Alkaline Beryllium Borate NaBeB₃O₆ and ABe₂B₃O₇ (A = K, Rb) as UV Nonlinear Optical Crystals

A new series of alkaline beryllium borates NaBeB3O6, α-KBe2B3O7, β-KBe2B3O7, γ-KBe2B3O7, and RbBe2B3O7 were synthesized by spontaneous crystallization with molten fluxes based on A2O−B2O3 (A = Na, K, Rb) solvent. For KBe2B3O7, three polymorphous phases were found and are referred to as α-, β-, and γ-KBe2B3O7 relative to their crystallization temperature from high to low. All of the materials are noncentrosymmetric except α-KBe2B3O7. NaBeB3O6 and α-KBe2B3O7 consist of a new anionic group [Be2B3O11]9−, which is similar to a naphthalene molecule that is rarely found in inorganic matter. β-KBe2B3O7, γ-KBe2B3O7, and RbBe2B3O7 consist of 2D alveolate beryllium borate layers [Be2BO5]∞, which are connected by strong covalent bonds. The UV−vis diffuse reflectance spectroscopy on powder samples indicated that the short-wavelength absorption edges of noncentrosymmetric materials are all below 200 nm. Second-harmonic generation (SHG) on powder samples was measured using the Kurtz and Perry technique, which indicated that NaBeB3O6, β-KBe2B3O7, γ-KBe2B3O7, and RbBe2B3O7 are all phase matchable materials, and their measured SHG coefficients were approximately 1.60, 0.75, 0.68, and 0.79 times as large as that of d36 (KDP), respectively.