On the nature of the dhcp to fcc transition under pressure in Pr and Pr-Th alloys

The results of electrical resistance (R), thermoelectric power (TEP) and X-ray diffraction measurements on praseodymium (Pr) and its alloys with thorium under pressure are reported. The maximum inR vsP curve exhibited by Pr persists only in the dhcp phase of PrTh alloy. X-ray measurements confirmed that in the alloys also the maximum inR vsP curve is due to the dhcp → fcc transition. Thus the behaviour of Pr and Pr-Th alloys is different from that of La and its alloys with Ce and Th where the maximum in theR vsP curve is electronic in origin and is exhibited by the dhcp, fcc and dist fcc phases.     

Opacities of high temperature highZ plasmas

A procedure is described for computation of opacities for highZ plasmas. Bound-bound, bound-free, free-free and scattering processes are considered. The inputs for these have been obtained by solving IEEOS form of Saha’s equation. Detailed calculations of opacities have been done for tungsten and uranium up to 10 keV of temperature. Preliminary results have been presented at the Nucl. Phys. and Solid State Phys. Symp., Dept. of Atom. Energy, held at Ahmedabad, India (December 1976).     

Reentrant high-conduction state in CuIr2S4 under pressure

The results of electrical resistance and angle dispersive X-ray diffraction measurements at high pressures and ambient temperature on the chalcogenide spinel, CuIr₂S₄ 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.     

Measurement of laser driven shock wave transit time through thin aluminium targets by optical shadowgraphy

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. PACS 52.50Jm; 52.50Lp; 52.25 Communicated by K. Takayama     

Equation of state theories of condensed matter up to about 10 TPa

A knowledge of the equation of state (EOS) of matter is necessary to describe the dependence of its thermodynamic properties on its microscopic internal structure. EOS data are also vital for a variety of applications, particularly those that involve numerical simulation of dynamic processes. This has motivated intense research on EOS of materials, both in experimental and theoretical areas. Various laboratories in the world are involved in the development of techniques for the generation of increasingly higher pressures using high power lasers, electric and rail guns etc. The range of these controlled dynamic pressures is currently reaching about10 TPa. On the theoretical side, the earlier empirical methods have now been replaced by sophisticated band structure calculations. In this article, we review these first principle theoretical attempts to calculate EOS over a range of densities and temperatures, where a variety of physical phenomena such as band crossings, band closures, phase transitions, nuclear and electronic thermal excitations, core ionization, liquid disorder etc. are encountered. The emphasis in this review is on shock Hugoniot equation of state theories.     

Shock Hugoniot calculations and melting in Pb

Shock Hugoniot calculations are carried out for lead (Pb) using both solid and liquid state theories. The Hugoniots in solid and liquid cases are in mutual agreement within the experimental uncertainties. However, the shock temperatures are quite different as computed from solid and liquid state theories. This fact can perhaps be used to detect melting along the shock Hugoniot provided the shock temperatures are accurately measured.     

Molecular dynamic studies on materials under laser shocks

Molecular dynamic (MD) simulations offer a powerful means of understanding the microscopic characteristics of shock-propagation through solids and fluids, especially for the short spatial and temporal scales relevant to laser-driven shocks. First-principles molecular dynamics can be directly compared with time-resolved experimental measurements, and methods based on empirical (embedded-atom) potentials fitted to first-principles quantum-mechanical calculations are effective for MD simulations of shock propagation through many millions of atoms. In comparison, thermodynamic approaches based on free-energy considerations do not provide detailed information about mechanical-relaxation or phase-transformation processes within the shock front. We illustrate these ideas by way of embedded-atom simulations of shock-wave propagation through copper crystals of different orientation.     

High-pressure study of adamantane: variable shape simulations up to 26 GPa

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.     

Determination of lead in water using laser ablation–laser induced fluorescence

The detection sensitivity of laser-induced breakdown spectroscopy (LIBS) is improved by coupling it with a laser-induced fluorescence method. A waterjet sample containing 500 ppm of Pb as an analyte was ablated by a 266 nm, frequency-quadrupled Q-switchedNd:YAG laser at an energy of ~ 260 μJ. After a short delay the resulting plume was re-excited with a 283.306 nm, nanosecond pulse dye laser at energies ranging from 45 to 100 nJ. The limit of detection (LOD) of lead in water was determined both by the single-pulse LIBS technique and Laser Ablation coupled with Laser-Induced Fluorecence (LA–LIF) method. It was found to be 75 ppm in the case of single-pulse LIBS and 4.3 ppm for LA–LIF. When the resonant pulse was detuned from the transition wavelength the LA–LIF signal disappeared demonstrating the resonant selectivity of this technique.