Cationic radius ratio and formation of K2NiF4-type compounds

A survey of known compounds which can assume K₂NiF₄-type structure at least over a certain P, T range was made. The cationic radius ratios r_A/r_B of this family of compounds were found to vary over a relatively narrow range of values around 2.00, in contrast to the much wider ranges known for other important A₂BX₄ structure groups, e.g., β-K₂SO₄, thenardite, olivine, etc., providing thereby a constraint on the crystal chemical variabilities in this group of compounds. Usefulness of the simple size parameter r_A(IX)/r_B(VI) in predicting formation of new K₂NiF₄-type compounds is discussed.