The temperature dependence of the reflectance in the range 30–1500 ${\mathrm{cm}}^{\mathrm{-}1}$, the transmission in the vicinity of 3500 ${\mathrm{cm}}^{\mathrm{-}1}$, and the Raman spectrum at 15 K of high-purity ${\mathrm{KTaO}}_3$ are compared with the same properties of reduced ${\mathrm{KTaO}}_3$ with oxygen vacancies. Phonons at critical points in the Brillouin zone are observed, while disruption of the Ta-O-Ta chains in the reduced material generates at all temperatures an overdamping of the ferroelectric soft mode, which is no longer detectable. The analysis of the transmittance of both samples in the vicinity of the OH stretching-mode frequency supports the conclusion that an interaction exists between the OH radicals and the ferroelectric soft mode in the high-purity ${\mathrm{KTaO}}_3$ samples.

• Received 2 August 1990

DOI:https://doi.org/10.1103/PhysRevB.43.7555