Nain(C2o4)(Hpo4)(H2o)5: An Ultraviolet Transparent Nonlinear Optical Oxalate-Phosphate with Large Birefringence

Abstract

The development of new types of crystalline phosphates is of importance owing to their promising applications in ultraviolet (UV) nonlinear optical (NLO) materials and devices. In this study, a new oxalate-phosphate, NaIn(C2O4)(HPO4)(H2O)5 (NICOP), has been obtained by the low-temperature hydrothermal method. NICOP crystallizes in noncentrosymmetric space group P212121. Its structure consists of two-dimensional [In(C2O4)(HPO4)]∞ layers, assembled into a pseudo-three-dimensional (3D) framework linked by alkali metal cations and hydrogen bonds. NICOP is the first second-harmonic generation-active (SHG-active) oxalate-phosphate with a phase-matchable SHG response of 0.2 × KH2PO4 (KDP). The ultraviolet cut-off edge is 254 nm, corresponding to a bandgap of 4.88 eV. Benefiting from its π-conjugated [C2O4] group, large experimental birefringence (0.098 @546 nm) is observed for NICOP. First-principles calculations were executed to demonstrate the microscopic origin of the optical performance. Our work suggests that the incorporation of π-conjugated [C2O4] group into phosphates can lead to the construction of promising multicomponent phosphate UV transparent NLO materials with large birefringence.

Keywords: nonlinear optics, ultraviolet, second-harmonic generation, oxalate-phosphate, birefringence