Polarized micro-Raman spectroscopy of oriented A(B′1/3B″2/3)O3 powders and microwave ceramics

Complex perovskites of the general formula A(B′1/3B″2/3)O3 (A = Ba or Sr, B′ = Mg, Mn or Zn and B″ = Nb or Ta) belong to an ordered trigonal structure and exhibit remarkable microwave dielectric properties, allowing their use as dielectric resonators. In this spectral region, the dielectric behavior is determined by the phonon characteristics of the materials. In view of these applications and of fundamental studies, ceramic materials are generally obtained and the phonon spectra are then unpolarized, without the benefits of selection rules. In this work, we report on our polarized micro-Raman spectroscopy of ordered trigonal A(B′1/3B″2/3)O3 ceramics under special experimental conditions. The model system used was BaMg1/3Nb2/3O3 (BMN). The materials were obtained either by conventional hydrothermal route followed by sintering or by microwave-hydrothermal process. The complete assignment of the vibrational modes of BMN ceramics was carried out through the analysis of the dependence of the Raman intensities with light beam polarization beside selection rules predictions. Scanning electron microscopy was employed to show the morphological characteristics of the hydrothermal powders, as well as to correlate the phonon vibrations with the dielectric properties in the microwave region.