Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals
The specific features of diffuse X-ray scattering in BaTiO{sub 3}, KNbO{sub 3}, and PbTiP{sub 3} perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO{sub 3}. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO{sub 3} and KNbO{sub 3} crystals, there exist quasi-one-dimensional 'soft' modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO{sub 3}, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO{sub 3}, KNbO{sub 3}, and PbTiO{sub 3} are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.
- OSTI ID:
- 21246931
- Journal Information:
- Journal of Experimental and Theoretical Physics, Vol. 108, Issue 3; Other Information: DOI: 10.1134/S106377610903008X; Copyright (c) 2009 Pleiades Publishing, Ltd; Country of input: International Atomic Energy Agency (IAEA); ISSN 1063-7761
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABSORPTION
APPROXIMATIONS
ATOMIC DISPLACEMENTS
BARIUM COMPOUNDS
CORRELATION FUNCTIONS
CRYSTALS
DIFFUSE SCATTERING
FERROELECTRIC MATERIALS
FINE STRUCTURE
LEAD COMPOUNDS
NIOBATES
NUCLEAR MAGNETIC RESONANCE
OPTICAL MODES
ORTHORHOMBIC LATTICES
PEROVSKITE
PHASE TRANSFORMATIONS
POTASSIUM COMPOUNDS
SHELL MODELS
TITANATES
X-RAY DIFFRACTION