MS16-P11 Structure- property relations and structural instabilities in high-temperature piezoelectric rare-earth calcium oxoborates RCa4O(BO... Marie Münchhalfen (Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany) Jürgen Schreuer (Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany) Christoph Reuther (Institut für Mineralogie, TU Bergakademie Freiberg, Freiberg, Germany) Jens Götze (Institut für Mineralogie, TU Bergakademie Freiberg, Freiberg, Germany) Erik Mehner (Institut für Experimentelle Physik, TU Bergakademie Freiberg, Freiberg, Germany) Hartmut Stöcker (Institut für Experimentelle Physik, TU Bergakademie Freiberg, Freiberg, Germany)email: marie.muenchhalfen@rub.de

Monoclinic (space group Cm) rare-earth calcium oxoborates (RCOB), RCa4O(BO3)3 (R = rare earth element), have been studied for about 25 years because of their promising non-linear optical properties. These crystal species are isostructural with calcium fluoroborate, Ca5(BO3)3F [1], whose structure in turn is closely related to the one of hexagonal fluorapatite [2]. The substitution of tetrahedral PO4 groups by planar BO3 groups causes distortions which is the reason for the pronounced polar properties of the rare-earth oxoborates. Recently, the members of the RCOB family gathered interest as potential candidates for high-temperature piezoelectric sensing applications since they combine favorable properties like high melting point at around 1770 K, no reported structural phase transitions, high piezoelectric sensitivity and high electric resistivity [3]. Furthermore, the RCOB structure offers different possibilities for cation substitution which in principle allow for tuning of physical properties. Their low symmetry results in a high number of degrees of freedom regarding the anisotropy of physical properties.

 

We studied heat capacity, thermal expansion as well as dielectric, piezoelectric and elastic properties of GdCa4O(BO3)3 between 100 K and 1473 K using differential scanning calorimetry, dilatometry and resonant ultrasound spectroscopy. Contrary to the reported lack of phase transitions, all investigated physical properties undergo reproducible discontinuities at around 1000 K. X-ray diffraction experiments on quenched samples indicate a gradual increase of cation disorder starting roughly at around 1000 K. Therefore, a glass-like transition from static to dynamical cation disorder is likely responsible for the observed discontinuities. First results indicate only a minor influence of the glass-like transition on the electromechanical properties.
 

Acknowledgments: The authors gratefully acknowledge financial support of the DFG (PAK921/1, SCHR 761/4: Structure/property relationships and structural instabilities of high-temperature piezoelectrics of the oxoborate family.

References:

[1] Shirong, L., Huang, Q., Zheng, Y., Jiang, A., Chen, C. (1989): Acta Cryst, 45, 1861-1863.

[2] Fletcher, J. G.; Glasser, F. P.; Howie, R. A. (1991): Acta Cryst, 47, 12-14.

[3] Yu, F., Hou, S., Zhao, X., Zhang, S. (2014): IEEE Trans. Ultrason., Ferroelect., Freq. Control, 61, 1344-1356.
Keywords: piezoelectricity, elasticity, order-disorder