Unprecedented v-shaped [SeO₂E] units stabilized in KM[Se₂O₅]₂[SeO₂] (M= Al and Ga) constructed by nanoconfinement strategy

Pyramidal [SeO₃E]²⁻ (E being a lone pair) selenite units with stereochemically active lone pairs (SCALP) have been widely used for the design of nonlinear optical (NLO) materials with large second harmonic generation (SHG) response. However, compared with the sp³ hybridization pyramidal [SeO₃E]²⁻ units, primitive V-shaped sp² hybridization [SeO₂E] units containing not only SCALP but also the delocalized π-electrons are more conducive to excellent NLO performances. But up to now, no crystalline selenites containing V-shaped [SeO₂E] groups have been reported because of the fact when the acid SeO₂ gaseous molecule reacts with a basic oxide molecule, the V-shaped [SeO₂E] unit typically transforms into the pyramidal [SeO₃E]²⁻ units. Herein, through constructing three-dimension (3D) ordered electrically-neutral nanopores in K-Al/Ga-Se-O framework to ‘adsorb’ SeO₂ gaseous molecule, the first selenites containing the [SeO₂E] units, KGa[Se₂O₅]₂[SeO₂] (I) and KAl[Se₂O₅]₂[SeO₂] (II) were designed and synthesized successfully. Theoretical calculations and experimental results indicate that they both exhibit excellent NLO properties, including the largest SHG response ((I)~10.2×KDP, (II)~11.5×KDP) and birefringence (Δn(I)=0.196@546 nm; Δn(II)=0.208@546 nm) in selenites that have no d⁰ transition metal (TM) and other stereochemically active lone pair cations, suggesting their immense potential as NLO crystals.