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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.

Using an updated data set of ballistic PKIKP travel time data at antipodal distances, we test different models of anisotropy in the Earth's innermost inner core (IMIC) and obtain significantly better fits for a fast axis aligned with Earth's rotation axis, rather than a quasi-equatorial direction, as proposed recently. Reviewing recent results on the single crystal structure and elasticity of iron at core conditions, we find that an hcp structure with the fast c axis parallel to Earth's rotation is more likely but a body-centered cubic structure with the [111] axis aligned in that direction results in very similar predictions for seismic anisotropy. These models are therefore not distinguishable based on current seismological data. In addition, to match the seismological observations, the inferred strength of anisotropy in the IMIC (6–7%) implies almost perfect alignment of iron crystals, an intriguing, albeit unlikely situation, especially in the presence of heterogeneity, which calls for further studies.

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.

Ab initio molecular dynamics simulations carried out for tetragonal and orthorhombic distortions of iron closely follow the results of static-lattice electronic-structure calculations in revealing that the body-centered cubic (bcc) phase of Fe is mechanically unstable at pressures of 0.3–1.5 TPa and temperatures up to 7000 K. Crystal-structural instabilities originate in the static lattice for the bcc configuration, and are consistent with recent results from both static and dynamic high-pressure experiments. Both theory and experiment thus show that the close-packed (hexagonal, hcp and face-centered cubic, fcc) crystal structures of iron are those relevant to the cores of Earth-like planets.

The high-pressure behavior of HgO-montroydite was investigated up to 36.5 GPa using angle-dispersive X-ray diffraction. The tetragonal phase of this material (HgO-II), a distortion of the NaCl structure, transforms into the cubic NaCl structure (HgO-III) above ~31.5 GPa. The transformation of mercury oxide from the orthorhombic Pnma (HgO-I) structure to a tetragonal I4/mmm structure (HgO-II) is confirmed to occur at 13.5 ± 1.5 GPa. Neither of the high-pressure phases, HgO-II nor HgO-III, is quenchable in pressure. The derived isothermal bulk modulus of HgO-II and its pressure derivative strongly depend on the assumed zero-pressure volume of this phase, but our elasticity results on HgO-II nevertheless lie significantly closer to theoretical calculations than prior experimental results, and the measured pressure of the phase transformation to the NaCl structure is also in agreement with recent theoretical results. The general accord with theory supports the existence of significant relativistic effects on the high-pressure phase transitions of HgO.

The high pressure behavior of mercury decomposed from mercury oxide up to 20.4 GPa was investigated using angular-dispersive X-ray diffraction. The results showed that liquid mercury solidified at 2.0 GPa and was resolved as α hexagonal, R-3m, a=3.3743±0.0007 A and c=6.8199±0.0013 A. When compressed up to 5.7 GPa, α mercury transformed into orthorhombic γ phase directly, which is not the case of transforming from an α structure to a body-centered tetragonal structure (β). The space group of orthorhombic γ phase was interpreted successfully as Pmmn, with a=2.7722±0.0010 A, b=4.0792±0.0028 A and c=6.8285±0.0029 A at 8.9 GPa.

We have measured the equation of state of the intermetallic compound AuIn 2 up to 20 GPa and Cd 0.8 Hg 0.2 up to 50 GPa using methanol–ethanol–water solution or argon as pressure media. In the experiments performed with argon as pressure medium, we minimized non-hydrostatic conditions by thermally annealing the sample. We present data revealing compressibility anomalies in AuIn 2 at 2.7 GPa and in Cd 0.8 Hg 0.2 near 8, 18 and 34 GPa with methanol–ethanol–water and argon. At pressures above 5 GPa the P – V data for AuIn 2 and Cd 0.8 Hg 0.2 from experiments preformed with argon as a pressure medium start deviating from those using methanol–ethanol–water, and the equation of state based on experiments in argon is stiffer compared with that in methanol–ethanol–water. This behavior is consistent with the relative merits of the two pressure transmitting media as documented in the literature. We also provide a brief summary of the results of electronic structure calculations that associate these anomalies with electronic topological transitions.

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.

