Preparation
Hafnium tetrafluoride may be prepared by passing anhydrous hydrogen fluoride over hafnium tetrachloride at 300°C:
HfCl4 + 4HF HfF4 + HCl
Another method of preparation involves thermal decomposition of ammonium fluorohafnate, (NH4)2 HfF6 [16925-24-9] in the absence of air:
(NH4)2 HfF6 HfF4 + 2NH3 + 2HF
Also, HfF4 can be prepared by treating metallic hafnium with 40% aqueous HF. The monohydrate formed may be heated at 350°C for several days under a flow of fluorine and nitrogen to yield anhydrous HfF4.
Hf + 4HF (aq) → HfF4•H2O + 2H2
Production Methods
Hafnium tetrafluoride is prepared by careful thermal
decomposition of ammonium fluorohafniate in an oxygen-
free atmosphere or by passing anhydrous hydrogen
fluoride over hafnium tetrachloride at 300℃. Direct synthesis
from the elements is incomplete because the product
merely forms a film on the metal. Ammonium fluorohafniate
can be prepared by crystallization from an aqueous
hydrofluoric acid solution by adding ammonium
fluoride.
