Peculiarities of the correlation functions, X-ray and neutron scattering in BaTiO3
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Based on molecular-dynamics (MD) simulations, we have calculated the static and dynamic correlation functions in a BaTiO3 crystal. The static correlation functions have been used to study the peculiarities of diffuse scattering in barium titanate showing the experimentally observed anomalous planes. Based on time-dependent pair correlation functions, we have calculated the phonon spectra of BaTiO3 and studied the central peak of inelastic scattering. The phonon frequencies calculated by the MD method agree well with those obtained previously in the quasi-harmonic approximation. We show that the central peak of inelastic scattering is associated mainly with the soft optic mode and has the same symmetry. The large anisotropy in the displacements of atoms in the soft mode allows the presence of peculiarities in both X-ray scattering and EXAFS spectroscopy to be explained. The characteristic shape of the EXAFS spectra is shown to be explained by the quasi-one-dimensional motion of the oxygen ions in the cubic lattice of BaTiO3. Our calculation of triple correlation functions shows that the titanium atom in the described model oscillates around the cubic cell center. Explaining the experimental data that have caused disagreement about the nature of the phase transition in BaTiO3 using the developed model gives grounds to treat the phase transition in barium titanate as a displacive one.
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