La₃ZrGa₅O₁₄: Band‐Inversion Strategy in Topology‐Protected Octahedron for Large Nonlinear Response and Wide Bandgap

Abstract The contradictory relationship between band gaps and the second‐harmonic generation (SHG) response constitutes a formidable challenge in the rational design of infrared nonlinear optical (IR NLO) crystals. In oxide‐based crystals, the incorporation of strongly distorted octahedra containing d° cations as central elements are a common approach to enhance SHG responses, while inadvertently leading to a significant decrease in band gaps due to the unfavorable energy level splitting. In this study, an innovative “4 d /5 s electron band‐inversion” strategy is introduced to enhance SHG response while preserving a wide band gap within the octahedron‐symmetry‐protected langasite structure. A novel high‐performance IR NLO crystal, La 3 ZrGa 5 O 14 (LGZr) is successfully synthesized, where the unoccupied 4 d orbitals of the Zr 4+ cation underwent a transition from the valence band to the bottom of the conduction band, and the ZrO 6 octahedra exhibited minimal distortion. Consequently, LGZr exhibited the largest SHG response observed to date (reaching up to 2.4 × La 3 Nb 0.5 Ga 5.5 O 14 ) and the broadest band gap (5.16 eV) within the langasite family. Furthermore, LGZr is revealed with a remarkable laser damage threshold (1.66 GW/∼cm 2 ) and broad IR transmission capabilities (∼7.8 µm) and supported the growth of centimeter‐sized single crystal. The “band‐inversion strategy” offers significant advantages to realize high‐performance IR NLO crystals.