Mixed-metal fluoride hydrates and their thermal-decomposition products: an investigation by X-ray, Mössbauer, and thermal analysis
A series of mixed-metal fluoride heptahydrates has been prepared and shown to have the general formula ABF5·7H2O (A = Mn2+, Co2+, or Zn2+; B = Fe2+ or Cr2+). Of these, MnFeF5·7H2O and FeCrF5·7H2O are reported for the first time. Thermal dehydration of AFeF5·7H2O and ZnCrF5·7H2O leads to an intermediate dihydrate ABF5·2H2O. The heptahydrates are isomorphous with one another and the patterns have been indexed with a triclinic unit cell, all the parameters of which have been calculated for the AB combinations NiFe, CoFe, NiCr, and CoCr. The X-ray pattern of the previously reported anhydrous phase A appears during dehydration of ZnCrF5·7 H2O and members of the AFeF5·7H2O series in flowina hydrogen fluoride. Dehydration of all the members of the iron(III) and chromium(III) series yields X-ray patterns of AF2 in flowing nitrogen and (except ZnCrF5·7H2O) both AF2 and BF3 in flowing nitrogen and (except ZnCrF5·7H2O) both AF2 and BF3, in flowing hydrogen fluoride. The heptahydrate X-ray pattern persists even after complete dehydration of the CoFe, CoCr, ZnFe, and ZnCr mixed-metal fluoride heptahydrates in flowing hydrogen fluoride, CoFe only in flowing nitrogen. Mössbauer spectra indicate that the Fe3+ ions occupy two different octahedral sites.