High resolution infrared spectroscopy of hydrogen impurities in potassium tantalate

Abstract

The high resolution infrared absorption related to the OH stretching mode (around 3500 cm⁻¹) in KTaO₃ is studied as a function of temperature. In two samples, we observed a splitting of each of the three high temperature peaks into up to four peaks at low temperature. The spectrum obtained is essentially the same for both samples, the small differences resulting from the different thickness of the two samples. The evolution of the absorption frequency as a function of temperature is followed for the three samples. The absorption frequency decreases with increasing temperature and a simple model, involving thermal expansion, is presented and successfully confronted to our datas. The multiplicity of the peaks is related to the presence of three inequivalent positions for the OH dipole giving rise to the three high temperature peaks. The low temperature splitting is related to the fluctuations of the ferroelectric soft modes as it was found in SrTiO₃.