Fully tricoordinated assembly unveils a pioneering nonlinear optical crystal (SbTeO₃)(NO₃)

Balancing the critical property requirements is key to surmounting the obstacles in the application of nonlinear optical (NLO) crystals. Tricoordinated units, characterized by nearly the lowest coordination number, are common in inorganic NLO-active oxides; however, crystals solely composed of such units are rare. Herein, by assembling three distinct tricoordinated units (SbO₃, TeO₃, and NO₃) into a single crystal, a pioneering fully tricoordinated NLO material, (SbTeO₃)(NO₃), was synthesized via a facile volatilization method. As the first reported tellurite-antimonite NLO crystal, (SbTeO₃)(NO₃) exhibits well-balanced properties: a strong phase-matched second-harmonic generation (SHG) effect (2.2 × KDP), short UV cutoff edge (253 nm) and moderate birefringence (0.081@546 nm). Unlike most deliquescent nitrates, (SbTeO₃)(NO₃) demonstrates exceptional water resistance (>30 days), attributed to its unique hydrophobic layers and stereochemically active lone pair (SCALP) electrons in the Sb³⁺ and Te⁴⁺ cations. Theoretical calculations reveal that the optical bandgap and SHG effect of (SbTeO₃)(NO₃) are collectively governed by the three tricoordinated motifs, with individual SHG contributions of 20.92%, 23.88%, and 55.12% from [SbO₃], [TeO₃] and [NO₃], respectively. This breakthrough underscores the efficacy of the fully tricoordinated assembly strategy in engineering NLO materials with optimally balanced properties.