M(B(SeO 3 ) 3 )H 2 O (M = Al, Ga): the first boroselenites with a unique sandwich like double-layer structure.

Exploration of new types of borates is important because of their promising applications in diverse fields. Two new boroselenites, namely, M(B(SeO ₃ ) ₃ )H ₂ O (M = Al, Ga), which represent the first IIIA metal boroselenite, were synthesized by hydrothermal reactions. M(B(SeO ₃ ) ₃ )H ₂ O (M = Al, Ga) possesses a unique sandwich like double-layer structure formed by two 2D [MSe ₂ O ₈ ] ^5- layers interconnected by 1D [BSeO ₅ ] ^3- chains. More interestingly, both compounds display large band gaps (4.86/4.79 eV) and moderate birefringences (Δ n = 0.063/0.064 at 1064 nm) based on density functional theory (DFT) calculations.