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.