Two-Stage Evolution from Phosphate to Sulfate of New KTP-type Family Members as UV Nonlinear Optical Materials through Chemical Cosubstitution-Oriented Design
KTiOPO4 (KTP) is a classic commercial nonlinear optical crystal, but the narrow bandgap (3.52 eV) prevents its practical application in the ultraviolet (UV) region. Many trials to widen the narrow bandgap for KTP failed in the past few decades. A chemical cosubstitution strategy was implemented to design new members of KTP-type family as potential UV NLO materials. Firstly, a novel centrosymmetric KTP-type compound NH4SbFPO4•H2O with sharply enlarged bandgap (5.01 eV) has been obtained through three-sites alivolent substitution. Secondly, noncentrosymmetric NH4SbF2SO4 has been synthesized by the introduction of more F- anions to destroy the crystal symmetry and SO42- to replace PO43- for balancing the charge in NH4SbFPO4•H2O, which realized the transformation from visible phosphate system to solar blind UV sulfate system for KTP-type family NLO materials. The preliminary experimental results indicated that NH4SbF2SO4 is a promising solar blind UV NLO material. The first-principles calculations indicated that the sharply enlarged bandgap benefited from the substitution of transition metal cations with main group metal cations and the introduction of F anions with high electronegativity.