Raman spectroscopy of Mg-Ta order-disorder in BaMg1/3Ta2/3O3

A complex perovskite-type material BaMg1/3Ta2/3O3 (BMT) with remarkable microwave dielectric properties was studied by microprobe Raman scattering in a wide temperature interval from 30 up to 1200 K. Two of the most appropriate practical forms of BMT materials, namely ceramics and single crystal fibers grown by the laser heated pedestal growth technique, were employed in the experiments. The way and degree of the B ion arrangement in Complex compounds (Mg and Ta in the present BMT) or, in other words, the nano-microstructural composition have remained the most important questions for synthesis and practical applications of these complex materials. X-ray diffraction yields a simple cubic structure and no B-site ordering for the BMT ceramics the <P(3)over bar m1> space group with 1:2 order for Mg and Ta ions and for fibers, which corresponds to the Pm3m symmetry on average. Comparison of the Raman spectra from both ceramics and fibers does not reveal any important difference. A careful consideration of some interdependent characteristics (the normal mode analysis and mode assignment, the mode positions and their behavior in comparison with related 1: 1 compounds, no appropriate splitting of the degenerate modes) led us to conclude that the Mg and Ta ions with 1:1 composition are arranged on nanoscale regions of the Fm3m space symmetry. The selection rules for Raman scattering are controlled by this short-range regular arrangement so one can distinguish between three possibilities (Pm3m, Fm3m and <P(3)over bar m1>) by Raman spectra. In this model, uncompensated charges should appear in both the 1:1 ordered regions and the rest of materials which becomes rich in Ta and pure in Mg. Several additional weak lines in the Raman spectra as well as slight splitting of the degenerate modes give evidence of some distortions in microstructures which could provide, at least partly, the charge compensation. These features seem to be mostly static. Dynamic changes were found on heating when the Raman spectra of BMT reduced in intensity up to the disappearance without any rearrangement in the B sublattice. We suppose that due to large phonon vibrations at high temperatures, the difference between two primary inequivalent oxygen octahedrons around Ta and Mg vanishes in average so that there is an equivalent change in the symmetry from Fm3m to Pm3m. No Raman-active modes of the first-order are allowed in the latter phase and this fact is in agreement with the experimental observation. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.