NH₄B₃PO₆(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, NH₄B₃PO₆(OH)₃, has been successfully designed and obtained. This compound belongs to the triclinic space group P1 (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 NH₄B₃PO₆(OH)₃ exhibits excellent potential as a DUV birefringent material.