Structure and dielectric characterization of a new A-site deficient La5/3MgTaO6 perovskite

The crystal structure of new A-site deficient La5/3MgTaO6 perovskite was investigated by neutron, X-ray and electron diffraction. Neutron and X-ray powder diffraction spectra were refined in the monoclinic image space group, with the parameters of the unit cell a=5.6304(2) Å, b=5.6226(2) Å, c=7.9434(2) Å and β=90.04(1)°. This structural model presumes a random distribution of the vacancies and takes into account both a rock salt-type Mg2+/Ta5+ cation ordering and a−a−c0 configuration of the octahedral tilting. Electron diffraction showed that the crystal structure is more complex at the local level due to a short-range vacancy ordering. This observation in combination with the symmetry analysis leads to the conclusion that the image symmetry with 2√2ap×2√2ap×2ap supercell is more adequate for the description of the crystal structure in the local level. Dielectric measurements performed in a wide frequency range were correlated with the crystal structure and compared with other ordered double perovskites. Far-infrared spectroscopy was used to characterize the lattice contribution to the dielectric response at the microwave frequencies. The complex dielectric function was evaluated and extrapolated down to the gigahertz range. The effect of the vacancies on the intrinsic dielectric losses is discussed based on the spatial phonon correlation model.