Monoclinically distorted perovskites, A₂ZnTiO₆ (A=Pr, Gd): rietveld refinement, and dielectric studies

Abstract

Double perovskite related oxide A₂ZnTiO₆A=Pr, Gd were synthesized by the solid state reaction method at 1523 K. The structure and microstructure of the compounds were studied by X-ray, SAED and FESEM. Rietveld refinement of the powder X-ray analysis shows that the compounds crystallizes in monoclinic space group P2₁/n (a⁺b⁻b⁻) with unit cell parameter √2a_p×√2a_p×2a_p (a=5.5026(1) Å, b=5.6305(1) Å, c=7.8149(1) Å, β=90.02(1)° for Pr₂ZnTiO₆ and a=5.3621(1) Å, b=5.6565(2) Å, c=7.6779(2) Å and β=90.264(2)° for Gd₂ZnTiO₆. Electron diffraction study confirms P2₁/n symmetry of the oxides. The monoclinic distortion is larger in Gd₂ZnTiO₆ than Pr₂ZnTiO₆ which is associated with the tolerance factor and the tilting angle of ZnO₆ and TiO₆ octahedra (ϕ=13.64° for Pr₂ZnTiO₆ and 16.51° for Gd₂ZnTiO₆). The compounds are highly ordered. The charge and size difference between B site cations are the driving force for the ordering of the B′O₆ and B″O₆ octahedra. Pr₂ZnTiO₆ shows a dielectric constant of 27 and dielectric loss of 0.003 while Gd₂ZnTiO₆ has a dielectric constant of 17 and dielectric loss of 0.005 measured at 1 MHz.