For A-site doping the dielectric constant of SrTiO3 ceramics decreases with cerium doping. Can any body known the decrease in the dielectric constant in term of Hardening and softening of soft phonon modes.
The dielectric constant is proportional to density of the material. The addition of CeO2 leads to a distortion at crystalline symmetry caused by the cell volume change, according to the solubility of CeO2 in SrTiO3.
The reduction of Ce + 4 will causes oxigen vacancies, which disturbes the lattice potential energy of the crystalline structure and increases the dielectric losses.
Donor ions enhance the reduction of TiO2, while acceptor ions inhibit this process. Ce ions play a critical role in doping of the grain boundary and modifying the permittivity of Composite. However, the ion that substitutes at grain boundaries and acts as an electron donor, increase the electron concentration across grain boundaries where ions act as electron acceptors, decrease it. “The ultimate victor of this important struggle, is the most important determinant of the dielectric properties improving of your dielectric materials“. So, if your doping change the donor/acceptor densities, then your electron concentration will increase in case of donor increasing, and your dielectric constant decreases.
On the other hand, based on IBLC model, an increase in grain size (tg) with decrease in grain boundary thickness (tgb) will cause in increase of the dielectric constant of materials with semiconductor grain-insulating grain boundary. So, the influence of a tight interface made by your doped materials can be another issue on dielectric constant and loss, too.
Please have a look at our paper: J. Appl. Phys. 117, 094103 (2015); http://dx.doi.org/10.1063/1.4914052
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