Hydroxyl-Driven Enhanced Birefringence in Borophosphates.

Borophosphates have become promising candidates for ultraviolet or deep-ultraviolet functional crystals. Through high-temperature solution method, four new borophosphates, K 2 B 2 P 2 O 9 , (NH 4 ) 2 BP 2 O 7 (OH), K 2 BP 2 O 7 (OH), and P 2 1 / c -(NH 4 ) 2 B 2 P 3 O 11 (OH), were acquired successfully. Single crystal X-ray diffraction suggests that K 2 B 2 P 2 O 9 , (NH 4 ) 2 BP 2 O 7 (OH), and K 2 BP 2 O 7 (OH) belong to the noncentrosymmetric space group, while P 2 1 / c -(NH 4 ) 2 B 2 P 3 O 11 (OH) belongs to the centrosymmetric compound. It is worth mentioning that K 2 B 2 P 2 O 9 , (NH 4 ) 2 BP 2 O 7 (OH), and K 2 BP 2 O 7 (OH) present the new fundamental building blocks [B 2 P 2 O 11 ], [BP 2 O 10 H], and [BP 2 O 9 (OH)], respectively, as far as we know. Compared with K 2 B 2 P 2 O 9 , (NH 4 ) 2 BP 2 O 7 (OH), K 2 BP 2 O 7 (OH), and P 2 1 / c -(NH 4 ) 2 B 2 P 3 O 11 (OH) exhibit a larger optical anisotropy, further confirming the positive effect of hydroxyl groups on birefringence. UV-vis-NIR diffuse reflectance spectra display that K 2 B 2 P 2 O 9 and (NH 4 ) 2 BP 2 O 7 (OH) have short UV cutoff edges. Meanwhile, theoretical calculations were conducted to comprehend their optical properties and electronic structures.