Dielectric relaxation mechanism of single crystal and polycrystal bismuth germanate

Abstract

We report the results of investigation on the mechanism of dark conductivity and dielectric relaxation of Bi12GeO20 single crystals and dense ceramics. Alternating current electric characterization was performed in the temperature interval from 30 °C up to 730 °C and frequencies from 1 Hz to 13 MHz. The samples presented ohmic behavior even at high temperatures, for applied potentials up to 2.2 V. The electric conductivity of single crystals was thermally activated, with potential barriers changing from 0.75±0.05 eV to 1.4±0.2 eV at 500 °C. The same conduction mechanisms were observed for ceramic samples, with activation energies of 0.9±0.1 eV and 1.5±0.1 eV for charge transport through grains and grain boundaries, respectively. These energy values were related to defects arising from substitutional Bi3++h+ in Ge4+ sites. In the ceramic samples, the higher resistivity of the grain interfaces leads to a Maxwell–Wagner polarization of the grain boundaries due to local charge rearrangement. As a result, the overall permittivity of ceramics was higher than that observed for single crystals at frequencies corresponding to the grain boundary response.

Dielectric relaxation in pure and Co-doped Bi12GeO20 single crystals

“…They found a reason for the dielectric relaxation in existence of impurity-vacancy-type quasi-dipoles. We think that by Macedo et al 6 adequately explained the origin of dielectric relaxation and the conductivity mechanism of single crystals or ceramics of Bi 12 GeO 20 . We also think that the same mechanism is valid also for Bi 12 GeO 20 doped with Co nanoparticles.…”

“…3. Above $300 K the ac conductivity at 1 Hz can be well described with the activation type temperature dependence, 0 / exp(ÀE=k B T), where E re-presents the apparent activation energy for the conduction process 6 and k B the Boltzmann constant. The values of the activation energies are E 1 ¼ ð0:55 AE 0:01) eV for pure and E 2 ¼ ð0:59 AE 0:01) eV for the doped sample.…”

“…The values of the activation energies are E 1 ¼ ð0:53 AE 0:03) eV for pure and E 2 ¼ ð0:64 AE 0:03) eV for a doped sample. The In order to describe the origin of dielectric relaxation and the conductivity mechanism of the single crystal or ceramics of Bi 12 GeO 20 we followed the explanation of Macedo et al 6 They explained conduction mechanism as the hopping of charge carriers released from two different energy traps, which are related to defects arising from substitutional Bi 3þ þ h þ in Ge 4þ sites. Namely, 6 in Bi 12 GeO 20 , the Ge 4þ cations are tetrahedrally coordinated, occupying the cube corners and body-centered sites and linked by hepta-coordinated Bi atoms.…”

“…The In order to describe the origin of dielectric relaxation and the conductivity mechanism of the single crystal or ceramics of Bi 12 GeO 20 we followed the explanation of Macedo et al 6 They explained conduction mechanism as the hopping of charge carriers released from two different energy traps, which are related to defects arising from substitutional Bi 3þ þ h þ in Ge 4þ sites. Namely, 6 in Bi 12 GeO 20 , the Ge 4þ cations are tetrahedrally coordinated, occupying the cube corners and body-centered sites and linked by hepta-coordinated Bi atoms. Bi/Ge antisite defect was suggested to be the most plausible intrinsic defect, and in view of the chemical instability of Bi 4þ , it was proposed that the Bi Ge defect would consist of a Bi 3þ ion associated with a hole.…”

“…Dielectric properties of this material were studied before. [2][3][4][5] Electrical conductivity and dielectric relaxation in BGO single crystals and ceramics were studied recently 6 in the temperature interval from 300 up to 1000 K. Microwave dielectric properties of ceramics of six sillenite compounds were investigated at 5.5 GHz and revealed the values of dielectric constant around 40 for all compounds. 7 For ceramic materials it has been shown that addition of nanoparticles could increase significantly the dielectric constant of a composite material close to the percolation threshold (nanoparticles close to each other -almost touching).…”