Double Second-order Jahn-Teller Strategy Driven NLO Response in Sulfate Crystal Nb2O2(TeO3)2(SO4) with a Large Birefringence

Abstract

Nonlinear optical (NLO) crystals serve as indispensable components in laser technology. Many modifications of sulfate-based NLO materials rely on introducing a single type of functional cation, which in some cases leads to an imbalanced improvement in either NLO response or birefringence. To enable simultaneous improvement of these key properties, we designed and synthesized a novel sulfate compound, Nb2O2(TeO3)2(SO4), using a double second-order Jahn-Teller (SOJT) strategy that introduces both Nb5⁺ (d0 cation) and Te4+ (lone-pair cation) into the sulfate framework. Remarkably, the introduction of distorted [NbO6] octahedra and [TeO3] pyramids enables the material to achieve a remarkable birefringence of 0.197 at 546 nm and a second-harmonic generation (SHG) response of 0.82 times that of KH2PO4 (KDP). Furthermore, the compound exhibits high thermal stability (> 600 °C), maintains chemical stability in both water and concentrated sulfuric acid, and possesses an ultraviolet (UV) cutoff edge at 305 nm. Theoretical analyses confirm that the synergy between the [NbO6] octahedra and [TeO3] pyramids is the key mechanism for the enhanced SHG response and birefringence. This work demonstrates the potential of Nb2O2(TeO3)2(SO4) as an UV-NLO material and validates the effectiveness of the multi-component synergistic strategy in designing advanced NLO crystals.