Ultrawide Bandgap and Outstanding Second‐Harmonic Generation Response by a Fluorine‐Enrichment Strategy at a Transition‐Metal Oxyfluoride Nonlinear Optical Material

Abstract

The development of nonlinear optical (NLO) materials has been hindered by competing microstructure requirements: the need to simultaneously engineer a large hyperpolarizability (a large second‐harmonic generation (SHG)) and a wide HOMO–LUMO gap (a wide band gap). Herein, a non‐centrosymmetric transition‐metal (TM) oxyfluoride K5(NbOF4)(NbF7)2 (KNOF) with an extremely high F/O ratio is constructed in high yield. KNOF exhibits an extremely wide band gap (5.88 eV) and a strong powder SHG response (4.0×KH2PO4)—both being the largest values for TM‐centered oxyfluorides—as well as a birefringence sufficient for applications. The dominant roles of the partially fluorinated [NbO2F4] and totally fluorinated [NbF7] groups in achieving the enlarged band gap in KNOF have been clarified by first‐principles calculations. Our results suggest that maximizing the fluorine content of oxyfluorides may unlock the promise of short‐wavelength‐transparent materials with exceptional NLO performance.

Isoreticular Design of KTiOPO4-Like Deep-Ultraviolet Transparent Materials Exhibiting Strong Second-Harmonic Generation

“…The transmittance spectra indicate that ZOF and HOF crystals are transparent into the deep-UV region, with absorption edges below 190 nm, corresponding to bandgaps of 6.53 eV (Figure a,b). These bandgaps are significantly wider than any previously reported for NLO d 0 -TM oxyfluorides ,− (Figure f). The magnitude of the distortions (Δ d ) of the [MO 2 F 6 ] polyhedra is calculated according to the equation: Δ d = 1/8 ∑ i [( d i – d av )/ d av ] 2 , where d i is the individual M–O/M–F bond length and d av is the average value, respectively.…”

“…First, the introduction of heavy 4d 0 /5d 0 -TM cations into the KTP-type structure can blue-shift the UV absorption edge in comparison with the 3d 0 Ti 4+ -containing KTP (350 nm) because oxygen-/fluorine-to-metal charge transfer in 4d 0 /5d 0 Zr 4+ /Hf 4+ cation-centered polyhedra is more difficult than with 3d 0 Ti 4+ cation-centered polyhedra. 46 Second, and benefiting from the large electronegativity of fluorine, we note that maximizing the replacement of oxygen by fluorine 49 Hf 4+ are more dispersed and at lower energy (near the bottom of the conduction band) compared to the 4d orbitals of Zr 4+ . As shown in the partial density of states curves of the respective elements in ZOF and HOF, the upper region of the valence band is mainly determined by F 2p orbitals, with a small contribution from O 2p orbitals.…”

“…First, the introduction of heavy 4d 0 /5d 0 -TM cations into the KTP-type structure can blue-shift the UV absorption edge in comparison with the 3d 0 Ti 4+ -containing KTP (350 nm) because oxygen-/fluorine-to-metal charge transfer in 4d 0 /5d 0 Zr 4+ /Hf 4+ cation-centered polyhedra is more difficult than with 3d 0 Ti 4+ cation-centered polyhedra . Second, and benefiting from the large electronegativity of fluorine, we note that maximizing the replacement of oxygen by fluorine promotes the formation of [MO 2 F 6 ] (M = Zr, Hf) polyhedra with a high F/O ratio (3:1), which facilitates a relatively wide UV transparency. Third, the bridging nature of the O(1) atom in ZOF and HOF can effectively eliminate dangling bonds on the O atoms, leading to an increase in the bandgap.…”

“…First, the introduction of heavy 4d 0 /5d 0 -TM cations into the KTP-type structure can blue-shift the UV absorption edge in comparison with the 3d 0 Ti 4+ -containing KTP (350 nm) because oxygen-/fluorine-to-metal charge transfer in 4d 0 /5d 0 Zr 4+ /Hf 4+ cation-centered polyhedra is more difficult than with 3d 0 Ti 4+ cation-centered polyhedra. 46 Second, and benefiting from the large electronegativity of fluorine, we note that maximizing the replacement of oxygen by fluorine 49 ] is more difficult than with higher-valence d 0 -TM cations; 46 the relatively lowvalence Zr 4+ and Hf 4+ show reduced covalent interactions with the oxygen/fluorine ligands, leading to a significant increase in the bandgap. We conclude that the wide bandgaps of ZOF and HOF can be attributed to the [ZrO 2 F 6 ] and [HfO 2 F 6 ] units as well as their corner-sharing connection modes.…”

“…The Δd for the [ZrO 2 F 6 ] and [HfO 2 F 6 ] polyhedra are 4.06 × 10 −4 and 3.41 × 10 −4 , respectively. The bandgap is inversely related to the magnitude of distortion of the d 0 -TM polyhedra in the ZOF/HOF and d 0 -TM oxyfluoride systems39,40,49,50,52,56 (Figure2c); the decrease in distortion with [ZrO 2 F 6 ] and [HfO 2 F 6 ] leads to an increase in the bandgap. These studies with high coordination number Zr 4+ /Hf 4+ -centered polyhedra confirm the feasibility of increasing the bandgap of d 0 -TM polyhedra by reducing the extent of distortion.To elucidate the origin of the wide bandgaps of ZOF and HOF, first-principles calculations were performed with density functional theory using the CASTEP package.…”