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The standard Gibbs free energy of formation of $MoO_{2- \delta}, \Delta_fG^0(MoO_{2-\delta})$, has been measured over a wide temperature range (925 to 1925) K using an advanced version of bi-electrolyte solid-state electrochemical cell incorporating a buffer electrode: $Pt\mid Mo + MoO_{2-\delta} \parallel$ $(Y_2O_3)ThO_2 \parellel (CaO)ZrO_2$ $\parallel O_2(0.1 MPa)\mid Pt$ The Gibbs free energy of formation of $MoO_{2-\delta}$, which is directly related to the measured cell e.m.f., can be represented by two linear segments: $\Delta_fG^0 (MoO_{2-\delta})$ $\pm 570/(J . mol^{-1}) = - 579, 821 + 170.003(T/K)$, in the temperature range (925 to 1533) K, and $\Delta_fG^0 (MoO_{2-\delta})$ $\pm 510/(J . mol^{-1}) = - 564; 634 + 160.096(T/K)$, in the temperature range (1533 to 1925) K. The change in slope at T = 1533 K is probably related to the phase transition of $MoO_2$ from monoclinic structure with space group $P2_1/c$ to tetragonal structure characteristic of rutile with space group $P4_2/mnm$. The enthalpy and entropy change for the phase transition are: $\DeltaH_{tr} = (15.19 \pm 2.1) kJ . mol^{-1}$; $\DeltaS_{tr} = (9.91 \pm 1.27) J . mol^{-1} . K^{-1}$ The standard enthalpy of formation of $MoO_{2- \delta}$ at T = 298.15 K assessed by the third-law method is: $\Delta_fH^0(MoO_2-{\delta}) = (-592.28 \pm 0.33) kJ . mol^{-1}$. The new measurements refine thermodynamic data for $MoO_2.$. |
