Disorder in BaTiO₃ and Ti EFG Tensors

Abstract

The motionally averaged electric field gradient tensors at the Ti-sites have been evaluated within the 8-site Ti disorder model for the various phases of BaTiO 3 . The results have been compared with the experimental data and with the observation of a non-zero Ti quadrupole coupling in the cubic phase. The pure 8-site "orderdisorder" model does not agree with the experiment. A good agreement is however obtained if in addition to the dynamic Ti disorder the "displacive" soft mode induced deformation of the unit cell shape is taken into account. The presence of the soft mode and the Ti disorder leads to a special type of phase transition with combined "displacive" and "order-disorder" character.

Structural evolution of Na0.5K0.5NbO3 at high temperatures

“…After this achievement, a series for the structural disorder of these structures was found [21][22][23]. Since then, a number of studies have been compiled on the basis of the displacive and order-disorder models from both experimental and theoretical points of view [6,7,[24][25][26][27][28][29]. Today, the consensus seems to be that the displacive and the orderdisorder natures are highly correlated, but that the latter becomes more prominent at higher temperatures.…”

Phase Transitions in Nanocrystalline Barium Titanate Ceramics Prepared by Spark Plasma Sintering

“…Studies on the crystal structure and phase transition of BT have been carried out using various experimental techniques 17–22 . Because of the close relationship between ferroelectricity and lattice dynamics, Raman spectroscopy provides a potentially valuable technique for the study of ferroelectric materials 23,24 . Based on the crystallography, there are four triply degenerate optical modes of vibration (3F 1u +1F 2u ) in cubic BT, which is the Pm 3 m group.…”