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ABSTRACT: We have used a shell model to study the phonon dynamics of multiferroic manganites RMnO3 (R= Tb, Dy, Ho). The calculated phonon dynamical properties, crystal structure, Raman frequencies and specific heat are found to be in good agreement with the available experimental data. Besides, the phonon density of states, elastic constants and phonon dispersion curves along high symmetry directions (sigma, delta and lambda) have also been computed. A zone-center imaginary Au mode is revealed in these phonon dispersion curves, which indicates the occurrence of metastability of the perovskite phase. The Gibbs free energy values of orthorhombic phase, when compared with those of hexagonal phase, indicate the possibility of coexistence of these two phases of these multiferroic manganites under ambient conditions. Comment: 13 pages, 9 figures
04/2010;
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ABSTRACT: The lattice dynamics of yttrium manganite (YMnO(3)) has been investigated by means of a shell model with pair-wise interionic interaction potential. The experimental data of crystal structure and Raman and infrared frequencies compare well with the lattice dynamical calculations. The phonon dispersion curves found along three high symmetry directions and the density of states of YMnO(3) have also been calculated from this model. The computed phonon density of states is used to derive the macroscopic thermodynamic quantities like the Debye temperature and specific heat. The crystal structure data computed from this model are in good agreement with the available experimental data measured by neutron powder diffraction. We have made a comparative study of the structures derived from the potential model calculations for both LaMnO(3) and YMnO(3). Symmetry vectors obtained through group theoretical analysis at the zone centre point were employed to classify the phonon frequencies obtained into their irreducible representations. The computed Raman and infrared frequencies have shown good agreement with the measured data.
Journal of Physics Condensed Matter 09/2009; 21(35):355402. · 2.55 Impact Factor
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ABSTRACT: The lattice dynamics of yttrium manganite (YMnO3) has been investigated by means of a shell model with pair-wise interionic interaction potential. The experimental data of crystal structure and Raman and infrared frequencies compare well with the lattice dynamical calculations. The phonon dispersion curves found along three high symmetry directions and the density of states of YMnO3 have also been calculated from this model. The computed phonon density of states is used to derive the macroscopic thermodynamic quantities like the Debye temperature and specific heat. The crystal structure data computed from this model are in good agreement with the available experimental data measured by neutron powder diffraction. We have made a comparative study of the structures derived from the potential model calculations for both LaMnO3 and YMnO3. Symmetry vectors obtained through group theoretical analysis at the zone centre point were employed to classify the phonon frequencies obtained into their irreducible representations. The computed Raman and infrared frequencies have shown good agreement with the measured data.
Journal of Physics Condensed Matter 08/2009; 21(35):355402. · 2.55 Impact Factor
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ABSTRACT: The lattice dynamical and allied properties of the multiferroic manganites SmMnO3, EuMnO3 and GdMnO3 were investigated in this work by means of a shell model with transferable pairwise interionic interaction potential. This shell-model potential is able to reproduce the available crystal structure and phonon frequencies. A zone center imaginary Au mode is observed in these lattice dynamics calculations that indicates metastability of the perovskite phase. Comparison of the Gibbs free energies in the orthorhombic and hexagonal phases points to the possible coexistence of the two phases of these manganites under ambient conditions.
New Journal of Physics 07/2009; 11(7):073041. · 4.18 Impact Factor
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ABSTRACT: The lattice mechanics of sodium nitrate with calcite structure is investigated by means of a rigid molecular ion model of the molecular solids employing the external mode formalism. The phonon dispersion curves calculated are in good agreement with those recently measured from the technique of neutron inelastic scattering. The values of the eigenvectors and the cohesive energy obtained for sodium nitrate also appear to be reasonably correct. The possibilities of further improvement and extension of the model are also discussed.Die Gittermechanik von Natriumnitrat mit Kalzit-Struktur wird unter Benutzung des Modells starrer Molekülionen der Molekülfestkörper sowie des Formalismus der externen Moden untersucht. Die berechneten Phononendispersionskurven sind in guter Übereinstimmung mit neueren Meßergebnissen der inelastischen Neutronenstreuung. Die erhaltenen Werte der Eigenvektoren und der Kohäsionsenergie für Natriumnitrat scheinen befriedigend korrekt zu sein. Die Möglichkeiten einer weiteren Verbesserung und Ausweitung des Modells werden ebenfalls diskutiert.
physica status solidi (b) 02/2006; 112(2):717 - 724. · 1.32 Impact Factor
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ABSTRACT: We have investigated the detailed phonon dynamics of the magnetoresistant lanthanum manganite (LaMnO3) using a shell model with pairwise interionic interaction potential. The investigations include the computation of the phonon density of states, the specific heat, the elastic constants, and the phonon-dispersion curves of LaMnO3. The symmetry vectors obtained through detailed group-theoretical analysis at the zone center point were employed to classify the phonon frequencies obtained into their irreducible representations. This analysis enabled direct comparison with the observed Raman and infrared spectra. Furthermore, we have computed the specific heat at constant volume at different temperatures and the phonon-dispersion curves along the three major symmetry directions for LaMnO3. The results have been found to present an overall better interpretation of the available experimental data as compared to those obtained from other theoretical model calculations.
Phys. Rev. B. 75(21).
