V.C. Sahni

New phase transition in LiKSO4

Abstract A subtle first order phase transition in LiKSO 4 has been discovered with the help of a temperature dependent study of the Raman intensity measurements of certain polar modes in different polarization configurations. The room temperature hexagonal C 6 6 (P6 3 ) phase transforms to trigonal C 4 3v ( P 31c) phase at 201°K while cooling; the reverse transformation (on heating) takes place at 242°K. The phase transition appears to be primarily associated with a cooperative reorientation of SO 4 tetrahedra in the crystal.

Evolved gas analysis of YBa2Cu3Ox

Evolved gas analysis of X-ray pure YBa2Cu3Ox has shown a substantial evolution of CO2, in addition to the known O2 liberation. This CO2 evolution was found to weaken the intergrain coupling in the compound, causing a low temperature (<350°C) degradation of its superconducting properties. The present investigations are an attempt to identify the sources of carboration and find out ways to eliminate them.

X-ray structural study of crystalline and quasicrystalline Al6 CuMg4

Abstract X-ray diffraction study of quasicrystalline and transformed crystalline phases of Al 6 CuMg 4 alloy is reported. The pattern of this quasicrystalline alloys has been indexed and compared with that of Al 6 CuLi 3 . Close structural relationship between crystalline and quasicrystalline phases of this alloy is discussed. Our study shows that in transformed crystalline phase, few lattice sites, which lie on the vertices of a triacontahedron are partially occupied. The implications of this result are discussed.

On the correlation energy density functional in momentum space

Abstract A generalization of the Coulson-March formula has been developed interconnecting the electron density and the momentum denstiy within the framework of Thomas-Fermi theory. This has been employed to derive a correlation energy density functional in momentum space and numerical results are presented. Some remarks on the density functional theory in momentum space are also made.

Raman study of (NH4)2CuCl4·2H2O II. order-disorder transition

Abstract Thc results of a Raman scattering study of the order-disorder phase transition in (NH 4 ) 2 CuCL 4 · 2H 2 O are presented. From the variation of intensity of some selected modes in the temperature range 300-100°K, we have established the temperature dependence of the long range order parameter m , defined as the difference between the fractional number of NH 4 ions in the two possible orientations. Since the ammonium network in (NH 4 ) 2 CuCL 4 · 2H 2 O is “quasi one-dimensional”, in contrast to the three-dimensional network in NH 4 Cl, we discuss the consequence of this feature on the evolution of m .

Raman study of (NH4)2CuCl4·2H2O—I: Dynamics and structure in low temperature phase

Abstract We report the results of a Raman scattering study of (NH 4 ) 2 CuCl 4 2H 2 O at 300, 205 and 100°K, in order to elucidate the dynamics and phase transition in this double salt. A group theoretical calculation of the symmetry vectors, in the high temperature phase ( D 4 h 14 ), is made and the various modes are identified. The deuterated compound (ND 4 ) 2 CuCl 4 ·2D 2 O has also been investigated to help in identifying the modes involving motion of the ammonium ions and water molecules. Through a careful analysis of the spectra at 100°K, the space group in the low temperature phase has been established as D 2 d 3 . The important consequence of this result is that this leads to parallel spatial ordering of ammonium tetrahedra in this compound in the low temperature phase.