation of hydrodynamic shock phenomena. The reliability of it is tested by comparing the isotherms and shock Hugoniots obtained from it with available experimental data. This is illustrated for three typical materials Al, Pt and Au, which are of considerable interest for inertially confined thermonuclear fusion studies. We observe that reliable equation of state (EOS) data can be generated for nonmetallic substances like CH2. The EOS is used in the simulations of laser-driven shocks in double-layered target aluminum on gold (Al-Au) using hydrodynamic code employing a simplified laser-absorption model. The output from it is compared with the recent laser-driven shock experiments of Koeniget al.

A new polymorph of HfMo2O8 (β-form) is synthesized under high pressure and high temperature conditions. The powder X-ray diffraction (XRD) data could be explained based on a monoclinic lattice (Space Group: C2/c No. 15) with the unit cell parameters as: a=11.415(3), b=7.906(2), c=7.438(2) Å and β=122.37(2)°, V=566.9(2) Å3. The detailed powder XRD data and analysis are reported herein.

Crystalline resorcinol [${\mathrm{C}}_{6}$${\mathrm{H}}_{4}$(OH${)}_{2}$] has been shown to amorphize at 40 kbars, solely using Raman scattering. The external modes vanish resulting in a spectrum similar to the density of states spectrum, but the internal modes persist indicating a breakdown of translational and orientational correlation between the molecules. The amorphous state recrystallizes after the pressure is released. We also provide additional information on the known \ensuremath{\alpha} to \ensuremath{\beta} phase transition.

We report a structural phase transition in ${\text{AuIn}}_{2}$ above 25 GPa. The inferred structure of this phase is post cotunnite $(P{112}_{1}/a)$. Combining the new results with previously observed behavior of ${\text{AuIn}}_{2}$, we propose a sequence of structural transitions from ${\text{CaF}}_{2}$ $(Fm3m)$ to amorphous phase at 24 GPa to monoclinic $(P{112}_{1}/a)$ structure at 25.6 GPa. During decompression the high-pressure phase is preserved to the pressure of 9.4 GPa and it finally transforms back to the $Fm3m$ phase. Analysis of volume reduction and bulk modulus increase reveals close similarity with the postcotunnite phase observed in ${\text{PbCl}}_{2}$. This study is important in understanding the behavior of metallic phases of various oxides of geophysical interest which exist in ${\text{CaF}}_{2}$ fluorite-type structure.

Experiments carried out demonstrate the production due tosoft X-rays of enhanced ablation pressure. The targets used were planar gold foils. Aluminium targets were also used to demonstrate the relative effect.

First principle predictions for the equation of state of gold using solid and liquid state theories are compared up to combined pressures and temperatures of 600 GPa and 17 000 K with static diamond anvil cell compression, ultrasonic measurements and shock Hugoniot data which include a recent laser driven shock Hugoniot points at 600 GPa. Excellent agreement between theoretical and experimental data is observed. The theoretically estimated 300 K isotherm agrees to within 2 GPa with the isotherm that has been measured to 70 GPa using the diamond anvil cell. The structural energy estimates show that the normal f.c.c. phase remains stable under pressure. The estimate of the shock Hugoniot temperature of gold at 600 GPa based on a liquid state model is consistent with the measurements of laser induced shock luminescence, which in fact provides an experimental determination of the temperature of gold above its Hugoniot melting point. The powerful means provided by theory in the prediction of material ...

X-ray diffraction measurements and Raman spectroscopy at room temperature document the equation of state and the frequency shifts for ${E}_{\text{g}}$, ${T}_{2\text{g}}$, and ${A}_{1\text{g}}$ vibrational modes of polycrystalline ${\text{LaB}}_{6}$ under pressure. The data exhibit smooth pressure dependencies, yielding a zero-pressure isothermal bulk modulus ${K}_{0T}=164(\ifmmode\pm\else\textpm\fi{}2)\text{ }\text{GPa}$ in good accord with independent ultrasonic measurements, and show no evidence of structural or electronic phase transitions up to at least 25 GPa.

Observations of the formation and propagation of a laser-driven strong shock in fused and a-quartz reveal anomalously high, transient shock velocities. These results disagree with calculations based on equilibrium equations of state. It is conjectured that at the shock front, a transformation of quartz to stishovite is preceded by shock-induced disorder accompanied by a significant entropy increase. This state remains dominant until sufficient relaxation has occurred.

