  The structure of BaMg1/3Ta2/3O3 (BMT) has been studied using X-ray
scattering. The phonons have been measured and the results are similar to those
of other materials with the perovskite structure such as PbMg1/3Nb2/3O3 (PMN).
The acoustic and lowest energy optic branches were measured but it was not
possible to measure the branches of higher energy, possibly this is because
they largely consist of oxygen motions. High-resolution inelastic measurements
also showed that the diffuse scattering was strictly elastic and not directly
related to the phonon spectra. A diffuse scattering was observed in BMT near
the (H\pm1/2, K\pm1/2, L\pm1/2) points in the Brillouin zone and this had a
characteristic cube shape. This arises from ordering of the B-site ions in BMT.
Additional experiments revealed a diffuse scattering in BMT similar in shape to
Bragg reflections at wave-vectors of the form (H\pm1/3, K\pm1/3, L\pm1/3). Such
reflections were also observed by Lufaso [Chem. Matt. 16 (2004) 2148] from
powders and suggest that this structure of BMT consists of 4 differently
oriented domains of a trigonal structure and results from a different ordering
of the B-site ions from that responsible for the scattering at the (H\pm1/2,
K\pm1/2, L\pm1/2) points. The results lead us to suggest that for BMT single
crystals the bulk has the properties of a cubic perovskite, whereas the surface
may have quite different structure from that of the bulk. This difference
resembles the behaviour of cubic relaxors like PMN and PMN doped by PbTiO3,
where significant surface effects have been reported.
