K R Rao

Inelastic neutron scattering from and lattice dynamics of α-KNO3

Coherent inelastic neutron scattering techniques are employed to measure several branches of the phoon dispersion relation in KNO3 in its orthorhombic (α-phase or phase II) form at room temperature. Group theoretical selection rules for external modes of the crystal have been used in the measurements along the three symmetry directions Σ(ξ00), Δ(0ξ0) and Λ(00ξ).Theoretical investigation of the lattice dynamics of the crystal is carried out on the basis of a rigid molecular-ion model using the external mode formalism. A two-body potential consisting of the Coulombic interaction and the Born-Mayer type short range interaction is assumed. The effective charges and radii of different atoms are determined by applying the stability criterion for the crystal. Dispersion curves are calculated, representation by representation, making use of group theoretical information. Comparison of theoretical results with experimental information on elastic constants, optical data and neutron results are made. Agreement between theoretical and the various experimental results may be considered very satisfactory.

SANS from micellar solutions of CTAB and sodium salicylate

SANS from 0.1M solution of CTAB without and with addition of varying concentrations of NaSal has been studied. The measured spectra show that on addition of NaSal two or more of CTAB micelles join together to form a bigger micelle. These micelles disintegrate to smaller ones on heating.

Phonon density of states in Tl2CaBa2Cu2O8

Abstract The neutron-weighted phonon density of states in the high-temperature superconductor Tl 2 CaBa 2 Cu 2 O 8 ( T c =107 K) is obtained from coherent inelastic neutron scattering measurements at the Dhruva reactor at 300 K. The phonon spectrum is qualitatively similar to that in the 90 K superconductor YBa 2 Cu 3 O 7 , and compares well with an independent lattice dynamical calculation.

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

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.

Study of radiation damage in stainless steel SS 302 by conversion electron Mössbauer spectroscopy

Abstract Conversion electron Mossbauer spectroscopy has been used to study radiation damage by 100 keV protons in stainless steel SS 302. For low dose the defect component is a quadrupole doublet with positive isomer shift. For higher dose, 5 x 1018 protons cm−2 and above, there is an additional single satellite line with larger isomer shift. The nature of radiation damage produced is discussed.

Neutron instrumentation for Dhruva utilisation

The paper reports the present state of neutron instrumentation at the 100 MW Dhruva reactor. It gives some illustrative examples of data taken on the recently commissioned instruments at the Dhruva and CIRUS reactors.

A computer simulation of LiKSO4

Molecular-dynamics computer-simulation of an ionic molecular solid LiKSO4 has been carried out at 300 and 1000 K using the atom-atom potentials obtained from lattice dynamical studies. We observe hopping of lithium ions to interstitial positions which is related to reorientations of sulphate tetrahedra.

Low-field neutron spin echo observation and development of a neutron spin echo spectrometer

Abstract The results of neutron spin echo (NSE) experiments conducted in the direct beam geometry using the BeO filtered incident spectrum have been discussed in the light of the results published in the literature. The design details of the NSE spectrometer to be installed at the Dhruva reactor are reported.

Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl2CaBa2Cu2O8 (Tl-2122,Tc=107 K) and YBa2Cu3O7 (Y-123,Tc=92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.

Mössbauer study of the stability of quasicrystalline phases: Al78Fe22 and Al82Fe18

A quasicrystal of Al-Fe with a stoichiometric composition very close to a crystalline phase is shown to transform to the crystalline phase with simple thermal cycling from 300 K through 78 K and 300 K.