A new alkaline beryllium borate KBe4B3O9 with ribbon alveolate [Be2BO5]∞ layers and the structural evolution of ABe4B3O9(A = K, Rb and Cs)

Abstract

A new alkaline beryllium borate, KBe₄B₃O₉, was synthesized by spontaneous crystallization with molten fluxes based on a K₂O–B₂O₃ solvent for the first time. From single crystal X-ray diffraction measurements, KBe₄B₃O₉ has been found to crystallize in the monoclinic space group P2/c. KBe₄B₃O₉ consists of 2D ribbon alveolate beryllium borate [Be₂BO₅]_∞ layers which are connected directly by strong covalent bonds. The structural evolution of ABe₄B₃O₉ (A = K, Rb and Cs) is discussed. The UV–Vis diffuse reflectance spectroscopy on powder samples indicates that the short-wavelength absorption edges of the aforementioned materials are all below 200 nm. A new method is reported for the synthesis of RbBe₄B₃O₉ and CsBe₄B₃O₉. The thermal behavior of KBe₄B₃O₉ and RbBe₄B₃O₉ demonstrates that they are all incongruently melting compounds.