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 $Li2O$ contents of 48.0, 48.5, and $49.0mol%$⁠. We selected $VLSAW$ for $Z$-cut $Y$-axis propagating $(ZY)$ $LiNbO3$ with the highest sensitivity to chemical composition changes $[0.0253Li2Omol%∕(m∕s)]$⁠, and also $VLSAW$ for $Y$-cut $X$-axis propagating $(YX)$ $LiNbO3$ $[0.0464Li2Omol%∕(m∕s)]$ 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 $48.477Li2Omol%$⁠. 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 $−0.0046(m∕s)∕mm$ from top to bottom. The $VLSAW$ variations for the entire examined region $(60×36mm2)$ exhibited a maximum change of $0.55m∕s$⁠, corresponding to the chemical composition change of $0.025Li2Omol%$⁠. The relationships between the chemical compositions and the acoustical physical constants (the elastic, piezoelectric, and dielectric constants, and density) were also provided.