ANALYSIS OF THE PHASE-TRANSITION SEQUENCE OF THE ELPASOLITE (ORDERED PEROVSKITE) PB2MGTEO6

The elpasolite compound lead magnesium tellurate Pb2MgTeO6 undergoes a structural phase transition at about 194 K. This phase transition has been characterized by calorimetry, by dielectric permittivity measure ments and by Raman spectroscopy. The main features of the Raman spectr a, collected between 35 K and room temperature, are consistent with gr oup-theoretical predictions, though the onset of the incommensurate su perstructure gives rise to a very complex splitting of the cubic lines . From the comparison between these results and the structures, it is concluded that the phase transition is only weakly first order and has a mainly displacive character. The thermal evolution of the Raman spe ctra and of the dielectric permittivity support the existence of an ad ditional phase transition at about 142 K.