Dielectric permittivity and Cr³⁺ impurity ion probe luminescence in SrTiO₃ sol-gel ceramics

In order to inspect bulk properties of the SrTiO₃ ceramics grains, dielectric permittivity and temperature shift of a zero-phonon R-emission line of the Cr³⁺ impurity-ion probes were studied in sol-gel SrTiO₃:Cr (3 × 10^-2%) ceramics of the average grain size ∼24 μm. We found that non-dispersive permittivity (100 Hz to 1 MHz) increases on cooling less steeply than in single crystals reaching ∼5000 at 30 K. At lower temperatures the dispersion appears and the permittivity saturates at ∼8 K (ε′ ≈ 7000, 400 Hz). However, the temperature shift of the R-line in ceramics remains practically identical to that for SrTiO₃ single crystals, in which R-line shifts with temperature is Δν ∼ 1/ε′ ∼ω_{TO 1}² . This indicates that the bulk of the grains in ceramics exhibits properties inherent to SrTiO₃ quantum paraelectric, which agrees with the "generalized brick-wall" approach developed recently to describe polarization response of granular high permittivity materials.