Refractive-index measurements of the beta'' aluminas

The refractive indices of selected monovalent and divalent beta'' alumina single crystals are determined using prism refraction techniques. The birefringence is found to vary from uniaxial negative to uniaxial positive depending on the electronic polarizability of the exchanged ion. Thus, beta'' alumina is a novel material in which the magnitude and polarity of the birefringence can be tuned simply by ion exchange. The refractive-index data are used to predict an isoindex point (n(e) = n(0)) for the mixed-system Na(+)-Ag(+) beta'' alumina compositions.