Extending Unique 1D Double-Chains to 2D Layers with Birefringent Gain by Introducing a Hydroxyl Group

It remains a significant hurdle for discovering birefringent materials in the deep ultraviolet (DUV, λ < 200 nm). It is well-known that the OH anions are recognized for their capability to eliminate the dangling bonds from terminal oxygen atoms, promoting the ultraviolet (UV) cutoff edge blueshift and regulating the crystal structure. Here, two new barium hydroxyborates, Ba3B11O18(OH)3(H2O) (BaBOH) and Na2BaB10O16(OH)2(H2O)2 (NaBaBOH), were designed and synthesized while displaying different dimensions. Remarkably, BaBOH presents novel one-dimensional (1D) [B22O37(OH)6]∞ double-chains formed by a new fundamental building block (FBB)[B11O21(OH)3]. NaBaBOH possesses a 2D [B10O16(OH)2]∞ layer with a less common FBB [B10O19(OH)2]. They enrich the structural diversity of hydroxyborates. Moreover, NaBaBOH exhibits a broad transparent window within the DUV spectral range (<190 nm) and possesses a favorable birefringence of 0.064. Furthermore, detailed summaries and structural comparisons have been implemented for all hydroxyborates containing alkali and alkaline-earth metals. This reveals that the OH group modulation strategy can be appropriately employed for the structural design.

Na2GaS2Cl: a new sodium-rich chalcohalide with two-dimensional [GaS2]∞ layers and wide interlayer space

By introducing halogens to the A/Ga/Q (A = Na, K; Q = S, Se) system, one new chalcohalide namely Na₂GaS₂Cl was successfully obtained. It crystallizes in the orthorhombic space group Cmcm (63). Na₂GaS₂Cl has a layered structure consisting of two dimensional [GaS₂]_∞ layers which are stacked in "face to face" and "back to back" arrays and separated by Na⁺ and Cl^- ions. Interestingly, supertetrahedral building units [Ga₄S₁₀] (T2) which are rarely found in metal chalcogenides and metal chalcohalides are formed in this structure. Moreover, the distances of two adjacent layers are around four times larger than the ionic radius of the Na⁺ ion, which is very likely to provide a perfect environment for the storage and migration of Na⁺ ions. In particular, the volume concentration of the Na⁺ cations in this compound is as high as 1.54 × 10²² cm^-3. The UV-vis-NIR spectroscopy measurement reveals that the optical band gap of this title compound is 3.06 eV. The electronic structural calculations on Na₂GaS₂Cl show that the band gap is mainly determined by the [GaS₄] groups and Na-Cl ionic bonding.