S. K. Sikka

Structure determination of Ls-threonine by neutron diffraction

The structure of the aminoacid, Ls-threonine [NH3+ CH(CHOHCH3)COO−], space groupP212121,a=13.630(5),b=7.753(1),c=5.162(2) Å,z=4, has been determined from neutron diffraction data using direct methods. The intensities of 1148 neutron Bragg reflections were measured from a single crystal. The structural parameters were refined by the method of least squares using anisotropic temperature factors. The finalR(F2) is 0.068. The structure was also refined from the x-ray data of Shoemakeret al (1950J. Am. Chem. Soc.72 2328); there is good agreement between the two sets of heavy atom parameters. The parameters of hydrogen atoms are of course more precisely determined in our neutron study. The molecular conformation and the hydrogen bonding scheme are discussed. Weighted average values of bond distances and angles from 14 aminoacid structures with ionized carboxylic groups studied by neutron diffraction at Brookheven and Trombay are also presented.

The equation of state and structural stability of titanium obtained using the linear muffin-tin orbital band-structure method

Room-temperature isotherms of titanium in HCP and omega -phases are calculated by the first-principles linear muffin-tin orbital energy band method. Comparison with experimental data shows excellent agreement. Structural phase stability analysis by the Andersen force theorem shows that the omega -phase is the lowest-energy phase at 0 K and normal volume. The possibility that the s to d electronic transition is the cause of shock discontinuity at 17.5 GPa.

HIGH pressure structural investigations of zirconium using LMTO method

Abstract The structural energy differences have been calculated for zirconium as a function of pressure at zero temperature using the Andersen force theorem and the linear muffin tin orbital method. The structures included are the following: α (hcp), the room temperature room pressure phase, ω- a three atom simple hexagonal, bcc and fcc. Our calculations show that the bcc structure would become energetically most favourable above 11 GPa. This results is in agreement with well known correlation between the crystal structure and the d-electron population in transition metals at normal volume. The diamond anvil cell based high pressure x-ray diffraction experiments are in progress to verify this result.

On orientation relations between α and ω phases in Zr by texture studies using neutron diffraction method

In the study reported here, the authors have used the neutron diffraction method to confirm the orientation relations between alpha and omega phases in Zr by utilizing the texture in as-grown, alpha-phase samples. As is now well established, the three most common crystal structures encountered in group IV elements are: alpha, beta, and omega. From a selected area electron diffraction study done on pressure-treated Ti and Zr foils, the variant I (illustrated in this paper) was observed. Further, the rocking curve, with the cylindrical axis vertical, for the (100)alpha reflection, indicated that there were only a few big grains present in the sample.

High pressure phase transitions in organic solids I:α → β transition in resorcinol

An experimental program has been started to study polymorphic phase transitions under pressure in organic solids using the Be gasketing technique developed by us. This allows us to obtain x-ray diffraction patterns of low symmetry organic solids with high resolution, employing CuKα radiation. The first organic solid studied is α-resorcinol. At 0.5 GPa, it transforms to its high temperature and denser modification, β-resorcinol. The transformation mechanism is discussed with the help of molecular packing calculations.

High pressure phase transitions in organic solids II: X-ray diffraction study ofp-dichlorobenzene at high pressures

X-ray diffraction experiments onp-dichlorobenzene at high pressures show a transition at ∼ 0.3 GPa, to a new phase, the diffraction pattern of which cannot be indexed on the anticipated low temperature monoclinic crystal structure. We have instead found an orthorhombic cell, very closely related to the low temperature monoclinic cell, for this new phase. This structure, which also occurs inp-diiodobenzene at ambient conditions, has cell constantsa =14.02,b = 6.06,c = 7.41Å andZ = 4. The space group is Pbca. This new phase has a non-β herring-bone structure, in contrast with the initialα phase which has aβ-structure with ribbon-like arrangement of molecules, with Cl-Cl contacts of ∼ 4A between adjacent molecules. This implies that with pressure the halogen-halogen interaction in this compound plays a less dominant role in crystal engineering.

Saha’s ionization equation for highZ elements

Saha’s ionization equation has been solved for highZ elements with the aim of providing input for opacity calculations. Results are presented for two elements, tungsten and uranium. The ionization potentials have been evaluated using the simple Bohr’s formula with suitable effective charges for ions. The reliability of the free electron density, ion concentrations, etc., obtained from the Saha’s equation solutions has been checked by comparing thePT andET computed from them with those given by the Thomas-Fermi-Dirac equation of state. The agreement between the two is good from temperatures above 0.2 keV.

Phenomenology of the Pokaranpne experiment

The phenomenology of the Pokaranpne experiment (yield - 12 kiloton oftnt) conducted in a shale-sandstone rock, 107 meters underground, is described with the aid of computations using a one-dimensional spherical symmetric rock mechanics computer code developed by the authors. The calculated values of cavity radius, spall velocity and extent of rock fracturing are in good agreement with the observed values. The principal mechanism for crater formation at Pokaran was spall and the relatively smaller crater dimensions and non-venting of radioactivity gases were due to lower kinetic energy transferred to the shale-sandstone rock.

Some investigations on shock wave induced phase transitions

This paper briefly presents the activities of our laboratory in the area of phase transition studies under shock waves. Both experimental and theoretical studies are carried out. For experimental investigations, a single stage gas gun and associated diagnostic techniques have been set up. For interpretation of shock wave data, theoretical methods based on density functional formalism and molecular dynamics techniques are employed. Some examples from our work are presented.

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.