Structural ordering transition and repulsion of the giant LO-TO splitting in polycrystalline BaSr1-xTiO3

Abstract

We report Raman and x-ray diffraction studies of polycrystalline BaxSr1-xTiO3 (BST) prepared by the sol-gel technique in the range of x = 0 to 1.0. Other than a tetragonal to cubic phase transition at composition x approximate to0.75 of the BST powders detected by x-ray diffraction and Raman spectroscopy, a new structural ordering transition around x approximate to0.4-0.5 is observed due to the atomic arrangement. By taking the anharmonic coupling between the three A(1) (TO) 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 BaxSr1-xTiO3 compounds.