First-principles study of electronic structures and nonlinear optical properties of AMoO3(IO3) (A = Li, Rb and Cs) crystals.

First-principles studies of the geometric structures, electronic structures and second-order nonlinear optical properties of polar alkali metal-Mo(VI)-iodate compounds (AMoO(3)(IO(3)) (A = Li, Rb and Cs)) have been performed within density functional theory and the independent particle approximation. Our results indicate that, for these compounds, due to the similarity in their anionic groups, the electronic structures and the prominent features of the frequency dependent second-order nonlinear optical susceptibilities are similar, especially for the isostructural Rb and Cs compounds. Also, the calculated SHG coefficients of these compounds are large, which confirms the high response in experimental SHG measurements. By comparing the absolute magnitude of the SHG coefficients, the order LiMoO(3)(IO(3)) > RbMoO(3)(IO(3)) > CsMoO(3)(IO(3)) is clearly established in the low photon energy range. Further analyses based on the spectral and spatial decomposition of the SHG coefficients reveal that the main sources of the SHG properties of these compounds are from the distorted MoO(6) and IO(3)(-) groups, and can be mainly attributed to the electronic transition from the nonbonding O 2p states (i.e. the lone-pair electrons of O atoms) to the Mo 4d and some I 5p states. It is noticeable that in these compounds, due to the difference in ion size and coordinate environment, the contributions of alkali metals to the SHG processes are very different: for large cations, such as Rb and Cs, they can be neglected, while for the very small Li, they should be included.