Zirconium Complex Fluorides

When potassium fluoride is added to an excess of zirconium fluoride solution a crystalline precipitate of the compound K₂ZrF₆ is obtained. This substance is potassium zirconifluoride, to which there correspond isomorphous silicifluorides, titanifluorides, and stannifluorides.

Potassium zirconifluoride is also formed by fusing zircon with potassium hydrogen fluoride. It crystallises from hot water in rhombic prisms, its solubilities at various temperatures being as follow:

Temperature ° C.	2°	15°	19°	100°
Grams of K2ZrF6 in 100 grams of H2O	0.78	1.41	1.70	25.0

By varying the proportions between potassium fluoride and zirconium fluoride the composition of the product may be altered. Thus by pouring a solution of zirconium hydroxide into a concentrated solution of potassium fluoride the salt K₃ZrF₇ or K₂ZrF₆.KF is obtained, which crystallises in regular octahedra; on recrystallisation from hot water, however, K₂ZrF₆ separates. When, on the other hand, excess of zirconium fluoride is employed the salt KZrF₅.H₂O is formed in monoclinic crystals.

Caesium resembles potassium in forming the stable zirconifluoride Cs₂ZrF₆, which can be recrystallised. By varying the proportions between the two components, however, CsZrF₅.H₂O or CsF.ZrF₄.H₂O and 2CsF.3ZrF₄.2H₂O can be obtained; the former, but not the latter, of these two salts can be recrystallised without decomposition.

The less electropositive sodium does not so readily form the salt Na₂ZrF₆. This salt is produced when 2 parts of sodium fluoride are added to 14 of zirconium fluoride, but cannot be recrystallised. The salt 5NaF.2ZrF₄, originally produced by Marignac, is formed under very wide conditions.

Besides these compounds, ammonium, lithium, and thallium form double salts with zirconium fluoride, as well as the bivalent metals Mg, Zn, Cd, Mn, Ni, and Cu.