The first silver bismuth borate, AgBi2B5O11

Abstract

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).