The electric activity of grain boundaries in donor-doped SrTiO₃ bicrystals is studied using a combination of variable temperature impedance spectroscopy, current-voltage measurements and scanning probe microscopy. The dominant transport mechanism changes from thermionic emission to diffusive at about 220 K. The temperature dependence of interface capacitance is interpreted in terms of field-dependent dielectric constant in the vicinity of the interfaces. Simplified expressions for the temperature dependences of interface potential, charge and capacitance for a weakly nonlinear dielectric are derived to describe the experiment. The complex nature of these materials requires incorporation of such effects as nonlinear polarization and multiple simultaneous transport mechanisms.

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