V. U. Nayar

Vibrational spectra of MII3Pb(P2O7)2 (MII = Ni, Co)

Abstract Raman and IR spectra of polycrystalline Ni3Pb(P2O7)2 and Co3Pb(P2O7)2 have been recorded and analyzed. The internal modes are assigned in terms of PO3 and POP vibrations. The results point to a bent POP bridge configuration in Co3Pb(P2O7)2 as in Ni3Pb(P2O7)2. In the cobalt compound, the P2O4−7 ions are distorted. Non-coincidence of the majority of the Raman and IR bands confirms a centrosymmetric structure for Ni3Pb(P2O7)2, and Co3Pb(P2O7)2. The POP bridge angle is slightly higher in the cobalt compound than in the nickel compound.

Vibrational spectroscopic studies of FeClMoO4, Na2MoO4 and Na2MoO4·2H2O/D2O

Abstract FTIR and Raman spectra of FeClMoO4 single crystal and polycrystalline Na2MoO4, Na2MoO4·2H2O and Na2MoO4·2D2O are recorded and analysed. The band positions for different modes suggest that MoO4 tetrahedron is more distorted in FeClMoO4. The larger splitting observed for the bending modes and partial retention of degeneracy of the asymmetric stretching mode indicate that angular distortion is greater than liner distortion in MoO42− ion in FeClMoO4 confirming x-ray data. The non-appearance of the ν1 and ν2 modes in the IR and partial retention of the degeneracies of various modes show that MoO42− ion retains Td symmetry in Na2MoO4. Wavenumber values of the ν1 mode indicate that the distortion of MoO4 tetrahedra in the four crystals are in the order FeClMoO4> Na2MoO4·2H2O>Na2MoO4·2D2O>Na2MoO4. The water bands suggest the presence of two crystallographically distinct water molecules in Na2MoO4·2H2O. They form strong hydrogen bonds.

Raman and infrared spectral analysis of thallium niobyl phosphates: Tl2NbO2PO4, Tl3NaNb4O9(PO4)2 and TlNbOP2O7

Raman and infrared spectra of Tl2NbO2PO4, Tl3NaNb4O9(PO4)2 and TlNbOP2O7 are reported. The observed bands are assigned in terms of vibrations of NbO6 octahedra and PO4 tetrahedra in the first two compounds and in terms of NbO6 octahedra and P2O74− anion in the third compound. The NbO6 octahedra in all the title compounds are found to be corner-shared and distorted. The higher wavenumber values of the ν1 (NbO6) mode and other stretching modes indicate that the NbO6 octahedra in them are distorted in the order TlNbOP2O7>Tl2NbO2PO4>Tl3NaNb4O9(PO4)2. The splitting of the ν3 (PO4) mode indicates that PO4 tetrahedra is distorted more in Tl2NbO2PO4 than in Tl3NaNb4O9(PO4)2. The symmetry of P2O74− anion in TlNbOP2O7 is lowered. Bands indicate that the P–O–P bridge in the above compound has a bent P–O–P bridge configuration.

INFRARED AND RAMAN SPECTRA OF CS2VOP2O7 AND SINGLE CRYSTAL RB2(VO)3(P2O7)2

Abstract Infrared (IR) and Raman spectra of Cs 2 VOP 2 O 7 and single crystal Rb 2 (VO) 3 (P 2 O 7 ) 2 have been recorded and analysed. The spectra are interpreted on the basis of P 2 O 7 4− and VO vibrations. The appearance of ν s P-O-P in both Raman and IR spectra suggests a bent P–O–P bridge for the P 2 O 7 4− ions in both compounds, in agreement with X-ray data. The observed bands point to an eclipsed configuration for the P 2 O 7 4− ions with C 2 ν symmetry in Rb 2 (VO) 3 (P 2 O 7 ) 2 . The appearance of P–O stretching modes in Cs 2 VOP 2 O 7 at higher wavemumbers than those in Rb 2 (VO) 3 (P 2 O 7 ) 2 indicates that the P–O bonds in the Cs compound is stronger than in the Rb compound. VO modes are also interpreted.

Infrared and polarized Raman spectra of (CH3)2NH2Al(SO4)2 · 6H2O

FTIR and single crystal Raman spectra of (CH3)2NH2Al(SO4)2 x 6H2O have been recorded at 300 and 90 K and analysed. The shifting of nu1 mode to higher wavenumber and its appearance in Bg species contributing to the alpha(xz) and alpha(yz) polarizability tensor components indicate the distortion of SO4 tetrahedra. The presence of nu1 and nu2 modes in the IR spectrum and the lifting of degeneracies of nu2, nu3, and nu4 modes are attributed to the lowering of the symmetry of the SO4(2-) ion. Coincidence of the IR and Raman bands for different modes suggest that DMA+ ion is orientationally disordered. One of the H atoms of the NH2 group of the DMA+ ion forms moderate hydrogen bonds with the SO4(2-) anion. Al(H2O)6(3+) ion is also distorted in the crystal. The shifting of the stretching modes to lower wavenumbers and the bending mode to higher wavenumber suggest that H2O molecules form strong hydrogen bonds with SO4(2-) anion. The intensity enhancement and the narrowing of nu1SO4, deltaC2N and Al(H2O)6(3+) modes at 90 K confirm the settling down of the protons in the hydrogen bonds formed with H2O molecules and NH2 groups. This may be one of the reasons for the phase transition observed in the crystal.

