Ultrasonic microspectroscopy of congruent LiNbO3 crystals

Experimental procedures for evaluating the compositional homogeneity of LiNbO3 single crystals were developed using the line-focus-beam ultrasonic material characterization system, and the true congruent composition was determined. First, the relationships among leaky surface acoustic wave (LSAW) velocities (VLSAW), chemical compositions, Curie temperatures, densities, and lattice constants were investigated for crystal evaluation, using X -, Y -, and Z -cut substrates prepared from three LiNbO3 crystals grown along the crystallographic Z axis with different Li2 O contents of 48.0, 48.5, and 49.0 mol %. We selected VLSAW for Z -cut Y -axis propagating (ZY) LiNbO3 with the highest sensitivity to chemical composition changes [0.0253 Li2 O mol %(ms)], and also VLSAW for Y -cut X -axis propagating (YX) LiNbO3 [0.0464 Li2 O mol %(ms)] that was advantageous for detailed evaluation of distributions in the chemical composition along the pulling direction as well as the diameter direction.The congruent composition was estimated to be 48.477 Li2 O mol %. Next, the homogeneities of the three above-mentioned crystals and a commercially available LiNbO3 crystal with a nominally congruent composition were evaluated using data for YX - LiNbO3 specimens. Consequently, compositional changes were observed clearly as VLSAW changes. The commercial crystal specimen had a gradient of -0.0046 (ms)mm from top to bottom. The VLSAW variations for the entire examined region (60×36 mm2) exhibited a maximum change of 0.55 ms, corresponding to the chemical composition change of 0.025 Li2 O mol %. The relationships between the chemical compositions and the acoustical physical constants (the elastic, piezoelectric, and dielectric constants, and density) were also provided.