Synthesis, characterization and dielectric relaxation of phase pure columbite MgNb2O6: Optimization of calcination and sintering

A tungsten bronze ceramic oxide, MgNb2O6, has been prepared by standard solid state reaction route by optimizing the calcination and sintering temperatures as well as heating/cooling rates during calcination/sintering, respectively. Material calcined at 1100 °C and sintered at 1125 °C gives pure columbite phase in orthorhombic structure with lattice parameters a=5.692Å, b=14.231 Å and c=5.023 Å as revealed from X-ray diffraction study. FESEM micrographs reveal nano-size grain formation with average grain size ≈80 nm. The dielectric permittivity and the loss tangent of the sample have been measured in the frequency range 1 kHz–1 MHz and temperature range 30–400 °C. The frequency dependent peak observed at ≈350 °C in dielectric response is associated with the onset of dielectric relaxation mechanism associated with the coupling of off-center Mg2+ and Nb5+ dipoles with thermally activated conduction electrons. Dielectric dispersion in the material is fitted with Jonscher's relation and values of coefficients a(T) and exponent n(T) were determined from experimental data. Electrical properties of the material were studied using impedance spectroscopy technique. Detailed analysis of impedance spectrum suggested that the electrical properties are strongly temperature dependent. The relaxation is polydispersive and conduction is mainly through grains. The frequency dependent ac conductivity at different temperatures indicated that the conduction is thermally activated process. AC conduction activation energies are estimated from Arrhenius plots and conduction mechanism are discussed.