Mixed-metal fluoride hydrates and their thermal-decomposition products: an investigation by X-ray, Mössbauer, and thermal analysis

Abstract

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.