• V M Padmanabhan

      Articles written in Pramana – Journal of Physics

    • Neutron diffraction study of ammonium tartrate (NH4)2C4H4O6

      V S Yadava V M Padmanabhan

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      A neutron diffraction study of ammonium tartrate has been carried out. Using the diffractometer in symmetrical setting, intensities of 750 reflections have been measured. The positions of all the hydrogen atoms have been determined. A good agreement is noticed between the present neutron and the earlier x-ray heavy atom parameters. The tartrate ion consists of two nearly identical planar halves, with an interplanar angle of 62°. Tre structure is stabilized by a net-work of hydrogen bonds. Details of hydrogen bonding and the ammonium ions environment are discussed.

    • Neutron, x-ray and lattice dynamical studies of paraelectric Sb2S3

      K R Rao S L Chaplot V M Padmanabhan P R Vijayaraghavan

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      Neutron and x-ray diffraction studies of Sb2S3 indicate extensive diffuse scattering in the plane perpendicular to the chain axis of polymer-like (Sb4S6)n molecules. The crystal structure of the paraelectric phase is said to be orthorhombic with space group D2h16 with four molecules per unit cell. The observed diffuse scattering may be due to static disorder or some dynamical effects. In this paper the authors have examined the possible dynamical origin by recourse to lattice dynamical studies. Dispersion relation of phonons along the three symmetry directionsa*,b* andc* is evaluated based on a lattice dynamical model incorporating Coulomb, covalent and a Born-Mayer-like short range interactions. Group theoretical analysis based on the group of neutral elements of crystal sites (GNES) was essential in order to examine and aid in the numerical computations. The group theoretical technique involving GNES extended to ‘pseudo-molecular’ systems is also discussed in this context.

      The phonon dispersion relation shows that there are rather flat TA-TO branches of very low frequency in thea andc directions which may give rise to diffuse scattering. The branches along theb-axis are quite dissimilar to those alonga andc axes because of anisotropy. Variation of the potential parameters leads to instability of the lowest TA-TO branch. This is suggestive of a temperatures or pressure-dependent phase transition. However since these modes are optically ‘silent’ one needs to carry out either high resolution neutron scattering or ultrasonic studies to confirm various aspects of the theoretical studies.

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