New Insights into the Crystal Structure of Hydrated Ca(BF₄)₂

What is it about?

This research presents the ab initio solution and refinement of the crystal structure of Ca(BF₄)₂·xH₂O using laboratory powder diffraction data. The study was motivated by the need for a definitive hydration state for Ca(BF₄)₂, important for applications in calcium-based rechargeable batteries, and addresses previous ambiguities in the value of x. Powder diffraction patterns were measured, indexed, and analyzed using McMaille, FULLPROF, and ESPOIR software, leading to the assignment of a triclinic cell and a structural model featuring [CaO₄F₄] square antiprisms interconnected with [BF₄] tetrahedra. Rietveld refinement indicated that all four water molecules are coordinated within the structure, justifying the revised formula Ca(H₂O)₄(BF₄)₂. The hydrogen-bonding network was proposed based on shortest O⋯F and O⋯O contacts, with the majority of hydrogen bonds linking adjacent ribbons and contributing to three-dimensional cohesion. The compound is found to be isostructural with Ca(ClO₄)₂·4H₂O, although no strontium analogue with similar hydration could be identified. The results provide a clarified structural model and hydration state, with implications for understanding the compound's stability and potential anisotropic properties.

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Why is it important?

This research provides an ab initio solution and refinement of the crystal structure of Ca(BF₄)₂·xH₂O from laboratory powder diffraction data, addressing longstanding uncertainties about the hydration state and structural arrangement of this compound. The findings are significant for the development of calcium-based rechargeable batteries, as they clarify the composition and hydrogen-bonding framework crucial for understanding the material's properties and behavior, particularly in electrochemical applications. Key Takeaways: 1. The study determines that the crystal structure of the hydrated phase is best formulated as Ca(H₂O)₄(BF₄)₂, with four water molecules directly coordinated in [CaO₄F₄] square antiprisms, thereby resolving previous ambiguities regarding the hydration number (x) in Ca(BF₄)₂·xH₂O. 2. Structural analysis reveals that the compound forms mono-dimensional infinite ribbons through sharing of F corners between [CaO₄F₄] square antiprisms and [BF₄] tetrahedra, with these ribbons interconnected by a complex network of H⋯F and H⋯O hydrogen bonds, completing a three-dimensional cohesion. 3. The research establishes the isostructural relationship between Ca(H₂O)₄(BF₄)₂ and calcium perchlorate tetrahydrate, Ca(ClO₄)₂·4H₂O, and highlights the potential for anisotropic physical properties due to the hydrogen-bonded ribbon architecture, suggesting avenues for further investigation relevant to material performance in battery technologies.