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ABSTRACT: High pressure behavior of the compound Bi <sub>12</sub> SiO <sub>20</sub> 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 B<sub>0</sub> , and its pressure derivative B<sup>′</sup> of Bi <sub>12</sub> SiO <sub>20</sub> are evaluated to be 36 GPa and 16.7 GPa, respectively. Mode Grüneissen parameters of various Raman active modes of Bi <sub>12</sub> SiO <sub>20</sub> are also reported. The stability of Bi <sub>12</sub> SiO <sub>20</sub> at high pressure is discussed in the light of the pressure-induced amorphization reported in bismuth-orthosilicate ( Bi <sub>4</sub> Si <sub>3</sub> O <sub>12</sub>) and -orthogermanate. Comparison of the observed phonon behavior with that reported for Bi <sub>4</sub> Si <sub>3</sub> O <sub>12</sub> reveals that two of the Raman modes in Bi <sub>4</sub> Si <sub>3</sub> O <sub>12</sub> have negative pressure dependencies clearly indicating dynamic instability while Bi <sub>12</sub> SiO <sub>20</sub> does not show any signatures of zone-center instabilities.
Journal of Applied Physics 11/2010; · 2.17 Impact Factor
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ABSTRACT: In situ high-pressure Raman spectroscopic studies on
ZrV2O7 suggest growth of disorder in the
high-pressure orthorhombic phase, inferred from the excessive broadening
of Raman peaks, and the sample gradually turns amorphous above 4GPa .
The samples pressure cycled from 10GPa exhibit irreversible
amorphization. On the other hand, the spectrum of the sample pressure
cycled from 4GPa is found to have all the characteristic peaks of
V2O5 indicating a possible decomposition of the
compound into a mixture of simple oxides. A partial decomposition is
indeed confirmed from the x-ray-diffraction pattern of the recovered
sample. A large decrease of the intensity of the VO4
tetrahedral stretching modes is consistent with the possible
coordination change of vanadium, from fourfold in
ZrV2O7 to fivefold in V2O5
. Nucleation of nano-/poorly crystallized grains of the daughter phase
even at ambient temperature is understood as due to the existence of the
fivefold coordinated crystalline phase, V2O5 .
Physical review. B, Condensed matter 04/2007; 75(17):174119.
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ABSTRACT: The Raman spectrum of the crystalline complex thiourea:diethyloxalate (2:1) is reported. The Raman wavenumbers in the complex are compared with those of crystalline thiourea (TU) and diethyloxalate (DEO), and the observed differences are understood from the point of view of the hydrogen bonding in the system. From the wavenumbers of the NH stretching vibrations, it is inferred that the strengths of hydrogen bonds in the complex are smaller compared to those in TU. The evolution of the Raman spectrum at high temperature reveals that the NH··· O hydrogen bonds, which are a key to the formation of the complex, are broken above 397 K. The spectrum corresponding to the crystalline complex reappears below 375 K when cooled from temperatures below melting. Copyright © 2006 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy 09/2006; 37(12):1447 - 1452. · 3.09 Impact Factor
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ABSTRACT: Room temperature Raman scattering measurements have been carried out on well characterized samples of MgB2-xCx. The Raman line corresponding to the E2g phonon mode shows progressive hardening from 620 cm-1 in pristine MgB2 to 775 cm-1 in the sample with carbon fraction x=0.2. The corresponding line width on the other hand, increases from a value of about 220 cm-1 to 286 cm-1 in samples with x = 0.1, beyond which it decreases to a value 167 cm-1 for x=0.2. From the average mode frequency and the line width obtained from Raman measurements and taking the values of N(0) obtained from the calculated variation in s hole density of states in MgB2-xCx, the electron phonon coupling strength to the E2g phonon, l2g, is evaluated using Allen’s formula. This remains large for low C fraction, but shows rapid decrease for x > 0.10. Using this value of l2g appropriately weighted, TC is obtained from McMillan’s equation. These values are in good agreement with the experimentally measured TC variation in MgB2-xCx. Comment: PDF format; containing text and figures. accepted for publication in Journal of Physics Condensed Matter
04/2005;
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ABSTRACT: 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.
Journal of Physics Condensed Matter 04/2005; 17(17):2633. · 2.55 Impact Factor
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ABSTRACT: 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.
The Journal of Chemical Physics 11/2004; 121(15):7320-5. · 3.33 Impact Factor
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ABSTRACT: Raman scattering measurements have been carried out in MgB2-xCx for x=0.0 -0.30. The broad Raman band around 600cm-1 range, which corresponds to the E2g mode in MgB2 is seen to narrow and reduce in intensity with increasing carbon content. For x>0.15, new set of bands appear at 1200 cm-1 possibly arising from the B-C, E2g modes. The decrease in Tc with carbon substitution seems to correlate with this change over.
10/2003;
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ABSTRACT: 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. © 2000 American Institute of Physics.
The Journal of Chemical Physics 04/2000; 112(15):6739-6744. · 3.33 Impact Factor
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ABSTRACT: 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.
