THERMODYNAMIC STABILITIES OF LAVO3 AND LAVO4 BY EMF METHODS

The e.m.f. of the galvanic cell: Pt, La2O3, LaF3, O-2 (1 atm) \ CaF2 \ O-2 ( 1 atm), LaF3, LaVO4, V2O5 (s or 1), Pt, was measured as a funct ion of temperature. The melting temperature and the standard enthalpy of fusion of V2O5 was derived to be 957 K and 68.5 kJ mol(-1) in agree ment with the literature. The standard Gibbs energies of formation, De lta G(f,ox)(o), of LaVO3 and LaVO4 from the constituent binary oxides were calculated to be: Delta G(f,ox)(o)(LaVO3) +/-3.3 (kJ mol(-1))= - 17.54-0.02652 T (K) and Delta G(f,ox)(o)(LaVO4) +/-0.78 (kJ mol(-1)) = - 125.93 + 0.02210 T (K). These values are valid over the range 960 t o 1073 K. Using the literature data on the binary oxides, the Delta G( f)(o) of LaVO3 was calculated to be - 1269 kJ mol(-1) at 1273 K (by ex trapolation) and found to exhibit a systematic trend with other LaMO(3 ) compounds (where M = Cr, Mn, Fe, Co, Ni and Cu) when plotted against the atomic number of the transition metal. A phase diagram of La-V-O system at 1073 K could be suggested based on the obtained results.