KTeOPO4 and NH4SnClSO4: Two New KTiOPO4 Derivatives as Birefringent Crystals Featuring Enhanced Optical Anisotropy

Abstract

Two novel KTiOPO4 (KTP) derivative crystals, KTeOPO4 and NH4SnClSO4, were successfully synthesized and characterized for the first time. Both compounds exhibit significantly enhanced optical anisotropy, with birefringence values of 0.1713 and 0.1845@1064 nm for KTeOPO4 and NH4SnClSO4, respectively, surpassing that of the parent KTP material. Structural analyses reveal that the large birefringence originates from the highly distorted [TeO5] and [SnO3Cl3] polyhedra, which feature stereochemically active lone-pair electrons. Experimental and computational results confirm that KTeOPO4 and NH4SnClSO4 possess a wide band gap, indicating potential applications in the ultraviolet region. This work demonstrates the effectiveness of incorporating lone-pair cations into KTP-type frameworks to engineer crystals with superior birefringence.