Temperature evolution of the crystal structures in La(Mg1/2Ti1/2)O3 perovskite: relation to the microwave dielectric properties

The crystal structure and phase transition of the perovskite La(Mg1/2Ti1/2)O3 (LMT) have been investigated using high-resolution neutron powder diffraction in the temperature range 60-1414 K. Between 1246 and 1310 K LMT undergoes the monoclinic-to-rhombohedral (P 21/n-R3-bar) crossover required by Landau theory to be a first order. The temperature dependences of the unit cell parameters and the octahedral tilt angles suggest a discontinuous phase transition. Specific variations of both the metal-oxygen bond lengths and the octahedra volume in LMT are correlated with the dielectric anomalies observed in the same temperature ranges. The uncommon dielectric behaviour in the vicinity of room temperature has been interpreted in terms of deformations of the TiO6 octahedra which change their polarizability and thereby contribute to the dielectric response of LMT. It is believed that in spite of a generally negligible effect, such a contribution to the relative permittivity can occur in compositions containing oxygen octahedra as structural elements