RAMAN-SPECTROSCOPY OF MG-TA ORDER-DISORDER IN BAMG1 3TA2/3O3/

A complex perovskite-type material BaMg1/3Ta2/3O3 (BMT) with remarkabl e microwave dielectric properties was studied by microprobe Raman scat tering in a wide temperature interval from 30 up to 1200 K. Two of the most appropriate practical forms of BMT materials, namely ceramics an d single crystal fibers grown by the laser heated pedestal growth tech nique, were employed in the experiments. The way and degree of the B i on arrangement in Complex compounds (Mg and Ta in the present BMT) or, in other words, the nano-microstructural composition have remained th e most important questions for synthesis and practical applications of these complex materials. X-ray diffraction yields a simple cubic stru cture 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 s pectra from both ceramics and fibers does not reveal any important dif ference. A careful consideration of some interdependent characteristic s (the normal mode analysis and mode assignment, the mode positions an d their behavior in comparison with related 1: 1 compounds, no appropr iate 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 disting uish 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 r ich in Ta and pure in Mg. Several additional weak lines in the Raman s pectra as well as slight splitting of the degenerate modes give eviden ce of some distortions in microstructures which could provide, at leas t partly, the charge compensation. These features seem to be mostly st atic. Dynamic changes were found on heating when the Raman spectra of BMT reduced in intensity up to the disappearance without any rearrange ment in the B sublattice. We suppose that due to large phonon vibratio ns at high temperatures, the difference between two primary inequivale nt oxygen octahedrons around Ta and Mg vanishes in average so that the re 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 t his fact is in agreement with the experimental observation. (C) 1997 P ublished by Elsevier Science Ltd. All rights reserved.