The new silver borate Ag3B5O9

An antimony(iii) borate with large birefringence exhibiting unwonted [B5O11] fundamental building blocks and dimeric [Sb2O6] clusters

By combining π-conjugated [B₃O₆] groups and Sb³⁺ cations with SCALP, we successfully obtained the second antimony(iii) borate SbB₅O₉ with a large birefringence (0.151 @ 1064 nm), which exhibits unwonted [B₅O₁₁] FBBs and dimeric [Sb₂O₆] clusters.

A re-interpretation of the structure of the silver borate, Ag16B4O10, in the light of the extended Zintl-Klemm concept

The borate Ag₁₆B₄O₁₀ was synthesized at high temperature and at elevated oxygen pressures [Kovalevskiy et al. (2020). Chem. Sci. 11, 962-969]. Its structure consists of [B₄O₁₀]^8- polyanions (isostructural to P₄O₁₀) embedded in an Ag matrix. According to the standard valences Ag⁺, B³⁺ and O^2-, the formula has an excess of eight e^- which the above authors proposed were located, pairwise, in four Ag₄ tetrahedra within the silver substructure. That conclusion was based on the semiconducting and diamagnetic properties, as well as the very small `attractors' of the Electron Localization Function (ELF) found at the centre of these Ag₄ tetrahedra. However, a large overlap of the projected density of states (DOS) of silver and oxygen indicated possible dispersion interactions between both atomic species. In this article, an alternative description is proposed based on the extended Zintl-Klemm concept. The anion [B₄O₁₀]^8- can be formulated as Ψ-[N₄O₁₀] P₄O₁₀, if it is assumed that the eight e^- are transferred to the four B atoms, so converting them into Ψ-N, this then makes sense of its similarity with P₄O₁₀, [N₄(CH₂)₆], adamantane and tetraisopropyladamantane. When the Ag atoms connect to the O atoms, they arrange as the H atoms do in hexamethylenetetramine (HMTA). If the two lone pairs of each of the bridging O atoms in Ψ-[N₄O₁₀] are equated to the C-H bonds in HMTA, then, this same equivalence exists between the C-H bonds and the O-Ag bonds in the compound Ag₁₆B₄O₁₀. The 24 Ag atoms surrounding each [B₄O₁₀]^8- group prolong the sphalerite structure of the borate anion by means of Ag-O bonds which also fit the sphalerite structure formed of AgO. The eight excess electrons might then be distributed between the Ag and the O atoms, so making sense of the mixing of the Ag and O states. The Ag atoms bonded to the O atoms of the [B₄O₁₀]^8- groups form a coat that interconnects the borate anions through Ag-O bonds. To establish the validity of this new proposal, the study needs to be extended to the compound Ag₃B₅O₉.