Crystal chemistry and crystallography of the Aurivillius phase Bi5AgNb4O18

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn 2Ti2NbO10 Dion-Jacobson Phases

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n = 2 systems and from proper mechanisms in n = 3 CsBi2Ti2NbO10. We report here a combined experimental and computational study on analogous n = 3 CsLn 2Ti2NbO10 (Ln = La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.

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).