The thermodynamic properties of substoichiometric yttrium sesquioxide

Abstract

The thermodynamic properties of the yttrium + oxygen system have been measured and shown to be consistent in the range of composition between the stoichiometric sesquioxide and the substoichiometric sesquioxide at its lower phase boundary. The enthalpy of formation and dissociation energy of YO(g) at T → 0 are − (8.7 ± 1.5) and (168 ± 1.5) kcal th mol −1 , respectively. The partial molar enthalpy of vaporization of YO(g) from Y 2 O 3 (s) at 2400 k is (159.2 ± 1.5) kcal th mol −1 calculated via the third-law procedure and (158.2 ± 0.8) kcal th mol −1 from a second-law measurement. The partial pressures of Y(g) and YO(g) at the lower phase boundary of the sesquioxide in the temperature range 1833 to 2100 K are given by the equatons: log 10 p (Y, g, l.p.b.) = − (17650 ± 260) K/ T + (4.081 ± 0.134) and log 10 p (YO, g, l.p.b.) = − (26460) ± 280)K/ T + (6.274 ± 0.144). From these results values for the standard Gibbs free energy of formation of the substoichiometric sesquioxide were calculated as a function of temperature and the corresponding composition of the lower phase boundary. At 2000 K the composition is Y 2 O 2.89 and the standard Gibbs free energy is −310 kcal th mol −1 which is 9 kcal th mol −1 more positive than that of the stoichiometric oxide. The melting temperature of the nearly stoichiometric oxide, Y 2 O 2.99 0 , is (2710 ± 10) K.