Deep-Ultraviolet Transparent Phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 Show Nonlinear Optical Activity from Condensation of [PO4]3- Units

It is challenging to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between deep-UV transparency and high NLO activity. Known deep-UV NLO materials are almost exclusively limited to borates, except few newly discovered phosphates despite their small NLO activities. Here we report two asymmetric phosphates, RbBa2(PO3)(5) (I) and Rb2Ba3(P2O7)(2) (II), which feature [PO3](infinity) chains and [P2O7](4-) dimers formed by condensation of [PO4](3-) units, respectively. Remarkably, I achieves the desired balance, with the shortest deep-UV absorption edge at 163 nm and the largest NLO activity of 1.4 x KDP (KH2PO4) in deep-UV NLO phosphates. According to first-principles calculations, the enhanced macroscopic SHG response of I can be attributed to the [PO3](infinity) chains which exhibit significantly larger microscopic SHG coefficients as compared with the [P2O7](4-) dimers.