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

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Salt-concentration effects on the structural dynamics of aqueous beryllium fluoride (BeF 2 ) must be elucidated to advance the separation and dehydration processes. This study systematically investigates BeF 2 -solution concentration-dependent structures and dynamics using nuclear magnetic resonance and ultrafast spectroscopy. Be 2+ primarily exists as the tetrahedral complex [BeF 2 (H 2 O) 2 ] in a BeF 2 solution, with small [BeF(H 2 O) 3 ] + and [BeF 3 (H 2 O)] − complex contents. At BeF 2 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 Be 2+ -coordinated peak from the SCN – substitution in [BeF x (H 2 O) 4 – x ] complexes. The Be 2+ 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 BeF 2 -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.

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