Two Mixed-Alkali-Metal Selenates as Short-Wave Ultraviolet Nonlinear-Optical Materials.

Two novel mixed-alkali-metal selenate nonlinear-optical (NLO) crystals, Na 3 Li(H 2 O) 3 (SeO 4 ) 2 ·3H 2 O ( I ) and CsLi 3 (H 2 O)(SeO 4 ) 2 ( II ), have been successfully synthesized by an aqueous solution evaporation method. Both compounds feature the unique layers constructed of the same functional moieties including SeO 4 and LiO 4 tetrahedra: [Li(H 2 O) 3 (SeO 4 ) 2 ·3H 2 O] ∞ 3- layers in I and [Li 3 (H 2 O)(SeO 4 ) 2 ] ∞ - layers in II . The titled compounds display wide optical band gaps of 5.62 and 5.66 eV, respectively, according to the UV-vis spectra. Interestingly, they exhibit significantly different second-order nonlinear coefficients (0.34 × KDP and 0.70 × KDP, respectively). Detailed dipole moment calculations manifest that the large disparity can be attributed to the difference in the dipole moment of the crystallographically independent SeO 4 and LiO 4 groups. This work confirms that alkali-metal selenate system is an excellent candidate for short-wave ultraviolet NLO materials.