RCOB: A Type of Optoelectronic Functional Crystal Material with Great Application Potential

Abstract

Rare-earth calcium oxy-borate crystals RCa4O(BO3)3 (RCOB, where R represents rare-earth elements) are a class of crystal materials with noncentrosymmetric structures. Owing to their excellent physicochemical properties─including congruent melting behavior, broad transparency in the ultraviolet–visible range, high resistivity, low dielectric loss, and moderate piezoelectric activity─they are recognized as important optoelectronic functional crystal materials. The electronic configurations of rare-earth ions and the distinct crystal structures induced by their ionic radii influence the refractive index, dispersion characteristics, and nonlinear optical coefficients of the crystals, making the RCOB crystals applicable to a variety of frequency doubling, nonlinear frequency conversion, and parametric amplification applications. The piezoelectric performance of RCOB crystals is closely correlated with the size of R3+: the closer the size of R3+ is to that of Ca2+, the better the piezoelectric performance. This review summarizes the growth processes, crystal structures, thermal properties, optical properties, and piezoelectric properties of these crystals and employs first-principles calculations to investigate the band structure of RCOB crystals. Due to their excellent thermal stability, outstanding nonlinear optical performance, and exceptional piezoelectric properties, RCOB crystals hold significant promise as optoelectronic functional materials.