Hierarchized band gap and enhanced optical responses of trivalent rare-earth metal nitrates due to (d–p)π conjugation interactions

Abstract

Introducing NO₃ groups or particular rare-earth metals is one strategy that may yield optical functional materials without influencing the transmittance in the ultraviolet (UV)/deep-UV region. In this work, a new crystal, Na₄La₂(NO₃)₁₀·2H₂O (NLN), containing these two units was obtained for the first time by slow evaporation method at room temperature with dimensions of 3 × 2 × 0.75 mm³. It possesses an unusual, high UV cutoff edge and therefore totally differs from alkali/alkaline-earth metal nitrates and rare-earth metal borates. K₂La(NO₃)₅·2H₂O (KLN), which resembles NLN structurally, undergoes a noticeable improvement in its linear optical properties compared with NLN and has a large second harmonic generation effect. To clarify the inner mechanism or synergy of NO₃ and rare-earth metals in influencing electronic structure and optical properties, trivalent rare-earth metal nitrates such as Na₄La₂(NO₃)₁₀·2H₂O and K₂La(NO₃)₅·2H₂O have been studied systematically from first-principles, and compared with related borates. This has revealed that the unique (d–p)π interaction generated between the trivalent rare-earth metal and oxygen of NO₃ narrows the large band gap, enhances the second harmonic generation (SHG) effect and strengthens the optical anisotropy when the coordination environment is non-uniform in electronegativity. Specifically, the hierarchical band gap is related to the strength of these interactions, which itself depends on the delocalization of O-2p orbitals in the groups. This insight offers a way to design new functional materials with special characteristics by carefully selecting the cations or anionic groups.