• R Mittal

      Articles written in Pramana – Journal of Physics

    • Inelastic neutron scattering and lattice dynamics studies in complex solids

      Mala N Rao R Mittal Narayani Choudhury S L Chaplot

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      At Trombay, lattice dynamics studies employing coherent inelastic neutron scattering (INS) experiments have been carried out at the two research reactors, CIRUS and Dhruva. While the early work at CIRUS involved many elemental solids and ionic molecular solids, recent experiments at Dhruva have focussed on certain superconductors (cuprates and intermetallics), geophysically important minerals (Al2SiO5, ZrSiO4, MnCO3) and layered halides (BaFCl, ZnCl2). In most of the studies, theoretical modelling of lattice dynamics has played a significant role in the interpretation and analysis of the results from experiments. This talk summarises the developments and current activities in the field of inelastic neutron scattering and lattice dynamics at Trombay.

    • Inelastic neutron scattering study of lattice dynamics in α-ZnCl2

      A Sen Mala N Rao R Mittal S L Chaplot

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      Inelastic neutron scattering experiments have been carried out to measure the phonon density of states in polycrystalline α-ZnCl2 at Dhruva, Trombay. Lattice dynamical calculations, based on an interatomic potential model, are accomplished to study phonons associated with this otherwise extremely hygroscopic compound. Our calculated data are found to be well-compatible with the available measured ones.

    • Inelastic neutron scattering in Zr2NiH1.9 and Zr2NiH4.6

      R Mittal S L Chaplot P Raj K Shashikala A Sathyamoorthy

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      In this paper we report the results obtained from inelastic neutron scattering measurements on Zr2NiH1.9 and Zr2NiH4.6 using triple-axis spectrometer at Dhruva reactor, Trombay. The spectrum up to 35 meV represents largely the lattice modes of Zr and Ni atoms. The vibrational frequencies of hydrogen atoms are expected predominantly at higher energies. The phonon spectra from 35–180 meV were recorded using a Be filter as analyser. In order to analyse the observed neutron spectra, we assume a set of Ein-stein modes due to the hydrogen atoms which are delta functions in energy. These delta functions are broadened by the resolution of the instrument. The vibrational frequencies obtained from the fitting of the observed phonon spectra have been assigned to various tetrahedral sites in both the compounds.

    • Inelastic neutron scattering and lattice dynamics of GaPO4

      R Mittal S L Chaplot A I Kolesnikov C -K Loong O D Jayakumar S K Kulshreshtha

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      We report here measurements of phonon spectrum and lattice dynamical calculations for GaPO4. The measurements in low-cristobalite phase of GaPO4 are carried out using high-resolution medium-energy chopper spectrometer at ANL, USA in the energy transfer range 0–160 meV. Semiempirical interatomic potential in GaPO4, previously determined using ab-initio calculations have been widely used in studying the phase transitions among various polymorphs. The calculated phonon spectrum using the available potential show fair agreement with the experimental data. However, the agreement between the two is improved by including the polarisability of the oxygen atoms in the framework of the shell model. The lattice dynamical models are also exploited for calculations of various thermodynamic properties of GaPO4.

    • Negative thermal expansion in framework compounds

      R Mittal

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      We have studied negative thermal expansion (NTE) compounds with chemi- cal compositions of NX2O8 and NX2O7 (N=Zr, Hf and X=W, Mo, V) and M2O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.

    • Inelastic neutron scattering and lattice dynamics of NaNbO3 and Sr0.70Ca0.30TiO3

      S K Mishra R Mittal N Choudhury S L Chaplot D Pandey

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      NaNbO3 and (Sr,Ca)TiO3 exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice dynamical models have been developed for SrTiO3 and CaTiO3 which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO3.

    • Lattice dynamics of strontium tungstate

      Prabhatasree Goel R Mittal S L Chaplot A K Tyagi

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      We report here measurements of the phonon density of states and the lattice dynamics calculations of strontium tungstate (SrWO4). At ambient conditions this compound crystallizes to a body-centred tetragonal unit cell (space group I41/a) called scheelite structure. We have developed transferable interatomic potentials to study the lattice dynamics of this class of compounds. The model parameters have been fitted with respect to the experimentally available Raman and infra-red frequencies and the equilibrium unit cell parameters. Inelastic neutron scattering measurements have been carried out in the triple-axis spectrometer at Dhruva reactor. The measured phonon density of states is in good agreement with the theoretical calculations, thus validating the inter-atomic potential developed.

    • Inelastic neutron scattering and lattice dynamics of ZrO2, Y2O3 and ThSiO4

      Preyoshi P Bose R Mittal N Choudhury S L Chaplot

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      Zirconia (ZrO2), yttria (Y2O3) and thorite (ThSiO4) are ceramic materials used for a wide range of industrial applications. The dynamical properties of these materials are of interest as they exhibit numerous interesting phase transitions at high temperature and pressure. Using a combination of inelastic neutron scattering and theoretical lattice dynamics we have studied the phonon spectra and thermodynamic properties of these compounds. The experimental data validate the theoretical model, while the model enables microscopic interpretations of the observed data. The calculated thermodynamic properties are in good agreement with the experimental data.

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