BiGa(SeO₃)₃: A Phase Matchable SHG Material Achieved by Cation Substitution

The search of novel second-harmonic generation crystals in gallium selenite systems results in three new compounds of MGa₂(SeO₃)₄ (M = Sr, Pb) and BiGa(SeO₃)₃. The isostructural MGa₂(SeO₃)₄ (M = Sr, Pb) are centrosymmetric (P1̅, no. 2). The structures represent a complicated three-dimensional skeleton containing two different types of Ga₄Se₄ 8-member polyhedral ring tunnels with the counter cations located at the broad ones. When the divalent counter cations were substituted by a trivalent lone pair cation of Bi(III), a novel noncentrosymmetric compound of BiGa(SeO₃)₃ was achieved. BiGa(SeO₃)₃ displays a 3D gallium selenite framework with 1D bismuth oxide chains filling in the 12-member polyhedral ring tunnels. Frequency doubling measurements showed that BiGa(SeO₃)₃ presented an apparent SHG signal comparable to the commercial KDP and can realize phase matching. The lone pair anionic groups of SeO₃^2- made the major contribution according to the SHG density analyses. BiGa(SeO₃)₃ exhibits a wide transparent range and large optical bandgap (3.90 eV). The laser-induced damage threshold of BiGa(SeO₃)₃ was estimated to be 98.68 MW/cm², about 44 × AGS.