Structure of iron in the pressure range of 40-100 GPa at temperatures of 1500-2400 K is controversial. A double hcp (dhcp) and an orthorhombic distortion of the hcp structure have been proposed in recent experiments. We have carried out first-principles electronic structure total-energy calculations in this high-pressure region to evaluate the phase stability of these new structures. Besides, a monoclinic structure with two atoms per unit cell, which could be in the transit path of e -Fe (hcp)-γ -Fe (fcc) transformation has also been considered. Our calculations reveal that the total energies of the fcc and the dhcp structures are close to each other (within 1 mRy/atom), and 6-8 mRy above that of hcp. The orthorhombic phase is 40-55 mRy/atom higher in energy. The monoclinic structure is also of higher energy with respect to the hcp phase by about 50 mRy/atom. The entropy energy difference between the orthorhombic and hcp structures is estimated to be ≅ 2 mRy/atom at 2200 K. Thus the experimental observation of the orthorhombic structure by Andrault et al. is not supported by our calculations. However, these calculations are in agreement with the recent double-sided laser-heated (for avoiding thermal gradients) diamond-anvil cell experiments of Shen et al., in which only hcp and fcc phases were observed in situ at high pressures and temperatures.

High-resolution angular-dispersive x-ray diffraction shows that the intermetallic compound AuIn 2 retains its initial CaF 2 structure to pressures of 23 GPa. We observe continuous broadening of diffraction lines with pressure, leading to amorphization at 24 GPa. The pressure-volume equation of state exhibits an anomaly around 2.7 GPa, in agreement with previous findings and not attributable to nonhydrostaticity. Instead, ab initio plane-wave self-consistent field calculations indicate that the observed anomaly is associated with topological changes in the electronic band structure under pressure.

Diamond anvil cell (DAC) based resistance measurements are carried out in AuAl{sub 2} to the pressures of about 25 GPa for the first time. The data indicate possibility of phase transition around 12 GPa. High pressure data obtained using Elettra synchrotron source confirm the transformation of AuAl{sub 2} from ambient CaF{sub 2} to orthorhombic phase. The high precision P-V data when transformed to universal equation of state does not show any deviation from linearity. The first principles electronic band structure calculations up to these pressures in the cubic phase do not reveal any change corroborating the experimental findings of nonobservation of electronic topological transition in this compound.

Summary form only given. Intense laser ablation induced shocks have been found use in the field of high pressure and high energy density physics studies viz. equation of state (EOS) measurements, phase transformations, understanding astrophysical high density plasma phenomena etc. Several megabars of pressures can now be generated, under laboratory conditions using modest laser intensities. To achieve higher shock pressure one must use short laser wavelength and higher intensity. However, at higher laser intensities, collective laser absorption processes dominate and consequently generate hot electrons and hard X-rays, leading to the preheat of the target material. This causes difficulty in generation of high target compression. To overcome this unwanted phenomenon we propose use of multilayered targets (two or more layers). From the laser irradiation side, the shock impedances of the layers are kept in an ascending order. Shock impedance mismatch at the interface of the layers cause a shock pressure jump, when an ablation shock is launched at the low impedance layer by a laser, reaches the interface. The pressure increase depends on the shock impedance mismatch of the layered materials. Our 1-D radiation hydro code simulations show that using a low density plastic layer on gold or aluminum substrate produce a shock pressure enhancement of 3 and 1.6 respectively at the interface. This data also has been corroborated by experiments in our laboratory. In this paper we propose to present our simulations and experimental data and show that in absence of the low impedance layer the laser intensity requirements are much higher. In addition, plastic being a low atomic number material generates very low X-ray yield and thus provides better ablation efficiency. We shall also present our simulation results using three layer targets. We believe that these results are very useful for studying materials in 20 to 40 megabar pressure range using very modest laser intensities

High pressure behaviour of YNi 2 B 2 C at room temperature is investigated by electrical resistivity, thermopower and X-ray diffraction incorporating imaging plate. These studies do not indicate any structural transition up to 16.4 GPa. Bulk modulus of the material is measured to be 200 GPa and estimated to be 270 GPa from the theoretical calculations based on TB-LMTO method.

We report the synthesis and characterization of a new polymorph (β ) of HfMo 2 O 8 . This phase is synthesized from the trigonal (a) HfMo 2 O 8 by application of high pressures and high temperatures. The sample obtained is characterized by powder x-ray diffraction (XRD) and the details of high-pressure and high-temperature phase are explained. The Rietveld analysis of the powder XRD data reveals that the β -HfMo 2 O 8 crystallizes in a monoclinic lattice with unit cell parameters a=11.4138(6), b=7.9105(4), c=7.4395(3), and β =122.35(0)° . The β -HfMo 2 O 8 is found to be isostructural to the room-temperature phase of β -ZrMo 2 O 8 . The coordination numbers of Hf and Mo atoms with oxygen atoms are eight and five, respectively. The nature and sharing of Hf and Mo polyhedra are entirely different in β modification compared to that in the α phase.

