Na 3 Ti 3 O 3 (SeO 3 ) 4 F: A Phase-Matchable Nonlinear-Optical Crystal with Enlarged Second-Harmonic-Generation Intensity and Band Gap.

A new phase-matchable nonlinear-optical material, Na 3 Ti 3 O 3 (SeO 3 ) 4 F, has been achieved by a facile hydrothermal reaction. The anionic skeleton displays a honeycombed 3D structure with Ti 6 Se 6 12-member polyhedral ring tunnels along the c axis. Na 3 Ti 3 O 3 (SeO 3 ) 4 F 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 Na 3 Ti 3 O 3 (SeO 3 ) 4 F is also broader than that of the isomorph. Also, the powder laser-induced damage threshold of Na 3 Ti 3 O 3 (SeO 3 ) 4 F is even 5.5 times larger than that of Ag 3 Ti 3 O 3 (SeO 3 ) 4 F. 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.