The first silver bismuth borate, AgBi₂B₅O₁₁

The first silver bismuth borate, AgBi₂B₅O₁₁ (silver dibismuth pentaborate), has been prepared via glass crystallization in the Ag₂O-Bi₂O₃-B₂O₃ system and characterized by single-crystal X-ray diffraction. Its structure is derived from that of centrosymmetric Bi₃B₅O₁₂ by ordered substitution of one Bi³⁺ ion for Ag⁺, which results in the disappearance of the mirror plane and inversion centre. Second harmonic generation (SHG) measurements confirm the acentric crystal structure. It is formed by [Bi₂B₅O₁₁]_∞ layers stretched along c and comprised of vertex-sharing B₅O₁₀ and BiO₃ groups which incorporate the Ag⁺ cations. The new compound was characterized by thermal analysis, high-temperature powder X-ray diffraction, and vibrational and UV-Vis-NIR (near infrared) spectroscopy. Its thermal expansion is strongly anisotropic due to the presence of rigid B₅O₁₀ groups aligned in a parallel manner. The minimal value is observed along their axis [parallel to c, α_c = 3.1 (1) × 10^-6 K^-1], while maximal values are observed in the ab plane [α_a = 20.4 (2) and α_b = 7.8 (2) × 10^-6 K^-1]. Upon heating, AgBi₂B₅O₁₁ starts to decay above 684 K due to partial reduction of silver; incongruent melting is observed at 861 K. According to density functional theory (DFT) band-structure calculations, the new compound is a semiconductor with an indirect energy gap of 3.57 eV, which agrees with the experimental data (absorption onset at 380 nm).