NH 4 B 3 PO 6 (OH) 3 : A Hydroxyborophosphate with Short DUV Cutoff Edge and Large Birefringence

Birefringent crystals, as core materials for polarization modulation, hold significant importance for the advancement of laser polarizing technology, high-precision instruments, and other optical communication fields today. However, balancing the deep ultraviolet (DUV) cutoff edge and birefringence remains a demanding challenge. Borophosphates have emerged as a promising research direction for optoelectronic functional materials, but the small birefringence of most borophosphates limits their applications. Here, a novel mixed-coordinated hydroxyborophosphate, NH 4 B 3 PO 6 (OH) 3, has been successfully designed and obtained. This compound belongs to the triclinic space group P 1 (No. 2), exhibiting pronounced birefringence (the experimental refractive index difference value is 0.057 at 546 nm) and a short DUV cutoff edge below 200 nm. These properties indicate that NH 4 B 3 PO 6 (OH) 3 exhibits excellent potential as a DUV birefringent material.