Prof. PAN Shilie and his Ph.D. students from Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences (XTIPC),  recently synthesized a new noncentrosymmetric polyborate, Li₅Rb₂B₇O₁₄, by solid–state reactions of stoichiometric amounts (Li:Rb:B=5:2:7) of Li₂CO₃, Rb₂CO₃ and H₃BO₃. And the single crystal of Li₅Rb₂B₇O₁₄ has been synthesized through spontaneous crystallization. A mixture of Li₂CO₃ (98.0%), Rb₂CO₃ (99.5 %), and H₃BO₃ (99.5%) was placed into a platinum crucible in the molar ratio corresponding to Li:Rb:B=6:5.5:12. Then the crucible was put into the furnace and heated to 750 °C, kept at this temperature for 24 h, then cooled at 5 °C/min to 570 °C and finally quenched to room temperature at a rate of 10 °C/min. Colorless block crystals were separated from the crucible for structural characterization.The crystal structure of Li₅Rb₂B₇O₁₄ was investigated by single–crystal X–ray diffraction on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 ？) at 296(2) K and integrated with the SAINT program. The infinite three–dimensional structure of Li₅Rb₂B₇O₁₄ constructs from bridged LiOn (n = 4, 5, 6) and RbO₁₀ polyhedra with two kinds of one–dimensional B–O chain fundamental building blocks through their vertex oxygen atoms.

Thermal analyses were carried out on NETZSCH STA 449 F3 Jupiter instrument at a temperature range of 30–850 °C with a heating rate of 10 oC·min–1 in an atmosphere of flowing N₂. The DSC curve of Li₅Rb₂B₇O₁₄ shows one endothermic peak at 637 °C on the heating curve, which tentatively suggests that Li₅Rb₂B₇O₁₄ melts congruently at 637 °C. UV–VIS–NIR diffuse–reflectance data were collected with a SolidSpec–3700DUV spectrophotometer with fluororesin as the standard, in the wavelength range from 190 to 2600 nm. The powder sample was obtained by spontaneous crystallization from a stoichiometric ratio melt. It exhibits a short–wavelength absorption onset at 190 nm. The feature of a short UV cut off edge is favorable in practical applications. The electronic property calculations were performed using the first principles plane-wave pseudopotential method implemented in the CASTEP package. The calculated band structures and the density of states of Li₅Rb₂B₇O₁₄ suggest that its indirect energy gap is 4.247 eV. Further investigation for the growth of large crystals and related physical property studies is under way.

The work has been supported by the National Natural Science Foundation of China，the “One Hundred Talents” program of Chinese Academy of Sciences. The article was published in CrystEngComm.

 View of the structure of Li₅Rb₂B₇O₁₄ and cations coordination environments down c axis(orange, Li; green, Rb; blue, B; red, O; purple, BO₄ group; light blue, BO₃ group). (Image by XTIPC)