NIR absorption of Nb⁴⁺ polarons in reduced SBN crystals

A broad absorption band around 0.72 eV, assigned to the absorption of Nb⁴⁺ polarons, is observed, even at room temperature and above, 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 spectral shape of the Nb⁴⁺ polaron absorption shifts to higher energies with decreasing temperature. The concentration of polarons can be estimated from the magnetic susceptibility and yields a value of the optical oscillator strength f ≈ 5 × 10^-2 for the NIR-absorption.