Raman and theoretical studies on structural evolution of Li2BeF4 and binary LiF-BeF2 melts.

The structural evolution of Li 2 BeF 4 from crystalline to molten states has been investigated by Raman spectroscopic technique with the assist of density functional theory (DFT). The experimental results of temperature-dependent Raman spectra of Li 2 BeF 4 showed that no solid-state phase transformation was taken place during heating process from 298 to 723 K. In the molten state, the structure of BeF 4 2− anion was affected by Li+ cation and environment. For the binary LiF-BeF 2 melts with varying LiF/BeF 2 ratio, the stretching bands of beryllium‑fluorine bonds in molten Raman spectra were deconvolved identified by Gaussian function. When the concentration of BeF 2 exceeding 33 mol%, the dimer Be 2 F 7 3− was formed and dominated the spectrum. It was characterized to have a linear Be–F–Be geometry with two BeF 4 moieties bridged by a single fluorine atom. A second anion Be 3 F 10 4− processing a triple chain structure formed when the content of BeF 2 increasing to 50 mol%. When the concentration of BeF 2 went beyond 60 mol%, the Raman intensity and band width gradually decreased. While the intensity and band width of Raman spectra gradually decreased with the concentration of BeF 2 exceeding to 60 mol%, which suggests that the band should be assigned to a hexatomic Be 6 F 18 6− ring which was similar to the situation in the pure BeF 2 network structure. • The structure of beryllium fluorides was studied by Raman and DFT calculations. • Different models and methods were established to simulate crystal and melt. • Fluoroberyllate species BeF 4 2−, Be 2 F 7 3−, Be 3 F 10 4−, and Be 6 F 18 6− were observed. [ABSTRACT FROM AUTHOR]