The high pressure and high temperature structural investigations on α and β phases of HfMo 2 O 8 are reported. α - HfMo 2 O 8 transforms to a monoclinic ( δ ) phase, followed by a second transition to a triclinic ( e ) phase at 1.30(5) and 2.70(5) GPa, respectively. α - HfMo 2 O 8 undergoes pressure induced amorphization above 24 GPa due to kinetically hindered phase transition to its high density polymorph β phase. The β - HfMo 2 O 8 also undergoes a complete pressure induced amorphization above 35 GPa due to the kinetic hinderance to decomposition. When annealed at ambient pressure, β - HfMo 2 O 8 reconverts to the trigonal α phase above 700 °C.

In an effort to set up microtron based photoneutron source and in order to optimize the neutron yield, photoneutron production from beryllium has been studied for different volumes of beryllium irradiated by different peak energy bremsstrahlung radiation. The theoretical estimation of neutron yield has been carried out using the MCNP simulation for 8.75, 8.15 and 7.58 MeV peak energy of bremsstrahlung radiation. The experimental measurements were carried out using two types of detectors: SSNTD CR-39 and custom designed Silver wrapped GM detector. The neutron yield corresponding to beryllium of volume 381.70 cm 3 are found to be 2.13E+09, 2.00E+09 and 1.74E09 n/s (MCNP calculation values) for electrons of energy 8.75, 8.17 and 7.48 MeV, respectively. The experimental results are compared with the MCNP simulated results and are good agreement.

The results of electrical resistance and angle dispersive X-ray diffraction measurements at high pressures and ambient temperature on the chalcogenide spinel, CuIr 2 S 4 are reported. The resistance increases gradually and reaches around 12 GPa a value that is approximately forty times the initial value. Above 15 GPa, the resistance decreases up to 30 GPa and on further pressure increase tends to saturate at a value slightly above the ambient pressure value. Thus, the material exhibits a reentrant high conducting phase under pressure. The behaviour of the electrical resistance exhibits a close correlation with the structural evolution with pressure.

Results of X-ray diffraction, electrical resistance, thermoelectric power measurements and electronic band structure calculations on NiSi2 under high pressure are reported. The thermoelectric power (TEP) changes sign near 0.5 GPa (from +30 to −20 μV/K). As the pressure is increased, the value of TEP increases further in magnitude and near 7 GPa it becomes −50 μV/K. The pressure vs. resistance curve measured up to 30 GPa using diamond anvil (DAC)-based technique exhibits a broad hump near 12 GPa and exhibits hysteresis on pressure release. The ADXRD patterns up to 42 GPa show a gradual irreversible loss of long-range order in NiSi2 with the diffraction lines progressively broadening under pressure. The FWHM of the diffraction lines show a rapid increase in the half-widths close to 0.5 GPa and also near 12 GPa. The computed band structure at a compression (without any disorder) corresponding to 12 GPa, exhibits an electronic topological transition (ETT). The rapid increase in disorder above 12 GPa implies that the ETT may be facilitating the structural disorder. It is suggested that the pressure drives the material through a region of entropic and energetic barriers and induces disorder in the material.

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.

Results of electronic band structure calculations, electrical resistance, thermoelectric power (TEP), and x-ray diffraction measurements, under pressure carried out on AuGa(2) to investigate its anomalous behaviour are reported. The first principles electronic band structure calculations confirm that a flat band close to the Fermi level along the Γ-X direction of the Brillouin zone is responsible for the unusual behaviour of AuGa(2). In synchrotron-based high-pressure x-ray diffraction measurements, it is observed to undergo a structural phase transition above 7 GPa. The TEP variation with pressure and the P-V data up to 7 GPa transformed to the universal equation of state (UEOS) indicate the existence of an electronic topological transition (ETT) near 3.2 GPa. Consistent with this, in electronic structure calculations carried out at reduced sample volume corresponding to 4 GPa, it is seen that the flat band crosses the Fermi level. The structure above 7 GPa is a distortion of the CaF(2) phase. This structure continuously evolves with increasing pressure. The continuous variation of electrical resistance across the transition is consistent with this.

We present the results of our studies using the first principles theories on materials in which controversies exist in the diamond anvil cell based high pressure data. The importance of theoretical simulations based on the first principles molecular dynamics or based on embedded atom potentials to study the dynamic response of condensed matter under shock is highlighted with results and their comparison with experimental data of time dependent X-ray diffraction. Finally, realizing the inadequacy of many variants of Thomas–Fermi–Dirac theories to simulate the matter in 10 to few hundred Mbar pressures, there is need to treat the LDA or GGA theories with due consideration of the core states becoming delocalized under these extreme pressures as a result of pressure and thermal ionization.

