Elastic, piezoelectric, and dielectric properties of rare-earth calcium oxoborates RCa_4O(BO_3)_3 (R= Er, Y, Dy, Gd, Sm, Nd, La)

摘要

Large single crystals of rare-earth calcium oxoborates RCa_4O(BO_3)_3 with R = Er, Y, Dy, Gd, Sm, Nd, La (RCOB) were grown by the Czochralski method. Complete sets of dielectric, piezoelectric stress, and elastic stiffness coefficients of the RCOB crystal species were determined at ambient conditions using a combination of resonant ultrasound spectroscopy and the substitution method. The results are inherently consistent and reveal clear crystal chemical trends, with the size of the trivalent cation playing an important role. In particular, the longitudinal and shear aggregate elastic stiffnesses, c_(11)~(iso) and c_(44)~(iso), decrease from about 165 to 154 GPa and, respectively, 45 to 41 GPa from the smallest to the largest R~(3+) cation of the investigated crystal species, while the dielectric coefficients ∈_(22) and ∈_(33) increase. However, the maximum longitudinal piezoelectric effect peaks with 8.6 pCN~(-1) at NdCOB, the species where the radius of R~(3+) best matches the one of Ca~(2+). Increasing differences in the size of R~(3+) and Ca~(2+) lead to anisotropic stresses in the crystal lattice, which are partially relaxed by an increasing degree of cation disorder.