Journal of Physics Condensed Matter 12/1998; 9(45):9745. · 2.55 Impact Factor
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ABSTRACT: Pressure-tuned resonance Raman scattering is investigated in the ambient and subsequent high-pressure phases of AgGaSe2 up to a pressure of 165 kbar using several fixed wavelengths of argon and krypton ion lasers. The nonpolar Raman active mode of A1 symmetry in the chalcopyrite phase exhibits resonances at 18 and 30 kbar for the incident wavelengths of 676.4 and 647.1 nm, respectively. From the observed resonances, the pressure coefficient of a direct interband transition involving a defect level is estimated to be 6.9 meV/kbar in the chalcopyrite phase. The observed resonances are fitted to those calculated incorporating the corrections for absorption and reflection. In the high-pressure orthorhombic phase above 51 kbar, AgGaSe2 has an indirect gap that is lower than the energy of the visible photon; however, enhancement of the intensities of Raman modes is observed with yellow-green wavelengths due to resonance with another direct interband transition. The pressure coefficient of this transition in the opaque phases is found to be smaller than that in the chalcopyrite phase. © 1996 The American Physical Society.
Phys. Rev. B. 06/1996; 53(23).
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ABSTRACT: High-pressure phase transitions in semiconductor mixed crystals Zn1-xMnxSe (x≤0.29) are investigated using Raman scattering and photoluminescence (PL) up to a pressure of 150 kbar. The samples are characterized for composition and crystal structure. A Raman mode is observed between TO and LO frequencies for samples with x≤0.097 and is assigned to a disorder activated zone boundary phonon. Apart from the PL band at 2.1 eV due to the 4T1→6A1 transition between the crystal-field split levels of Mn2+, a PL band is observed at 1.94 eV in the samples with x=0.063 which is assigned to a native defect forming a complex with Mn2+. The intensity of the PL band reduces drastically with pressure and completely disappears at about 15 kbar. A posible model for this behavior is presented. The pressure dependence of the phonon frequencies and the behavior of PL suggest three transitions P1, P2, and P3 for the mixed crystals with x=0.29, whereas those with x=0.063 and 0.097 show only P2 and P3 transitions. In the sample with x=0.29, TO mode splits into two components across P1 and the low-frequency component exhibits softening. The crystals turn opaque across P2 and the PL signal disappears suggesting that the band gap changes from direct to indirect. Intensities of Raman lines decrease and completely disappear across P3. The difference in the behavior of the phase transitions for the low and the high values of x is understood on the basis of their different crystal structures.
Phys. Rev. B. 10/1995; 52(15).
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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.
Solid State Communications.
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ABSTRACT: High-pressure Raman spectroscopic investigations are carried out on Li2SO4·H2O up to a pressure of 18.1 GPa. Changes in the Raman spectra suggest two structural transitions occurring around 2.6 and 7.0 GPa, respectively. Discontinuous changes in the internal mode frequencies of sulfate ions around 2.6 GPa indicate that the transition involves reorientation of SO4 ions. A comparison of the behavior of the high-pressure phase in this compound with those in other Li-based binary sulfates indicates that the orientational disorder in these compounds at high pressures arises due to dissimilar cations.
Physica B: Condensed Matter.
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Journal of Solid State Chemistry. 180(10):2824-2829.
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ABSTRACT: Results of high-pressure Raman-scattering measurements carried out on resorcinol up to pressures of 14.5 GPa are reported. Changes observed in the Raman spectra suggest transition to a new phase γ around 5.6±0.2 GPa besides the well-known isosymmetric α⃗β transition at 0.5 GPa. The β⃗γ transition is found to be reversible with hysteresis when released from peak pressures of about 11 GPa. Raman line shapes at high-pressures suggest that the new phase γ is disordered. Rapid pressurization of α-resorcinol, with the suppression of α⃗β transition, leads to a new phase δ above 3 GPa which turns out to be more ordered, unlike the γ phase, at comparable pressures. A luminescence peak centered around 2.25 eV is observed in the high pressure phase γ as well as δ above 6 GPa. Above 11 GPa, intensity of the Raman bands weaken drastically and submerge in the luminescence background. The ambient pressure Raman spectrum of resorcinol is not recovered when released from pressures above 11 GPa.
Phys. Rev. B. 65(5).
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ABSTRACT: Order–disorder transition in triethylenediamine has been investigated by Raman spectroscopy. Large increase in linewidth accompanied by discontinuous decrease in the mode frequencies are observed across the transition to the disordered cubic phase at 351 K. Except for a few modes, the magnitude of discontinuity in frequency is much less compared to that expected for the given volume change. Most of the doubly degenerate modes show abrupt increase in linewidth at the transition temperature, which is understood to be due to dynamic disorder in the cubic phase.
Journal of Molecular Structure. 789:195-199.
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ABSTRACT: Phase transitions in ternary chalcopyrite AgGaSe2 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.
Journal of Physics and Chemistry of Solids.
