RbB7O10(OH)2 and NaB11O16(OH)2: alkali metal hydroxyborates with two-dimensional layered structures and large birefringence

Abstract

Hydroxyborates have attracted much research attention due to their rich structural and outstanding optical properties. Herein, two new alkali metal hydroxyborates, RbB₇O₁₀(OH)₂ and NaB₁₁O₁₆(OH)₂, with two-dimensional (2D) layered structures have been synthesized in a closed system using a high-temperature solution method. The UV-vis-NIR diffuse reflectance spectrum of RbB₇O₁₀(OH)₂ shows that it has a UV cutoff edge below 200 nm. Theoretical calculations indicate that both compounds exhibit large band gap (7.40 and 7.41 eV) and large birefringence (0.074 and 0.105 at 1064 nm) values, respectively. Additionally, the origin of the birefringence in these title compounds has been analyzed using the response electron distribution anisotropy (REDA) method.