• R Mukhopadhyay

      Articles written in Pramana – Journal of Physics

    • Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

      S L Chaplot R Mukhopadhyay P R Vijayaraghavan A S Deshpande K R Rao

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      Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.

    • Phase transformation and electrical resistivity of tetracyanoethylene under pressure

      P Ch Sahu K Govinda Rajan Mohammad Yousuf R Mukhopadhyay S L Chaplot K R Rao

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      This paper reports the phase transformation behaviour of tetracyanoethylene (TCNE) under pressure as revealed by AC electrical resistivity, its time evolution and X-ray diffraction studies. An irreversible transformation from monoclinic to cubic phase occurs at 2.1±0.1 GPa and is indicated by a sharp resistivity drop at this pressure. The time evolution of resistivity studies indicate that this transformation occurs via an intermediate phase having resistivity higher than either of the two crystalline phases. Finally, the kinetics of phase transformations obtained by time evolution of resistivity is compared with the X-ray studies on the pressure quenched TCNE.

    • 1H relaxation study in (NH4)2SbF5

      B Bandyopadhyay A Ghoshray R Mukhopadhyay R M Kadam

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      1H spin-lattice relaxation rate (T1−1) has been measured using inversion recovery technique in polycrystalline (NH4)2SbF5 system in the temperature range 140–400 K. From the plot of log (M0M) againstτ, we have estimated two differentT1 corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT1 in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.

    • Rotational excitations of NH4+ in (NH4)xK1−xI

      S L Narasimhan R Mukhopadhyay

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      We propose a gaussian model for the distribution of rotational tunneling lines which constitute the experimentally observed inelastic neutron scattering spectra in the range 0 to 0.6 meV from the mixed system (NH4)xK1−xI. The intensity profiles generated by this model are in remarkable agreement with the experimentally observed ones.

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