Crystal structure and phonon properties of noncentrosymmetric LiNaB4O7

A new borate, LiNaB4O7, has been synthesized and characterized by single-crystal X-ray structure determination. The material crystallizes in the orthorhombic system, noncentrosymmetric space group Fdd2, with unit cell dimensions a=13.325(2), b=14.099(2), c=10.243(2) Å, Z=16, and V=1924.3(7) Å3. Like Li2B4O7, the structure is built of two symmetrically independent, interpenetrating image polyanionic frameworks built from condensation of the B4O9 fundamental building block, which is comprised of two distorted BO4 tetrahedra and two BO3 triangles. The interpenetrating frameworks produce distinct tunnels that are selectively occupied by the Li and Na atoms. Large single crystals exhibiting an optical absorption edge with λ<180 nm have been grown via the top-seeded-solution-growth method. The SHG signal (0.15× potassium dihydrogen phosphate (KDP)) is consistent with the calculated components of the SHG tensor and the approximate centrosymmetric disposition of the independent and interpenetrating image frameworks. A complete analysis of polarized IR and Raman spectra confirms a close relationship between the title compound and Li2B4O7.