TABLE OF CONTENTS Beryllium fluoride melts to form a very viscous liquid. Crystalline BeF 2 contains beryllium-fluorine tetrahedra in quartz and cristobalite forms, i.e., the crystalline forms of BeF 2 closely resemble those of SiO 2. Be 2+ ions are almost identical in size with Si 4+ ions, while F - ions are almost identical in size with O 2- ions. It is not surprising to find that vitreous beryllium fluoride closely resembles vitreous silica, with a network made up of tetrahedra connected at all four corners. The similarity between these glasses is so striking that beryllium fluoride has been called a weakened model for silica.
Clear glasses can be formed in a number of binary systems using BeF 2 and either alkali or alkaline earth fluorides. Since these glasses do not display any visual signs of phase separation, they were originally considered to be homogeneous. Structural models were proposed which are directly analogous to those for alkali silicate glasses, with the replacement of non-bridging oxygens by non-bridging fluorines, or NBF. Later studies using transmission electron microscopy, which can detect phase separation with a much finer scale than that obvious to the naked eye, reveal that essentially all of these glasses are phase separated, with BeF 2-rich and modifier fluoride-rich phases. If these glasses are phase separated over most of the glassforming region, then discussions of their structures in terms of models for homogeneous glasses...
