Influence of stoichiometry on defect-related phenomena inLiNbO3

The influence of ${\mathrm{LiNbO}}_3$ stoichiometry ([Li]/[Nb] ratio) on a number of physical phenomena, related to the presence of lattice defects, has been investigated. Positron-lifetime spectra from crystals with different compositions reveal two states for positrons (lifetimes of 230 and 250 ps). The long-lived state is attributed to positrons trapped at cation vacancies, most likely Nb vacancies. Luminescence spectra under x-ray excitation show the same main emission band, peaking at about 2.9 eV for all compositions, although the yield is drastically reduced on increasing Li deficiency. The optical-absorption spectra induced by x-ray irradiation at 80 K is a broad, structured band that can be decomposed into three components peaked at about 1.6, 2.3, and 3.3 eV, for all stoichiometries. However, the production efficiency of each component is dependent on composition. Crystals reduced by heating in a vacuum show a complex optical-absorption spectrum that can be decomposed in two main Gaussian components peaked at about 3.2 and 2.6 eV, with some contribution of another band at about 1.6 eV. The amount of reduction, as measured by the induced absorption, markedly increases with Li deficiency.