Zirconium tetrafluoride

Zirconium(IV) fluoride describes members of a family inorganic compounds with the formula (ZrF4(H2O)x. All are colorless, diamagnetic solids. Anhydrous Zirconium(IV) fluoride' is a component of ZBLAN fluoride glass.

Structure
Three crystalline phases of ZrF4 have been reported, α (monoclinic), β (tetragonal, Pearson symbol tP40, space group P42/m, No 84) and γ (unknown structure). β and γ phases are unstable and irreversibly transform into the α phase at 400 °C.

Zirconium(IV) fluoride forms several hydrates. The trihydrate has the structure (μ\sF)2[ZrF3(H20)3]2.

Preparation and reactions
Zirconium fluoride can be produced by several methods. Zirconium dioxide reacts with hydrogen fluoride and hydrofluoric acid to afford the anhydrous and monohydrates:
 * ZrO2 + 4 HF  ->  ZrF4  +  2 H2O

The reaction of Zr metal reacts at high temperatures with HF as well:
 * Zr + 4 HF  ->  ZrF4  +  2 H2

Zirconium dioxide reacts at 200 °C with solid ammonium bifluoride to give the heptafluorozirconate salt, which can be converted to the tetrafluoride at 500 °C:
 * 2ZrO2 + 7 (NH4)HF2 -> 2 (NH4)3ZrF7  +  4 H2O  +  NH3
 * (NH4)3ZrF7 -> ZrF4  +  3 HF  + 3 NH3

Addition of hydrofluoric acid to solutions of zirconium nitrate precipitates solid monohydrate. Hydrates of zirconium tetrafluoride can be dehydrated by heating under a stream of hydrogen fluoride.

Zirconium fluoride can be purified by distillation or sublimation.

Zirconium fluoride forms double salts with other fluorides. The most prominent is potassium hexafluorozironate, formed by fusion of potassium fluoride and zirconium tetrafluoride:
 * ZrF4 + 2 KF -> K2ZrF6

Applications
The major and perhaps only commercial application of zirconium fluoride is as a precursor to ZBLAN glasses.

Mixture of sodium fluoride, zirconium fluoride, and uranium tetrafluoride (53-41-6 mol.%) was used as a coolant in the Aircraft Reactor Experiment. A mixture of lithium fluoride, beryllium fluoride, zirconium fluoride, and uranium-233 tetrafluoride was used in the Molten-Salt Reactor Experiment. (Uranium-233 is used in the thorium fuel cycle reactors.)