Oxygen vacancy induced unusual behavior of the grain boundary and dielectric behavior of WO3 ceramic

In this study, the structural, electrical and dielectric behavior of WO3 synthesized through a novel sol-gel autocombustion synthesis method has been investigated. The X-ray diffraction (XRD) study of WO3 nanoparticles and bulk revealed a monoclinic (P21/n space group symmetry) crystal structure with a crystallite size of approximately 53.02 and 182.86 nm, respectively. The electron charge-density distribution through the maximum entropy method (MEM) showed a slight rotation in WO6 octahedra in the WO3 bulk relative to that in the WO3 nanoparticles. The size of WO3 nanoparticles was observed nearly spherical and in the range of 40-60 nm through transmission electron microscope (TEM). The fracture surface of WO3 bulk exhibited a porous microstructure with 510 nm average grain size through scanning electron microscope (SEM). Direct current (DC) conductivity measurements confirmed an oxygen vacancy induced semiconductor-to-metal transition at approximately 170 degrees C, specifically associated with the grain boundary. Temperature-dependent dielectric loss and alternative current (AC) conductivity measurements provided further evidence of this oxygen vacancy induced transition. The Kohlrausch-Williams-Watts (KWW) parameter ((3) for the grain and grain boundary (GB) was observed in the range of 0.6328 to 0.96419 and 0.99618 to 0.95197, respectively, indicating non-ideal Debye relaxation. The thermally activated peak at low temperature in the temperature-dependent dielectric constant confirmed the polaronic nature of the peak with 0.27 eV activation energy. The frequency exponent increases with the increasing temperature and depicts that the hopping mechanism occurs through a small polaron in WO3 material.