T. Sakuntala

The effect of annealing on the properties of nanoparticles dispersed in oxide glass containing Zn

The optical and vibrational properties of nanoparticles dispersed in oxide glass containing Zn are investigated as functions of the annealing temperature using photoluminescence, optical absorption and Raman spectroscopy. Upon annealing the samples, a blue-shift of the optical absorption and photoluminescence is observed, in contrast to the expected red-shift due to particle growth. This can be understood if one invokes the inclusion of Zn into the particle which more than compensates the red-shift due to particle growth. The observed increase in the LO phonon frequency upon annealing also confirms this. The particle size, obtained from the analysis of low-frequency Raman spectra in terms of confined acoustic phonons, shows considerable growth upon annealing above . Overtones up to 3-LO are identified. Furthermore, the LO phonon and its overtones are found to exhibit strong resonance enhancement as a function of exciting wavelength. However, upon annealing, the Raman intensities reduce dramatically due to the shift of the electronic transition energy away from that of the incident photon.

Phase stability of YbVO4 under pressure: In situ x-ray and Raman spectroscopic investigations

The phase stability of YbVO4 under pressure has been investigated using synchrotron based angle dispersive x-ray diffraction and Raman spectroscopic techniques up to 34.5 and 26.5 GPa, respectively. The results indicate that the compound transforms from the ambient pressure zircon structure to the scheelite structure above 5.9 GPa with 11.8% volume discontinuity. The coexistence of the two phases is observed over a large pressure range. At 15.8 GPa, the (011) peak of the scheelite phase develops asymmetry, and the pattern at further high pressures could be fitted to a fergusonite-type monoclinic structure. On reducing the pressure, the fergusonite phase reverses back to the scheelite phase; the latter phase could be recovered as a metastable phase at ambient pressure. The refined structural parameters along with the equation of state are given for various phases of YbVO4. Changes in the vibrational properties across these transitions, particularly across the scheelite↔fergusonite transition, have been inv...

Pressure induced amorphisation in LiKSO4 : a Raman scattering study

Abstract High pressure phase transitions in LiKSO4 are investigated upto a pressure of 152 kbar using Raman spectroscopy. Present Raman scattering results suggest that the sulphate ions in the high pressure phases occupy three nonequivalent sites leading orientational disorder. The fractional occupancies of different sites change discontinuously across phase transitions. Above 130 kbar, the orientational disorder appears to change slowly from a discrete type to a continuous type as the sample undergoes crystalline to amorphous transition which is complete at 150 kbar. The reverse transition to crystalline phase takes place around 100 kbar.

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.

High pressure stability of bismuth sillenite: A Raman spectroscopic and x-ray diffraction study

High pressure behavior of the compound Bi12SiO20 is investigated using in situ Raman spectroscopic and synchrotron-based angle dispersive x-ray diffraction techniques. Results indicate that the compound remains stable in the ambient pressure cubic structure up to 26 GPa. From the structural studies, bulk modulus B0, and its pressure derivative B′ of Bi12SiO20 are evaluated to be 36 GPa and 16.7 GPa, respectively. Mode Gruneissen parameters of various Raman active modes of Bi12SiO20 are also reported. The stability of Bi12SiO20 at high pressure is discussed in the light of the pressure-induced amorphization reported in bismuth-orthosilicate (Bi4Si3O12) and -orthogermanate. Comparison of the observed phonon behavior with that reported for Bi4Si3O12 reveals that two of the Raman modes in Bi4Si3O12 have negative pressure dependencies clearly indicating dynamic instability while Bi12SiO20 does not show any signatures of zone-center instabilities.

Raman spectroscopic study of high pressure phase transitions in AgGaSe2

Abstract Phase transitions in ternary chalcopyrite AgGaSe 2 are investigated up to a pressure of 160 kbar using Raman scattering. Pressure dependence of phonon frequencies suggests three phase transitions occurring around 30, 51 and 83 kbar, respectively. The band gap is found to increase with an increase in pressure up to 51 kbar. The phases above 51 kbar are opaque to visible light. Comparison of phase transitions in this compound with other chalcopyrites indicates a reduced stability of the tetragonal structure with an increasing mass of chalcogen anion.

High pressure behavior of ZrGeO4: A Raman spectroscopic and photoluminescence study

High pressure behavior of ZrGeO4 has been investigated using Raman and photoluminescence (PL) spectroscopies up to 25 GPa in a diamond anvil cell. Under the application of pressure, the GeO4 librational mode exhibits softening, suggesting dynamical instability of the scheelite structure. Qualitative changes are noted in the Raman spectrum above 12 GPa, suggesting a possible transition around this pressure. High pressure PL behavior of Eu3+-related crystal field transitions indicates a clear change in the site symmetry of Eu3+ around 12 GPa, strongly supporting structural transition to a lower symmetry phase at this pressure.

Amorphization-decomposition behavior of HfW2O8 at high pressure

Structural stability of HfW2O8 is investigated at high pressure using Raman spectroscopy. Irreversible amorphization is found to occur when pressurized to above 3 GPa under hydrostatic conditions. The Raman spectrum of the pressure-amorphized sample closely resembles that of amorphous WO3. On the other hand, spectrum of the recovered sample subjected to uniaxial compression of about 5 GPa showed bands characteristic of crystalline HfO2 besides the parent cubic phase and amorphous phase. Ex situ x-ray diffraction measurements on the pressure-cycled samples also indicated the presence of monoclinic HfO2. These results suggest that the observed pressure-induced amorphization (PIA) under hydrostatic compression is indeed due to hindered decomposition, which is facilitated under uniaxial compression. Thus, the behavior of HfW2O8 appears to be different from that of ZrW2O8, which exhibited only PIA and not pressure-induced decomposition at ambient temperature.

Decomposition of binary sulphate KHSO4 at high pressure

Abstract Pressure induced decomposition (PID) is known to occur only at elevated temperatures. Here we report its first occurrence at ambient temperature in a binary sulphate, KHSO4 at 4kbar. Raman spectroscopy is used for identifying the decomposition products as K3H(SO2 4) and H2SO4 from their characteristic spectra. One of the decomposition products being a liquid is argued to be the reason for the occurrence of the phenomenon at ambient temperature. Other possible decomposition routes are also examined.