Abstract
A set of $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ (PMN) relaxor ceramics with different porosities (66--96% of theoretical density) was processed from the pure perovskite PMN powder by varying the sintering temperature (930--980 K) and isostatic pressure. The effective room temperature permittivity at 100 Hz varied correspondingly from \ensuremath{\sim}4000 to \ensuremath{\sim}15 000. We measured and fitted the room temperature infrared (IR) reflectivity of corresponding polished ceramics, from which the effective dielectric function was obtained. The results were compared with the low-frequency permittivity data and discussed using several effective medium models. The best results were obtained using the Lichtenecker model (with the exponent \ensuremath{\alpha} \ensuremath{\approx} 0.2), which fits quite well the IR spectra as well as the static permittivity for all the porosity values. Also the simulation using the finite element method supported the same model. It allows us to conclude that the samples show quite complex shape and topology of the pores, but all of the pores show similarity with a weak nonzero percolation (open porosity).
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