Study of Cleavage Fracture Strength of Cation-Tuned Yttrium Calcium Oxyborate Crystals

The strategy of rare-earth cation substitution in yttrium calcium oxyborate (YCOB) crystals has attracted enormous interest over the years in the search for isostructural compounds with improved optical, thermal, and piezoelectric properties. However, our understanding of the importance of these rare-earth (R) cation-tuned R_xY_1–xCa₄O(BO₃)₃ (R:YCOB) crystals in the cleavage fracture strength is limited. Herein, to evaluate the effect of cation substitution on the cleavage fracture behavior of YCOB crystals, the cleavage fracture strengths of R:YCOB (R = Sc, Nd, Sm, and Er) crystals with varying substitution ratios were measured using a three-point bending test. The results showed that the cation-tuned YCOB crystals exhibit larger bending strength, with the highest values observed on the cleavage planes (201) and (101) in Sc_0.05YCOB and Er_0.3YCOB crystals, respectively, which are 32.1% and 28.7% higher than those of pure YCOB crystals with the same orientations. Building on the scanning electron microscopy results, we integrate the solid solution strengthening theories of alloys and propose that the introduction of R³⁺ ions inevitably causes lattice distortion in YCOB crystals, which increases resistance to dislocation movement, thereby hindering crack propagation and ultimately enhancing bending strength. It is believed that these findings may open new avenues of research to overcome the cleavage fracture of larger YCOB crystals.