Disclosed herein is a method for producing potassium fluoroniobate crystals
by which highly pure, large-sized potassium fluoroniobate crystals can be
obtained in high yield and which is advantageous from the viewpoints of
material cost and material-dissolving operation; and potassium
fluoroniobate crystals. This production method comprises the first and
second steps (a) and (b) of (a) adding a potassium-containing electrolyte
to a starting material comprising niobium to precipitate potassium
oxyfluoroniobate and/or fluoroniobate as coarse crystals, and separating
the coarse crystals by filtration, and (b) dissolving the coarse crystals
in a recrystallization solvent that is an aqueous solution comprising 12
to 35% by weight of hydrofluoric acid and that has been heated to a
temperature of 50.degree. C. or more, and cooling the solution to
40.degree. C. or lower at a cooling rate of less than 20.degree. C./h to
precipitate potassium fluoroniobate as crystals.