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 “4d/5s 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, La₃ZrGa₅O₁₄ (LGZr) is successfully synthesized, where the unoccupied 4d orbitals of the Zr⁴⁺ cation underwent a transition from the valence band to the bottom of the conduction band, and the ZrO₆ octahedra exhibited minimal distortion. Consequently, LGZr exhibited the largest SHG response observed to date (reaching up to 2.4 × La₃Nb_0.5Ga_5.5O₁₄) 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/∼cm²) 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.