We report the results of an X-ray diffraction study of CdAl 2 Se 4 and of Raman studies of HgAl 2 Se 4 and ZnAl 2 Se 4 at room temperature, and of CdAl 2 S 4 and CdAl 2 Se 4 at 80 K at high pressure. The ambient pressure phase of CdAl 2 Se 4 is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch–Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl 2 Se 4 and ZnAl 2 Se 4 reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order–disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl 2 S 4 and CdAl 2 Se 4 , which shows a very weak temperature dependence of the Raman-active phonon modes.

Phase contrast imaging is a new method of radiography in which the information of change in phase of the X-rays as it passes through the object gets reflected in the intensity. This leads to a better sensitivity and contrast than the conventional absorption radiography. In this paper we discuss the simulation studies of phase contrast imaging using monochromatic and polychromatic X-ray point source for simple two- and three-dimensional objects like circular and spherical objects (made up of carbon-fiber, polystyrene and lung tissue). The advantages of refraction contrast images are discussed in terms of contrast and resolution, and a comparison is made with absorption images. The result obtained shows considerable improvement in contrast with phase contrast imaging as compared to conventional absorption radiography. These results also guide us in proper selection of source to object distance, object to detector distance, etc. These results are proposed to be used in our experiment on phase contrast imaging with microfocus X-rays. The technique is going to be very useful in improving the resolution in the X-ray imaging for the composites, and in detection of cracks at micron level resolution. Moreover, if the doses can be controlled by proper selection of the detector or the source, it can have clinical application in the mammography.

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.

Laser driven shock wave transit time in thin aluminium targets was experimentally estimated by determining the shock emergence time at the rear of thin aluminium foils of varying thickness from 5 to 35 μm. A 20 J, 5 ns Nd:glass laser was focused to produce laser intensity of 1012 to 5 × 1013 W/cm2 on the targets which were placed in vacuum. Target foil movement was measured to an accuracy of 10 μm using optical shadowgraphy technique. This technique was used to accurately measure the shock transit time by recording the optical shadowgrams at various instants of time and thus identify the instant at which the foil is just set into motion. Shock transit time measured in foils of different thickness can give the value of shock velocity at a given laser intensity. Target motion recorded by shadowgraphy can also give the target foil velocity from which shock pressure can be estimated. Experimental values of shock transit time, shock velocity and shock pressure were observed to agree well with the values using one-dimensional multi-group radiation hydrodynamic simulations.

We report simulations of adamantane by carefully combining ab initio and empirical approaches to enable simulations with internal degrees of freedom on crystalline adamantane up to a pressure of 26 GPa. Two sets of simulations, assuming the adamantane molecule as a rigid (RB) and flexible body (FB), have been carried out as a function of pressure up to 26 GPa to understand changes in the crystal structure as well as molecular structure. The flexible body simulations have been performed by including 6 lowest frequency internal modes (obtained from DFT calculations performed with Gaussian98) out of the total of 72. The calculated variation in c/a and V/V(0) from the RB and FB calculations as a function of pressure have been compared with the experimental curve. Other relevant thermodynamic and structural properties reported are the radial distribution functions, deviation in the position of a given type of atom with respect to its position at standard pressure, delta(s), cell parameters, volume, and energy. With an increase in pressure, three additional peaks are seen to develop gradually at three different pressures in the center of mass (com)-com radial distribution function (rdf). We attribute these changes to structural transformations (probably second-order phase transitions) which is consistent with the three phase transitions reported by Vijayakumar et al. for adamantane in the pressure range of 1 atm-15 GPa. Our simulations also show that these additional peaks in the rdf's are associated with the differences between opposite and parallel spin neighbors of Greig and Pawley as well as the crystallographic directional dependence of intermolecular distances in the first three shells of the neighbors. Also, the structural quantities from the RB calculation show considerable deviation from the FB calculation for pressures greater than 5 GPa, which suggests that the rigid body assumption for molecules may not be valid above this pressure.

Results of electrical resistance measurements on MgB 2 at ambient temperature up to 25 GPa are presented. An abrupt reduction of nearly 30% in resistance around 18 GPa is observed. Band structure calculations in the presence of a frozen-in distortion of the E 2g phonon mode reveal that one of the closed Fermi sheets corresponding to the σ-band opens along the Γ – A direction at this pressure. It is suggested that the anomaly observed in the resistance is due to this phonon mediated electronic topological transition (ETT).

