• S K Sikka

      Articles written in Pramana – Journal of Physics

    • Structure determination of Ls-threonine by neutron diffraction

      M Ramanadham S K Sikka R Chidambaram

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      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.

    • Saha’s ionization equation for highZ elements

      B K Godwal S K Sikka

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      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.

    • Opacities of high temperature highZ plasmas

      B K Godwal S K Sikka

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      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.

    • Phenomenology of the Pokaranpne experiment

      R Chidambaram S K Sikka Satish C Gupta

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      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.

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

      Surinder M Sharma V Vijayakumar S K Sikka R Chidambaram

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      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

      Hema Sankaran Surinder M Sharma S K Sikka R Chidambaram

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      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.

    • Shock Hugoniot calculations and melting in Pb

      B K Godwal S K Sikka R Chidambaram

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      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.

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

      V Vijayakumar B K Godwal S K Sikka R Chidambaram

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      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.

    • Pressure induced amorphization of AlPO4

      Hema Sankaran Surinder M Sharma S K Sikka R Chidambaram

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      AlPO4 has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO4 reported by Jayaraman and coworkers (1987).

    • Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

      M S Somayazulu Nandini Garg Surinder M Sharma S K Sikka

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      Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2.

    • High pressure study of phase transitions inα-FePO4

      Chitra Murli Surinder M Sharma S K Kulshreshtha S K Sikka

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      High pressure behaviour of FePO4 in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO4 tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.

    • Spectroscopic study ofβ-Ni(OH)2 under pressure

      Chitra Murli V Sugandhi Nandini Garg Surinder M Sharma S K Sikka

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      Infrared absorption and Raman study ofβ-Ni(OH)2 has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm−1 around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.

    • Electronic structure of MgB2

      P Modak R S Rao B K Godwal S K Sikka

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      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 24.7 K.

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