On the basis of their short ultraviolet (UV) absorption edges, phosphates are ideal candidates for deep-UV nonlinear optical (NLO) applications. However, their often-weak second-harmonic generating (SHG) responses reduce their NLO applications. It has been demonstrated that the SHG response in polyphosphates or orthophosphates could be enhanced by highly polymerized P-O groups or aligned nonbonding O-2p orbitals of isolated PO4 units. Herein, we report on the design and synthesis of two pyrophosphates, K4Mg4(P2O7)3 and Rb4Mg4(P2O7)3, with potential NLO applications. Both materials exhibit relatively large SHG responses with 1064 nm radiation, 1.3× and 1.4× KH2PO4 (KDP) for K4Mg4(P2O7)3 and Rb4Mg4(P2O7)3, respectively. In addition, absorption edges below 200 nm were observed for both materials. For K4Mg4(P2O7)3, single crystal vacuum-UV transmission measurements revealed an absorption edge of 170 nm. First-principles electronic structure calculations identify that the NLO responses arise from the presence of the corner-connected [Mg4P6O21] double layers. We also investigated these compounds using hybrid density functionals, which are found to produce much better agreement with the experimental optical results. Finally, we detail the structural distortions giving rise to the NLO responses. Our results indicate that phosphates with low polymerized P-O groups, such as pyrophosphates, may exhibit large SHG responses if their structures are properly designed.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Materials Chemistry