KPb2(PO3)5: a novel nonlinear optical lead polyphosphate with a short deep-UV cutoff edge

Abstract

It is a challenge to explore deep-ultraviolet (DUV) nonlinear optical (NLO) crystals in a Pb-containing system due to the red-shift caused by the Pb atom. Here, three new NLO crystals APb₂(PO₃)₅ (A = K, Rb and Cs) were synthesized. Their single-crystal X-ray diffraction patterns indicate that they are isostructural and crystallize in the asymmetric space group of Pn (No. 7). Interestingly, KPb₂(PO₃)₅ displays moderate powder second harmonic generation (SHG) intensities (0.5× KDP) in type I phase matching behaviors with a short UV cut-off edge of 177 nm, which is the shortest UV cut-off edge among Pb-containing NLO crystals. To better understand the blue-shift of the band gap of KPb₂(PO₃)₅, the theoretical calculations by density functional theory were done. In addition, RbPb₂(PO₃)₅ shows a SHG response of 0.3× KDP and is phase matchable. According to first-principles calculations, compared with RbBa₂(PO₃)₅, KPb₂(PO₃)₅ has a noticeable enhancement in birefringence (0.021 and 0.009 at 1064 nm for KPb₂(PO₃)₅ and RbBa₂(PO₃)₅, respectively). Furthermore, the relationship between its structure and NLO properties was also discussed by calculation of dipole moments.