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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.
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