S.K. Deb

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.

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.

High pressure Raman scattering studies on adamantane

High pressure behavior of adamantane is investigated up to pressures of 26 GPa at ambient temperature using Raman spectroscopy. A detailed study of changes in the Raman spectrum of the C–H stretching modes across the disorder–order transition around 0.5 GPa is reported. Pressure dependence of the internal mode frequencies suggest two more subtle transitions around 2.8 GPa and 8.5 GPa, respectively, which are reversible. Evolution of spectra at higher pressures show evidence for another structural transition above 24 GPa.

Pressure induced phase transitions in hydroquinone.

High pressure behavior of alpha-hydroquinone (1,4-dihydroxybenzene) has been studied using Raman spectroscopy up to pressures of 19 GPa. Evolution of Raman spectra suggests two transitions around 3.3 and 12.0 GPa. The first transition appears to be associated with the lowering of crystal symmetry. Above 12.0 GPa, Raman bands in the internal modes region exhibit continuous broadening suggesting that the system is progressively evolving into a disordered state. This disorder is understood as arising due to distortion of the hydrogen-bonded cage across the second transition around 12 GPa.

Pressure-induced transitions in tetracyanoethylene: a Raman scattering study

We report the results of high-pressure Raman scattering studies of the cubic and monoclinic polymorphs of tetracyanoethylene (TCNE). The evolution of the Raman spectrum at high pressures suggests that the cubic form is stable up to about 8 GPa. Subsequent pressurization leads to a gradual loss of transparency, and the sample becomes opaque to visible light above 14 GPa. In the monoclinic samples, qualitative changes are observed in the Raman spectrum above 3.6 GPa which indicate a subtle phase transition around this pressure. These changes are reversible when the pressure is reduced from peak values of about 4.5 GPa. At still higher pressures, the sample progressively becomes black, similar to what is observed in cubic TCNE. The Raman spectrum of the sample above 7 GPa is indicative of polymerization of TCNE. The spectrum of the pressure cycled opaque phase shows broad features characteristic of an amorphous phase, which is understood as being due to random cross-linking of TCNE in the pressure-reducing cycle.

Raman scattering study of microstructure of n-type porous silicon

Abstract We report a detailed Raman scattering study of porous silicon film prepared on n -type silicon substrate. We observe large enhancement of Raman scattered signal and also that with increase in laser power the Raman line shape shows low frequency asymmetry, decrease in frequency of the peak position and reduction in signal enhancement. Our results could be explained consistently only by considering a two layered model of the microstructure of these films. We argue that this is a simple and non-destructive technique to look at the layered nature of the microstructure of these materials — so far seen only by high resolution electron microscopy.

High-pressure Raman scattering studies on hexamethylenetetramine

Abstract High-pressure Raman spectroscopic measurements have been carried out on hexamethylenetetramine (C6N4H12) up to pressures of 20 GPa. The evolution of Raman spectra at high pressures shows that the system undergoes two structural transitions around 1.4 and 12.5 GPa. The second phase transition around 12.5 GPa is associated with a large hysteresis indicating the first-order nature of the transition. Excessive broadening accompanied by considerable decrease in intensity of the internal modes above 15 GPa suggests that the system could be disordered above this pressure.

Excitonic resonance raman scattering from optic polar phonons in AgGaS2

The low temperature optical properties of the chalcopyrite semiconductor AgGaS2 is dominated by well defined excitonic level. We report here exciton mediated resonance Raman scattering from B2(LO) and E(LO) polar phonons at 235 cm−1 and 230 cm−1 respectively as a function of temperature over 20K to 200K. The variation resonance enhancement of these modes with temperature could be explained assuming an exciton width of the form Г (T)= a + bT + cT2 ie, the exciton decaying into one and two acoustic phonons. The agreement between the experiment and theory is very good. We also argue that the enhancement is due to Frohlich forbidden LO phonon-exciton coupling.