Accurate equation of state of AuIn 2 was obtained by x-ray diffraction measurements with ELETTRA synchrotron source to pressures over 20 GPa. Below 5 GPa, the P-V data when transformed to universal equation of state (UEOS), shows a deviation from linearity, confirming an electronic transition, consistent with the anomaly observed earlier in fusion and electrical data. The present high-resolution data also reaffirm a structural phase transition beyond 9 GPa. Full potential electronic band-structure calculations reveal a Lifshitz transition at the observed anomaly in the UEOS. Its transition pressure was found to be sensitive to the exchange-correlation terms.

In 0.25 Sn 0.75 alloy stabilized in the primitive hexagonal phase at ambient conditions is studied in a diamond anvil cell at high pressures with angle-dispersive x-ray diffraction using the synchrotron radiation at ELETTRA. The ambient primitive hexagonal phase was found to undergo a reversible transformation to a mixture of body-centered-tetragonal and hexagonal-close-packed phases above 11.6 GPa and not to the Cmca structure assigned in Si and Ge. The occurrence of the bct phase shows similarity to the high-pressure behavior of Sn. A transformation mechanism from the ph to hcp phase via the bct structure is also described.

Results of c/a variation under pressure for zinc from total-energy calculations that use first-principles linearized-muffin-tin-orbital calculations are presented. c/a decreases with compression and is found to change slope around V/${\mathit{V}}_{0}$\ensuremath{\approxeq}0.92, to approach the ideal value of c/a=1.63. This behavior is compared with existing experimental data and with full-potential linearized-augmented-plane-wave calculations.

The electronic structure of thorium is computed to study the recently reported (Phys. Rev. Lett. 67, 3563 (1991)) high-pressure fcc-bct structural transition. We find that elemental thorium is unique in that unoccupied {ital f} levels become populated at ultrahigh pressures, with the Fermi level intercepting the {ital f} band at a volume fraction of 0.6. The total energies show that the resulting occupation of {ital f} bands by about 1 electron drives the fcc-to-bct transition near 80-GPa pressure, and thorium then resembles the {ital f}-band metal cerium. The 6{ital p} electrons must be treated as band states in estimating the transition pressure and 0-K isotherm.

We present results of our electronic structure calculations and high pressure experimental studies of the angle dispersive X-ray diffraction, electrical resistivity, and thermoelectric power on Zn, Cd, AuIn2 and YNi2B2C. We focus on some of the observed anomalies at high pressures and find that the isostructural electronic topological transitions are responsible for them.

A first-principles computation of tetragonal distortion, structural stability, and equation of state has been carried out for indium, using the linear muffin-tin orbital electron-band-theory technique. Results on the variation of tetragonal distortion (c/a) with compression show a broad maximum around V/V 0 ≅ 0.8 (19 GPa) in agreement with experimental data. We do not find 5s5p-to-5d electron transfer as speculated to be the cause of the maximum in c/a. However, our results reveal that the core overlap resulting from broadening of the 4d states is the probable reason for the turnover in the c/a ratio. Within the atomic-sphere approximation we find the face-centered orthorhombic phase to be marginally stable beyond 56 GPa in accord with the experiments of Takemura et al.

We have studied the behavior of Pb at high pressure using the first-principles pseudopotential total-energy method, with spin-orbit effects treated within perturbation theory. Our calculations agree with recent experiments that indicate that Pb undergoes a phase transition to the bcc structure at pressures on the order of 100 GPa.

The ac resistance measurements on the trigonal (α-) phase of ZrMo2O8 and HfMo2O8 carried out up to 5GPa using the toroid anvil apparatus are reported. The ac resistance data in both the materials are found to be strongly frequency dependent and show a transition between two power law regimes with a crossover frequency (ωc) near 1kHz. For both the compounds slope changes and anomalies are observed in the ac resistance data at the structural transition pressures. The activation volumes at room temperature are obtained by analyzing the ac conductance data in terms of the pressure dependent activation volume model. The main conduction mechanism at 12Hz is believed to be the diffusion of oxygens ions, whereas hopping of point defects is responsible for ac conductuance above 33kHz. At 1KHz, a crossover from hopping to diffusion of point defects is observed at the α-δ (monoclinic) phase transition pressure.