M.L. Bansal

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.

Raman scattering study of LiKSO4—phases II and III

Detailed Raman scattering investigation of LiKSO4 in phases II and III across the transition temperatureTc ≃ 700 K is reported. Abrupt change in frequency and line width of the external and internal modes have been observed. Analysis of the results suggests lithium positional disorder and sulphate orientational disorder in the high temperature phase (II). The results also throw some light on the existence of twin domains in the crystal.

Rotational correlation function in methyl iodide

Abstract The rotational correlation function in CH 3 I has been determined from a Raman measurement of the v 3 (A 1 ) band profile at 296 K. The spectral data satisfy the second-moment criterion. The torque acting on the molecule has been obtained.

Calculation of Error in Depolarization Ratio Measurement Due to Finite Collection Angle in Laser Raman Spectroscopy

We calculate the error introduced in the measurement of the depolarization ratio (ρ) of strongly polarized lines due to the finite angle of collection employed in a laser Raman spectrometer. The mathematical formulation takes explicit account of both the collection lens and the polarization analyzer, placed to select the polarization of the scattered beam. It is shown that for the commonly employed geometry where the analyzer is rotated by 90° to select the polarized and depolarized intensities, keeping the polarization of the incident beam normal to the mean scattering plane, the relative error (Δρ/ρ) does not exceed 8%, even for a collection cone of half angle as large as 25° for ρ as small as 0.01. The present work also shows that the results in the literature which suggest that the relative error in the depolarization ratio is independent of ρ are incorrect.

Raman scattering investigation of the orientational dynamics of methyl iodide

Abstract A Raman scattering study of the v 3 vibration—rotation band in methyl iodide as a function of temperature and dilution (in cyclohexane) has been performed. All the data satisfy the second moment criterion. Detailed information about rotational correlation function, angular velocity correlation function, various correlation times and mean-square torque has been obtained. The correlation function, in the pure liquid, decays slowly with decrease in temperature whereas it decays faster with decreasing concentration in cyclohexane. It has been shown, from a consideration of the second moment as a function of concentration, that the contribution of collision-induced scattering to the v 3 band of methyl iodide is negligible. Applicability of a simple “damped librator model”, with a view to understanding certain aspects of the rotational motion in methyl iodide, is discussed.

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 .

Universal scaling law for first order phase transformation: LiKSO4

We have measured the kinetics of phase transition in LiKSO4 (C66→C3v4;Tc∼201 K) by monitoring the time-dependent change in Raman intensity of selected vibrational lines at various temperatures in the transformed phaseC3v4. We find that the observed growth curves for several temperatures display a nearly universal shape when plotted against a suitably scaled time parameter. We interpret our results in terms of a phenomenological model of nucleation and growth.

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.