VAPORIZATION BEHAVIOR AND THERMODYNAMIC STABILITY OF HFMO2O8

The standard Gibbs energy of formation for hafnium molybdate (HfMo2O8) was derived from its vapour pressure measured in the temperature range 1023 to 1185 K, employing the transpiration technique. HfM(2)O(8) vaporizes incongruently according to the reaction n(HfMo2O8)-->n(HfO2)+2(MoO3)(n) (n=3,4,5). The standard Gibbs energy of formation of HfMo2O8 calculated from the partial pressure of(MoO3)(3) in the vapour phase above the HfMo2O8+HfO2 mixture could be expressed by the relation Delta(f)G degrees(HfMo2O8) (kT/mol)=-(2498.78+/-4.06)+(0.6039+/-0.0037)T (1023<T/K<1185). The values of Delta(f)G degrees(HfMo2O8) obtained in this work are about 60 kJ/mol more positive than those reported recently from the solid electrolyte galvanic cell measurements.