Optical Anisotropy and Electrostriction in the Anodic Oxide of Niobium

Abstract

The anodic oxidation of niobium in sulfuric acid electrolyte is studied using a self‐nulling ellipsometer to follow the optical changes that occur when the field in the oxide film is switched. The oxide is optically anisotropic with the anodizing field applied, and the degree of anisotropy, the ratio between index changes parallel and transverse to the field, is found to equal 2.2 by a procedure that requires consistency from the analysis of field‐switching transients for a wide range of oxide film thicknesses. The film thickness increases and its refractive index values decrease when the field is applied. The dependences are not the parabolic dependences predicted by the theory of electrostriction, and the significance of this finding is discussed. Although the galvanostatic charge‐discharge transients that are used to study the changes in the low‐frequency dielectric constant exhibit significant hysteresis, they show that the decrease in dielectric constant with increasing field observed at optical frequencies occurs also at low frequencies. The results are compared with the dependence of dielectric constant on field deduced from open‐circuit transient analysis, and the significance of the discrepancy is discussed. It is concluded that the possibility that the oxide film has a significantly polar structure remains an open question.