P. S. Narayanan

Laser Raman spectroscopic study of ferroelastic LiCsSO4

Lithium caesium sulphate has been reported to undergo a phase transition from the room temperature orthorhombic phase with space groupP cmn to a final phase with space groupP 22/n. Though a sharp anomaly in its physical properties has been found at 202.0;K, it was found that there was a need for careful investigations in the vicinity of 240 and 210.0;K. Since the changes in the crystal structure involve primarily a rotation of the SO4 tetrahedron about thec-axis and as this may be reflected both in the intensity and polarisation of the internal as well as external phonon modes, the laser Raman spectra of oriented single crystals of LiCsSO4 at different temperatures were investigated. For correlation and definite identification of the spectral features, its infrared absorption spectrum was also studied. An analysis of the intensities and polarizations of the internal modes of the sulphate ions reveals the change in symmetry of the crystal. The integrated intensity and peak height of thev1 line, plotted against temperature show anomalous peaks in the region of the phase transition. Differential scanning calorimetric study gives the enthalpy change ΔH across the phase transition to be 0.213 kJ/mol.

Thermal expansion and phase transition in NaVO3

The present x-ray study has been undertook in order to correlate the phase transition in sodium metavanadate NaVO3 crystal with its structural aspects. The thermal expansion behaviour of NaVO3 was studied from room temperature up to 500 C, well beyond the transition temperature.

LiNbO_3

LiNbO_3

and their influence on microhardness

crystals were grown by Czochralski pulling method. By suitably optimizing the after-heater configuration a crystal to crucible diameter ratio of 64% was achieved. The boules were poled at high temperature for obtaining single domain crystals. Variation of microhardness with applied load was evaluated. Hardness studies on different domains revealed that the negative domains have higher microhardness than the positive ones

Effect of pressure on the ferroelectric properties of telluric acid ammonium phosphate

Telluric acid ammonium phosphate (TAAP) is a new class-m ferroelectric with a transition temperature of 45 degrees C. Recent Raman spectroscopy studies of this crystal suggest that the phase transition takes place predominantly by the order-disorder nature of the hydrogen bonds of the type N-H-O. The hydrogen-bonded ferroelectrics are known to be very sensitive to external pressure and deuteration. In view of this, the effect of hydrostatic pressure on ferroelectric properties of TAAP has been studied up to 6 kbar. It was found that Tc shifts to lower temperatures under the influence of external pressure. At low pressures, Tc decreases more rapidly than at high pressures. The pressure derivative evaluated from the linear portion of the curve is -2.69 degrees C kbar-1. The effect of pressure on the spontaneous polarisation is to reduce its value although the continuous nature of the transition does not undergo any change up to the maximum pressure employed. These results and the shift of Tc to higher temperature upon deuteration support the order-disorder nature of the hydrogen bonds during the phase transition.

Optical phase conjugation by two-wave mixing in iron-doped lithium niobate

A novel technique to generate forward phase conjugate wave by two-wave mixing (TWM) in photorefractive iron-doped lithium niobate crystal has been demonstrated. An optical beam from a positive transparency was forward phase conjugated by TWM technique. The experimental scheme was then extended to a specific interferometric application.

Laser Raman spectra of mixed crystals of [(NH4)1−x Kx]2 SO4

The Raman spectra of mixed crystals of [(NH4)1−x Kx]2 SO4 in the region 50–3400 cm−1 at 293 K and below 223 K have been reported. At room temperature 293 K, as the concentration of K+ ion increases in the crystal up to 50%, the frequencies of the totally symmetric vibrations of SO42− and NH4+ ions increase and thereafter the frequency of SO42− vibration decreases and attains the value in K2SO4. This change in frequency up to 50% of potassium concentration is due to the breaking of hydrogen bonds of the type N-H...O. The behaviour of Raman intensities of Ag(v1) mode of SO42− for various concentrations (x=0, 0·03, 0·11, 0·5, 0·85) suggest that the phase transition changes from first order type to one of second order. The phase transition in mixed crystals of [(NH4)1−x Kx]2 SO4 can be a cooperative phenomenon arising from a coupling between (NH4)+ ions through hydrogen bonds with the distorted SO42− ions in the low temperature phase.

Polarization switching in radiation damaged ferroelectric crystals

The polarization switching processes in radiation damaged ferroelectrics were studied by the Merz method. In irradiated triglycine sulphate and sodium nitrite, the switching time depends exponentially on the applied electric field. Irradiation increases the importance of nucleation and sideways motion of the domain walls in polarization switching.

Two beam coupling in linbo3:Fe

Abstract Some interesting new features of temporal fluctuations in the intensities of optical beams (λ=488nm) interfering to form a volume grating in an iron doped Lithium Niobate crystal have been studied. These intensity fluctuations have hitherto not been studied in detail and may help in identifying the carrier transport mechanism during the formation of the volume grating in photorefractive crystals.