NIR absorption of Nb⁴⁺ polarons in reduced SBN crystals

A stable broad absorption band around 0.72 eV, assigned to the absorption of Nb⁴⁺ polarons, is observed in pure SBN, SBN : Cr or SBN : Ce crystals after previous reduction treatment at T > 800 °C in Ar, vacuum or H₂. The absorption spectrum of Nb⁴⁺ polarons in reduced SBN crystals shows a considerable dichroism, which exists even far above room temperature. The peak position and the integrated absorption of Nb⁴⁺ polarons are measured as a function of temperature. The maximum of the Nb⁴⁺ polaron absorption shifts to higher energies with decreasing temperature. A comparison of optical absorption and recent literature values [1] of the magnetic susceptibility and therefore concentration of polarons yields values of the optical oscillator strength estimated to be f ≈ (5 ± 0.6) × 10^-2.