IR and polarized raman spectra of a single crystal Rb3H(SeO4)2 in the ferroelastic phase

Abstract Infrared and polarized Raman spectra of a single crystal Rb 3 H(SeO 4 ) 2 and the polycrystalline spectra of its deuterated analogue are recorded and analysed. Bands indicate the existence of SeO 4 4− ions and HSeO 4 ions, in agreement with a non-centrosymmetric dimer in the crystal. The difference between ν(Se-OH) and ν(Se-OH) bands is a measure of the hydrogen bond strength and it decreases on deuteration. The presence of an absorption continuum along with a transmission window shows that the proton motion is coupled with those of the oxygen atoms in the SeO 4 2− ions. The ABC bands in the OH stretching region also support strong hydrogen bonding in the crystal. The lower wave numbers for the stretching OH mode show that the dimer formation is quite anharmonic, as judged from the isotopic shift.

Wavelength Shift of Cladding Mode Resonances in a Mechanically Induced LPFG by Twisting the Fiber

Abstract The long-period fiber grating is mechanically induced over a twisted fiber and its characteristics are investigated. The amplitude as well as the wavelength shift of the resonance is studied in response to the applied twist and pressures. These resonances decrease in amplitude and shift to shorter wavelength side as the applied twist increases. The spectral responses of a grating assembly formed by two grating sections in series, one section with a twisted fiber and other with an untwisted fiber, are also investigated. Shearing stress and photo-elasticity causes the fiber to be circularly birefringent and the mechanically induced grating formed over the twisted fiber region causes the appearance of two resonances shifted away from the resonances of the untwisted grating section. At higher twist rates, resonant wavelength shift becomes insensitive to applied pressures, showing a reduction in the induced linear birefringence. The wavelength shift is almost symmetric with respect to the applied twist rate in clockwise and counterclockwise directions.

Infrared and polarized Raman spectra of Cu(HSeO3)2 · H2O single crystal

Abstract Infrared and polarized Raman spectra of Cu(HSeO3)2 · H2O single crystal have been recorded and analysed. The appearance of non-degenerate SeOH stretching vibrations in the αxz and αyz polarizations of Raman spectra indicate distortion of the HSeO3− ion in the Cu(HSeO3)2 · H2O crystal. The low wavenumber values obtained for the symmetric and asymmetric stretching vibrations of the HSeO3− ion are consistent with the strong hydrogen bonding and the influence of Jahn-Teller distortion as predicted in X-ray diffraction data. The shifting of the stretching and bending vibrations of the hydroxyl groups and water molecules from the free state values also confirms the strong hydrogen bonding in this crystal. Broad bands observed for both stretching and bending regions become sharp in the Raman spectrum recorded at 77 K. A doublet appears for the SeOH stretching mode at this temperature indicating the settling of protons in an ordered position and the absence of intrabond proton tunnelling.

Vibrational spectra of Te(OH)6 · 2NH4H2PO4 · (NH4)2HPO4

The IR and (polarized) Raman spectra of Te(OH)6 · 2NH4H2PO4 · (NH4)2HPO4 were recorded and analyzed. It was found that the hydrogen atoms are not strongly bonded to the oxygen atoms of phosphate groups and hence the phosphate ions exist as PO3−4 ions rather than as HPO4 and H2PO4 groups. The stretching and bending frequencies of PO4 and NH4 groups occupy a wide range of spectrum due to the presence of a large number of these groups in the unit cell.

Raman spectroscopic investigation on the phase transitions in Rb3H(SeO4)2 and Rb3D(SeO4)2 single crystals

Abstract A temperature dependent Raman spectroscopic investigation of Rb 3 H(SeO 4 ) 2 and Rb 3 D(SeO 4 ) 2 single crystals is carried out from liquid nitrogen temperature up to 620 K. The observed spectra show that the phase transitions are due to restructuring of the system of hydrogen bonds. The ferroelastic phase transition observed in the protonated compound at 451 K is of second order, which corresponds to a rise of symmetry from the room temperature monoclinic to the trigonal phase. The higher wavenumber for the ν(SeO) band at the onset of the transition indicates a positional disorder of protons. In the high temperature phase, the protons are in a highly disordered state followed by fast tunneling. The absence of a frequency shift in the low temperature spectra of the deuterated compound and the invariance of the symmetry during the transition show that the antiferroelectric transition at 95 K is second order and of the order-disorder type.