Piezoelectric and elastic properties of the nonlinear optical material bismuth triborate, BiB₃O₆

Abstract

Piezoelectric, pyroelectric, dielectric, elastic and thermoelastic properties of BiB₃O₆, point group symmetry 2, are reported. The eight independent components of the piezoelectric tensor could be derived from first-order longitudinal and transverse electrostrictive effects, employing a Michelson interferometer. For the determination of the elasticity tensor ultrasonic resonances of plane-parallel plates of different orientations and, in addition, the resonant ultrasound spectra (RUS) of rectangular parallelepipeda were measured. For the evaluation of the elasticity tensor from these resonant frequencies the piezoelectric coupling had to be taken into account. BiB₃O₆ exhibits a maximum longitudinal piezoelectric effect of 39.5×10^-12 mV^-1 along the twofold axis, a value about 17 timeslarger than that of α-quartz. The longitudinal elastic stiffness attains a minimum of about 5×10¹⁰ N m^-2 along the same direction and a maximum perpendicular to the twofold axis of about 33×10¹⁰ N m^-2, one of the largest elastic anisotropies observed in ionic crystals. This behaviour corresponds to the earlier observed extreme anisotropy of thermal expansion. The phenomena can be qualitatively interpreted by structural details, especially the preferential orientation of the lone electron pair of Bi³⁺.