Raman spectra and lattice dynamics of disordered complex perovskite BaMg$_{1/3}$Ta$_{2/3}$O$_3$

In relaxor ferroelectrics the chemical and the displacive ionic disorders coexist and may cause a relaxation of the selection rules for Raman scattering. We performed a Raman scattering study of BaMg$_{1/3}$Ta$_{2/3}$O$_3$ (BMT), which is chemically disordered cubic perovskite showing no evidences for displacive disorder. Polarized Raman spectra from a single crystal of BMT were collected in the temperature range of 5 $-$ 550 K. We are going to discuss the symmetry assignments of the observed modes and their temperature evolution. Simplified shell-model for the lattice dynamics of BMT will be presented. The results for BMT will be compared to the well-known observations for the Raman spectra from related relaxor ferroelectrics PbMg$_{1/3}$Ta$_{2/3}$O$_3$ and PbMg$_{1/3}$Nb$_{2/3}$O$_3$. In particular, the lowest Raman line observed in BMT is at $\sim$110 cm$^{-1}$, whereas the doublet line in PbMg$_{1/3}$Ta$_{2/3}$O$_3$ is observed around 50 cm$^{-1}$. Also, we found out that the width of well-isolated A$_{1g}$ line of BMT is approximately two times narrower than that observed in relaxors.