Two New Crystals in Li_<i>m</i>Cs<i>_n</i>B_{<i>m</i>+<i>n</i>}O_{2(<i>m</i>+<i>n</i>)} (<i>m</i> + <i>n</i> = 5, 7; <i>m</i> > <i>n</i>) Series: Noncentrosymmetric Li₅Cs₂B₇O₁₄ and Centrosymmetric Li₄CsB₅O₁₀

With the introduction of an alkali metal into the B–O framework, two new alkali metal borate crystals, Li5Cs2B7O14 and Li4CsB5O10, have been obtained for the first time. Both compounds obey a general formula of LimCsnBm+nO2(m+n) (m + n = 5, 7; m > n). The two crystals have different three-dimensional (3D) framework structures composed by LiOn (n = 4, 5), CsO10, BO3, and BO4 units. Li5Cs2B7O14 crystallizes into the polar and noncentrosymmetric space group Ama2, while Li4CsB5O10 belongs to the nonpolar and centrosymmetric space group P21/c. Detailed structure comparison analysis indicates that the different arrangements of the anionic groups in LimCs7–mB7O14 (m = 4, 5) and LimCs5–mB5O10 (m = 3, 4) may arise from the cation size effects, bond-valence requirements, and differences of coordination environment. In addition, in order to get better understandings of electronic structures and linear optical properties, we also carried out first-principle theoretical studies.