La3Ni2B2N3, which is similar to YNi2B2C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher TC, we have studied the compound Th3Ni2B2N3, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th3Ni2B2N3 as found for La3Ni2B2N3, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La3Ni2B2N3. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th3Ni2B2N3 to be a high temperature superconductor with a Tc in excess of 30 K.

We report the results of the high-pressure angle dispersive powder X-ray diffraction measurements obtained up to 31 GPa on the monoclinic and the tetragonal phases of NbOPO 4 . The tetrgonal phase was obtained by quenching the monoclinic phase from 5 GPa and 833 K. The lattice parameters of the monoclinic phase show an anomaly at 0.6 GPa, but the unit cell volume follow an almost linear compressional behavior. The bulk modulus B 0 and its pressure derivative B 0 ′ are estimated to be 66(3) GPa and 1.6(8), respectively, for the monoclinic phase. The irreversible pressure induced amorphization observed at ambient temperature in the monoclinic phase is due to the kinetic hindrance to bond reconstruction that is required for transformation form the monoclinic to the tetragonal phase. In the case of tetragonal NbOPO 4 , the compression is found to be anisotropic.

We review various anomalies seen in Raman spectroscopy, superconducting transition temperature T c and electrical resistance at room temperature and high pressure in the new high temperature superconductor MgB2. In order to explore the possibility of the anomalies being caused by an electronic topological transition (ETT), we have carried out detailed electronic structure studies of the band structure under pressure. Our calculations are done in the presence of a frozen-in phonon displacement that mimics the effects of thermal vibrations of the B-atoms at room temperature. Our results clearly establish the occurrence of a phonon-assisted ETT near 18 GPa. The dimensionless electron–phonon coupling λ is also found to be anomalous in its pressure variation at the same pressure. We also indicate why a detailed explanation of the phonon Raman anomaly needs the inclusion of effects arising from a non-linear electron–phonon coupling.

We calculate the phonon linewidth of a Raman-active zone centre phonon in a correlated metal. The correlated metal is described by the half-filled single-band Hubbard model on a hypercubic lattice in d = ∞ . The electronic self-energy and Green function are determined within the dynamical mean field theory (DMFT) for various values of U in the metallic regime and then used to calculate the phonon linewidth. We consider only the lowest order contribution corresponding to a process in which the phonon decays into a particle–hole pair. At weak correlation we find that the electronic contribution to the phonon linewidth is negligibly small but it grows with increasing correlation strength and then reaches a maximum before showing a decrease.

The ambient structural details and the results of room temperature high pressure angle dispersive X-ray diffraction and electrical resistance measurements on the quasi-one-dimensional sulfide, InV 6 S 8 , to a pressure of 25 GPa are reported. The material does not undergo a phase transition in this pressure range, though an anomaly in the c/a ratio has been observed around 10 Gpa. A fit of the Murnaghan equation of state to the V/V 0 versus pressure data, with the value of the derivative of B 0 with respect to pressure, B' 0 , fixed at 4 has yielded a value of the bulk modulus, B 0 , of 110 GPa. We also present data of the pressure dependence of the lattice constants, a and c, the ratio c/a, and the resistance at room temperature.

Results of the low frequency dielectric constant (e), dielectric loss and x-ray powder diffraction (XRD) measurements on orthorhombic Al 2 (WO 4 ) 3 under pressure are reported. The dielectric constant and the dielectric loss both show a sharp peak at about 0.5 GPa, indicating a ferroelastic phase transformation. This phase is found to be identical to the low temperature monoclinic phase of Al 2 (WO 4 ) 3 . Another monoclinic modification of Al 2 (WO 4 ) 3 is also observed from our XRD studies at 3.4 GPa. Al 2 (WO 4 ) 3 amorphizes above a pressure of 18 GPa, and this pressure induced amorphization is attributed to the lack of thermal activation for decomposition.

We present the Hugoniots of Al, Ta, Mo and W in their solid as well as liquid phases. The liquid phase calculations are carried out on the basis of the corrected rigid spheres (CRIS) model. The 0 K isotherm of the solid phases, which are the necessary inputs for our computations, have been obtained by full potential first principles electronic structure calculations with generalized gradient approximation (GGA) for the exchange–correlation terms. The melting curve as a function of pressure was obtained according to the recently published model based on dislocation mediated melting, and also compared with that using Lindemann criterion. Though the adiabatic pressure–volume curve is affected little by melting, the pressure–temperature curve shows substantial change.

