H.C. Gupta

Lattice dynamic investigation of the zone center wavenumbers of the cubic A2Ti2O7 pyrochlores

A short-range force constant model was applied for the first time to investigate the phonons in A2Ti2O7 (A = Y, Sm, Gd, Yb) pyrochlores in their cubic phase of space group Fd3m. The calculations of phonons were made with five stretching and four bending force constants. The calculated values of Raman and infrared wavenumbers are in good agreement with the observed values. Copyright

A lattice dynamical investigation of the Raman and the infrared frequencies of the cubic A2Sn2O7 pyrochlores

Abstract A short range force constant model has been applied for the first time to investigate the Raman and the infrared frequencies in A 2 Sn 2 O 7 (A=La, Sm, Yb, Lu) pyrochlores in their cubic phase of space group Fd 3 m . The calculations of zone center phonons have been made with six stretching and three bending force constants. The calculated values of Raman and infrared frequencies are in good agreement with the observed ones.

Zone center phonon frequencies in tetragonal zirconia: lattice dynamical study and new assignment proposition

Abstract A normal mode calculation of the zone center phonons in the tetragonal structure of ZrO 2 compound has been performed with particular emphasis on the low frequency modes. A restricted set of stretching and bending force constants have been introduced and lead to a good agreement between theoretical and experimental Raman and IR phonon frequencies. The E g symmetry of the two low frequency Raman modes which are involved in the softening process observed with increasing pressure were confirmed and none were left for a low frequency A 1 g totally symmetric mode as proposed in the literature.

A lattice dynamical investigation for the zone-centre phonon frequencies of the NaMnF3 perovskite

Abstract A short range force constant model has been applied to investigate the lattice dynamics of NaMnF 3 in its orthorhombic phase. The calculated zone-centre Raman frequencies agree fairly well with the measured values. The infra red measurements have been assigned the proper species for the first time on the basis of the present calculations.

A lattice dynamical investigation of the zone center frequencies of the orthorhombic NdGaO3 perovskite

Abstract A short range force constant model has been applied for the first time to investigate the phonons in NdGaO 3 perovskite in the orthorhombic phase. The calculations of phonons have been made with 10 stretching and 10 bending force constants. The calculated values of Raman and infrared frequencies are in good agreement with the observed ones. The infrared frequencies have been assigned for the first time.

Long wavelength optical lattice vibrations in mixed chalcogenide spinels Zn1−xCdxCr2S4 and CdCr2 (S1−xSex)4

Abstract The composition dependence of the long wavelength optical lattice vibrations of mixed spinels Zn1−xCdxCr2S4 and CdCr2 (S1−xSex)4 was studied using the de Launay-type angular force constant model. One-mode type behaviour is predicted for all the vibrational modes in mixed Zn1−xCdxCr2S4 and CdCr2 (S1−xSex)4 spinel systems. Good agreement between the theoretical and the experimental values was obtained.

A lattice dynamical investigation of Raman and infrared wavenumbers at the zone center of the orthorhombic NdNiO3 perovskite

Abstract A short-range force constant model has been applied for the first time to investigate the phonons in NdNiO 3 perovskite in the orthorhombic phase. The calculations with the ten stretching and ten bending force constants provide a good agreement for the observed Raman and infrared wavenumbers. The infrared wavenumbers have been calculated for the first time. All Raman and infrared wavenumbers are assigned to their specific mode of vibrations.

Study of interaction forces and phonons in some spinels and mixed systems

Abstract A detailed study of zone centre phonons in mixed spinel structure compounds is presented, using an angular force constant model considering interatomic interactions up to third neighbours only. For the first time the dynamical matrix for spinel structure compounds AB 2 X 4 of the order of 42 × 42 has been solved analytically at the zone centre. Using these analytical expressions and the measured zone centre frequencies, the evaluation of the force constants has been carried out first in the case of CdCr 2 S 4 , CdCr 2 Se 4 and ZnCr 2 S 4 . It has been found that the second neighbour interatomic interaction between B-X atoms is more significant than the first neighbour interaction between A-X atoms in these normal spinels. The physical aspects for the same are discussed. The computed phonon frequencies of CdCr 2 S 4 , CdCr 2 Se 4 and ZnCr 2 S 4 at the zone centre agree well with the Raman and the infrared measurements. Finally, the analysis is carried out to study the zone centre phonons in mixed crystals of these spinels.

Lattice dynamics of orthorhombic perovskite YMnO3

Abstract A short-range force constant model has been applied to investigate the phonons in YMnO 3 perovskite in the orthorhombic phase. The calculations with six stretching and two bending force constants provide a satisfactory agreement for the available Raman and infrared frequencies. Based on the present calculations, the infrared frequencies have been assigned for the first time.

Zone center wavenumbers of the orthorhombic LaGaO3 perovskite

A short-range force constant model was applied for the first time to investigate the phonons in LaGaO3 perovskite in the orthorhombic phase. The calculations with 10 stretching and 10 bending force constants provide good agreement for the observed Raman and infrared wavenumbers. The infrared wavenumbers at 329, 308, 296 and 165 cm−1 were assigned to the B1u mode and those at 510, 416, 353 and 275 cm−1 to the B3u mode. Copyright