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.