Fe/Nb Co-doped rutile-TiO nanocrystalline powders prepared by a combustion process: preparation and characterization and their giant dielectric response.

Fe/Nb co-doped TiO (FeNb-TO) nanocrystalline powders were prepared by a combustion process. A pure rutile-TiO phase of powders and sintered ceramics with a dense microstructure was achieved. Both co-dopants were homogeneously dispersed in the ceramic microstructure. The presence of oxygen vacancies was confirmed by Raman and X-ray photoelectron spectroscopy techniques. The low-frequency dielectric permittivity enhanced as co-doping concentration increased. The thermally activated giant-dielectric relaxation of FeNb-TO ceramics was observed. Removing the outer-surface layer had a slight effect on the dielectric properties of FeNb-TO ceramics. Density functional theory (DFT) calculation showed that, in the energy preferable configuration, the 2Fe atoms are located near the oxygen vacancy, forming a triangle-shaped FeVTi defect complex. This defect cluster was far away from the diamond-shaped 2Nb2Ti defect complex. Thus, the electron-pinned defect-dipoles (EPDD) cannot be formed. The giant-dielectric relaxation process of the FeNb-TO ceramics might be attributed to the interfacial polarization associated with electron hopping between Ti/Ti ions inside the grains, rather than due to the surface barrier layer capacitor (SBLC) or EPDD effect.