in water prepn. of metal carbonate hydrate (dissolving of metal in HCl or HNO3, evapn. to dryness, redissolving and addn. of carbonate), careful addn. of the F-compd. to an aq. slurry of the metal carbonate; removal of solvent, careful addn. of a layer of EtOH and cooling of the mixt. to 4°C, crystn. after several days, collection of crystals by filtn., recrystn. from aq. soln.;