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.

3D Visualization of the interior of an object has been one of the important goals of NDT, which has many applications. It can be used for examination of defects and cracks in their full three-dimensional perspective and reverse engineering applications. This paper describes the development and characterization of a high dynamic range cooled CCD-based cone beam tomography system for industrial applications and in particular for the NDT of materials used in the nuclear industry. The novel feature of this system is its ability to distinguish minor variations in the attenuation coefficients with very high resolution. The paper also discusses the development of software implementation of Feldkamp formulation for reconstruction, testing with mathematically simulated phantom, and hardware used in the work. The system is characterized in terms of MTF both in the mid-plane and off-midplane are also discussed in this paper. We will present results of experiments conducted on a variety of samples of nuclear and industrial use.

We adopt first principles methods for Zn-Mg clusters (related to Al-Zn-Mg quasicrystals) to estimate the energy cost for disorder in the glue region by giving small displacements to only the glue atoms, while keeping the atoms in the icosahedral cages frozen at the ideal icosahedral positions. We compare these estimates of cluster calculations with the energy changes for a structural transition in solids, and find that the glue region disorder costs at least an order of magnitude less energy. These results support the proposition of the cluster-disordered glue model, that the glue region in a quasicrystal may be disordered.

The effect of high pressure on the short-range order in the stable Al63.5Cu24Fe12.5 quasicrystal has been studied using Mossbauer spectroscopy. In agreement with the earlier studies, the system shows no structural transition up to the highest pressure of 24.2 GPa used in the present study. Variations in both the isomer shift (IS) and the quadrupole splitting (QS) of two non-equivalent iron sites with pressure suggest changes in the short-range order in the system. A linear variation of IS with pressure can be understood in terms of the compressibility of the structure around the Fe atoms. An increase in the quadrupole splitting with pressure suggests that in addition to the compression, a distortion of the local arrangement of Al atoms around an Fe atom also takes place. At 24.2 GPa, the Mossbauer lines exhibit significant broadening, indicating an increase in the static disorder in the system with pressure.

We report the results of electrical resistance measurements at high pressures on Cs 2 MoS 4 and KTbP 2 Se 6 . The results of high pressure X-ray diffraction study of Cs 2 MoS 4 are also presented. Interestingly, in the case of Cs 2 MoS 4 the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct–indirect band crossing. In the case of KTbP 2 Se 6 ,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs 2 MoS 4 .

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.

We report first-principles full potential linearised augmented plane wave calculations of the electronic band structure of the compound MgB2C2 and its hole-doped derivatives Mg0.5Li0.5B2C2, Mg0.5Na0.5B2C2, Mg0.9Na0.1B2C2 and Mg0.5K0.5B2C2. The parent compound MgB2C2 is a band insulator, which on hole doping, is predicted to turn metallic with a large density of states at the Fermi energy. Its band dispersion shows a flat band feature close to the Fermi energy, reminiscent of MgB2. Based on our estimates of changes in the density of states at the Fermi level, we predict that hole-doped MgB2C2 is a potential candidate for high-temperature superconductivity.

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 report room temperature electronic structure calculations on zinc at various compressions. Calculations show that the ln H scaling of the equation of state (EOS) in different forms shows changes of slope near 10 GPa indicating subtle electronic structure changes. The L-point electronic topological transition (ETT) is found to be very sensitive to the exchange-correlation terms. Even though elevated temperature is expected to reduce the signatures of an ETT, it also alters the proximity of the eigenvalues to the Fermi level, and this effect enhances the possibility of an ETT at the L-point in Zn. The weak c/a anomaly around 10 GPa persists in the 300 K calculations also.

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.

For centuries, scientists and engineers dreamed of being able to peer into the interiors of microscopic and macroscopic objects. A variety of imaging techniques, both invasive and non-invasive, have been developed to accomplish this goal. The present article provides an overview of our work on non-invasive imaging techniques being used and developed for basic and applied sciences. The details of various developments in setting up X-ray and neutron imaging techniques for applications in the Indian nuclear programme, development of two- and three-dimensional tomography for industrial applications, digital medical imaging, X-ray diffraction imaging for exploring the condensed matter at normal and high pressure conditions, X-ray holography for atomic resolution imaging, phase-sensitive imaging and imaging of transient phenomenon are discussed in this paper.