Some effects of melt stoichiometry on the optical properties of barium sodium niobate

Abstract

Single crystals of the compound barium sodium niobate (Ba 2NaNb 5O 15), currently of interest for its non-linear optical properties, undergo an apparent symmetry alteration when grown from melts rich in niobium pentoxide. The structure although still of the “tungsten-bronze” type changes from the usual orthorhombic biaxial form to a tetragonal uniaxial one. The symmetry change occurs rather abruptly when crystals are grown from melts containing 52.8 mole% niobium pentoxide and persists up to 58 mole%, the limits of the present study. Unlike the orthorhombic form which appears to have a consistent Curie temperature of about 560 °C, the ferroelectric Curie temperature of the uniaxial form shows a nearly linear relationship to the niobium pentoxide content in the melt. Curie temperatures ranging from 560 °C to 420 °C were observed and correspond to a Curie temperature depression of about 20 °C for each one mole% excess niobium pentoxide in the melt. Associated with the depression of the Curie temperature is a depression in the second harmonic phase matching temperature. Crystals grown at a nominal composition of 53 mole% niobium pentoxide appear to phase match for second harmonic generation at approximately -80 °C as compared to ∽ + 90 °C for the usual stoichiometric barium sodium niobate.