A Lithium Calcium Borate Nonlinear Optical Crystal with Deep-UV Cutoff Edge and Moderate Second-Harmonic Generation Response

Nonlinear optical (NLO) crystals with transparency extending into the deep-ultraviolet (DUV) region have received intense research interest. However, designing DUV NLO crystals with well-balanced comprehensive performance remains challenging. Here, we report a new mixed alkali and alkaline-earth metal polyborate NLO crystal, LiCaB 9 O 15 (LCBO), synthesized via spontaneous nucleation. LCBO belongs to the chiral orthorhombic space group P 2 1 2 1 2 1 (No. 19) and features a three-dimensional framework formed by the spiral alignment of [B 3 O 7 ] groups. This unique structure results in a deep-UV cutoff edge below 200 nm with a large bandgap of 5.7 eV. Moreover, LCBO exhibits a moderate second-harmonic generation response (SHG) of 0.8 × KDP and a suitable birefringence of 0.033 at 1064 nm. First-principles calculations trace the optical characteristics of LCBO primarily to the B–O sp hybrid orbitals in [B 3 O 7 ] groups. The combination of these properties positions LCBO as a potential candidate for short-wave UV NLO applications and contributes to the expanding structural diversity in metal polyborate chemistry.