NH₄PO₃HCH₃ and NH₄PO₃CH₂NH₃: Two Phosphate Derivatives with Deep-UV Cutoff Edges and Moderate Birefringence

To maintain moderate birefringent properties without compromising the wide optical transmittance window characteristic of phosphate materials, a crucial approach focuses on designing innovative functional units with great performance. This research has demonstrated that [PO₃HCH₃] and [PO₃CH₂NH₃] units serve as effective birefringent-active components in the DUV regions. This study successfully prepared two novel crystalline materials, NH₄PO₃HCH₃ and NH₄PO₃CH₂NH₃, which demonstrated both deep-UV transparency and moderate birefringence. These properties exceed those of conventional MgF₂ crystals and the majority of known phosphate compounds in the DUV region. Computational investigations indicate that the observed birefringence is from the [PO₃HCH₃] and [PO₃CH₂NH₃] building blocks, highlighting the potential of tetrahedral coordination environment engineering as a viable strategy for developing new optical functional units.