Concentration-Dependent Structures and Ultrafast Dynamics within Aqueous BeF ₂ : From Ion Complexes to Extended Networks

Salt-concentration effects on the structural dynamics of aqueous beryllium fluoride (BeF₂) must be elucidated to advance the separation and dehydration processes. This study systematically investigates BeF₂-solution concentration-dependent structures and dynamics using nuclear magnetic resonance and ultrafast spectroscopy. Be²⁺ primarily exists as the tetrahedral complex [BeF₂(H₂O)₂] in a BeF₂ solution, with small [BeF(H₂O)₃]⁺ and [BeF₃(H₂O)]^- complex contents. At BeF₂ concentrations exceeding 8.3 mol %, an extended Be-F network becomes prominent. Fourier transform infrared spectroscopy with a thiocyanate (SCN^-) vibrational probe reveals two SCN^- peaks: a bulk-water-associated peak and a Be²⁺-coordinated peak from the SCN^- substitution in [BeF_<i>x</i>(H₂O)₄-_<i>x</i>] complexes. The Be²⁺ peak full width at half-maximum exhibits nonlinear broadening at concentrations exceeding 8.3 mol %, consistent with network formation. Polarization-selective pump-probe measurements demonstrate that increased salt concentration accelerates vibrational energy relaxation and suppresses rotational diffusion. The rotational diffusion time is related to the macroscopic viscosity via the Stokes-Einstein-Debye model. The Be-F complexes cause non-Newtonian behavior of the BeF₂-solution viscosity. At concentrations exceeding 8.3 mol %, the formed network structures exponentially increase the viscosity, negating the classical Jones-Dole model. These macroscopic viscosity changes can be explained microstructurally. A coherent molecular mechanism linking the microscopic coordination structure to macroscopic transport properties is established.