Optical absorption of KTa1-xNbxO3 single crystals.

The optical absorption at photon energies near the fundamental absorption edge was measured in the cubic phase of ${\mathrm{KTa}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ crystals, both in a ``pure'' sample and in an (Fe,Ti)-doped case. In the first case the absorption constant followed the exponential Urbach rule, with a single steep slope, while in the second case two distinct slopes of lower values were clearly discernible, each in its own separate energy range. The range of temperatures applied to the samples was between 77 and 373 K. The functional temperature dependence of the slopes indicates that in both cases strains and impurities have a predominant influence through their constant electrical fields. On the other hand, the temperature shift of the optical-gap energy has been ascertained in both cases as being due to an internal Franz-Keldysh effect of the electrical fields created by the 25-meV ${\mathrm{LO}}_{1}$ phonons.