KBi(IO)(OH) and NaBi(IO): from the centrosymmetric chain to a noncentrosymmetric double layer.

Two new alkali-metal bismuth iodates KBi(IO)(OH) (1) and NaBi(IO) (2) have been synthesized by a hydrothermal method via tuning of alkali-metal ions. Compound 1 crystallizes in the space group P1[combining macron] and features a [Bi(IO)(OH)] chain with eight-membered Bi-O-I-O-Bi-O-I-O rings. The [Bi(IO)(OH)] chain consists of paddlewheel-like dimeric Bi(OH)(IO) units linked by IO pairs. Compound 2 in the polar space group Cc features a [Bi(IO)] double layer with BiI eight-membered rings (8-MRs). NaBi(IO) exhibits a strong second harmonic generation (SHG) response of ∼5.0 × KDP (KHPO) and a wide transmittance window (0.4-12 μm), and structure analyses and theoretical calculations reveal that polarization results mainly from the proper alignment of the IO groups. The calculated electronic structure based on the DFT method indicates that the BiO and IO groups dominate their optical properties.