Structural ordering transition and repulsion of the giant LO-TO splitting in polycrystalline (formula presented)

We report Raman and x-ray diffraction studies of polycrystalline (formula presented) (BST) prepared by the sol-gel technique in the range of (formula presented) to 1.0. Other than a tetragonal to cubic phase transition at composition (formula presented) of the BST powders detected by x-ray diffraction and Raman spectroscopy, a new structural ordering transition around (formula presented) is observed due to the atomic arrangement. By taking the anharmonic coupling between the three (formula presented) modes into account, the observed repulsion of the giant LO-TO splitting is a result of decreasing cell dimension and strengthening the electron-phonon coupling. The results help us to explain the weakening of the ferroelectricity in the BST tetragonal phase as x decreases. The anomalous inconsistency in our experimental results and theoretical prediction are attributed to the lack of detailed information of dynamical effective charges in the (formula presented) compounds.