Fluoride Model Systems: III, The System NaF–BeF2 and the Polymorphism of Na2BeF4 and BeF2

A revised equilibrium diagram is presented for the system NaF–BeF 2 , a weakened model of the system CaO–SiO 2 . The compound Na 2 BeF 4 has four structural modifications corresponding to the γ, β, α', and α forms of Ca 2 SiO 4 . Inversion temperatures were found to be as follows: α→α’= 320°C., γ→α’= 225°C., α’→β (metastable) = 115°C. The quartz form of BeF 2 was found to have a high‐low inversion at 220°C. The high‐quartz form exists in equilibrium with liquid in binary fluoride systems up to the melting temperature of BeF 2 , as closely as can be determined. The cristobalite form appears to have only a metastable existence, and a tridymite form has not been found. The compound NaBeF 3 has a structure similar to β‐CaSiO 3 below 343°C., with a different structure (not that of α‐CaSiO 3 ) above this temperature.