Structural Evolution in BaSn₂F₅X (X = Cl, Br, I): A Family of Alkaline Earth Metal Tin Mixed Halides

As the first family of Sn-based alkaline earth metal mixed halides, three new compounds, BaSn₂F₅X (X = Cl, Br, and I), are synthesized by hydrothermal method. These compounds are crystallized in the centrosymmetric space groups of P2₁/c, P4/nmm, and Pmma for BaSn₂F₅Cl, BaSn₂F₅Br, and BaSn₂F₅I, respectively, and their microscopic frameworks are all composed of the fundamental structural unit [SnF₄]^2- and its derivatives ([SnF₄Cl]^3- and [SnF₅]^3- groups). Interestingly, the structures in BaSn₂F₅X are significantly changed from one-dimensional (1D) to two-dimensional (2D) and then to 1D motifs as X varies from Cl, Br, to I. Structural analysis combined with theoretical calculations reveals that the structural diversities are caused by the difference of ionic radius and electronegativity of X^- anions as well as the orientation of the lone-pair electrons on Sn²⁺ cations. Moreover, the optical, electronic, and thermal properties for these three compounds are determined. This work provides a representative example to show how microscopic ions influence the structures, thus in favor of the design for new mixed halides, a type of important functional materials with many optoelectronic applications.