Thermochemical properties of the Fe-analogue of cryolite,Na_3FeF_6

摘要

Relative enthalpies for low-and high-temperature modifications of Na3FeF6 and for the Na_3FeF_6 melt have been measured by drop calorimetry in the temperature range 723-1318 K.Enthalpy of modification transition at 920 K,DELTA_(trans)H(Na_3FeF_6,920 K)=(19±3)kJ mol~(-1) and enthalpy of fusion at the temperature of fusion 1255 K, DELTA f usH(Na_3FeF_6,1255 K)=(89±3)kJ mol~(-1) have been determined from the experimental data.Following heat capacities were obtained for the crystalline phases and for the melt,respectively:Cp(Na_3FeF_6,cr,alpha)=(294±14)J(mol K)~(-1),for 723<=T/K<=920, Cp(Na_3FeF_6,cr,beta)=(300±11)J(mol K)~(-1) for 920<=T/K<=1233 and Cp(Na_3FeF_6,melt)=(275±22) J(mol K)~(-1) for 1258<=T/K<=1318.The obtained enthalpies indicate that melting of Na_3FeF_6 proceeds through a continuous series of temperature dependent equilibrium states,likely associated with the production of a solid solution.