Na₃Ti₃O₃(SeO₃)₄F: A Phase-Matchable Nonlinear-Optical Crystal with Enlarged Second-Harmonic-Generation Intensity and...

A new phase-matchable nonlinear-optical material, Na₃Ti₃O₃(SeO₃)₄F, has been achieved by a facile hydrothermal reaction. The anionic skeleton displays a honeycombed 3D structure with Ti₆Se₆ 12-member polyhedral ring tunnels along the c axis. Na₃Ti₃O₃(SeO₃)₄F presents a strong second-harmonic-generation (SHG) response of about 6 × KDP, which is twice that of its isostructural Ag compound. Interestingly, the band gap of Na₃Ti₃O₃(SeO₃)₄F is also broader than that of the isomorph. Also, the powder laser-induced damage threshold of Na₃Ti₃O₃(SeO₃)₄F is even 5.5 times larger than that of Ag₃Ti₃O₃(SeO₃)₄F. Theoretical calculations revealed that the fully filled Ag 4d and empty Ag 5s states have made the difference in the band gap and SHG response of the two isostructural compounds. Our work has provided a practical way of improving the SHG efficiency and band gap simultaneously, which is usually considered to be a pair of negatively correlated parameters.