Thermodynamic evidence for phase transition in MoO2−δ

The standard Gibbs free energy of formation of MoO2−δ, ΔfG∘(MoO2−δ), 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∣Mo + MoO2−δ∥(Y2O3)ThO2∥(CaO)ZrO2∥O2(0.1 MPa)∣PtThe Gibbs free energy of formation of MoO2−δ, which is directly related to the measured cell e.m.f., can be represented by two linear segments:ΔfG∘(MoO2-δ)±570/(J·mol-1)=-579,821+170.003(T/K),ΔfG∘(MoO2-δ)±570/(J·mol-1)=-579,821+170.003(T/K),in the temperature range (925 to 1533) K, andΔfG∘(MoO2-δ)±510/(J·mol-1)=-564,634+160.096(T/K),ΔfG∘(MoO2-δ)±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 MoO2 from monoclinic structure with space group P21/c to tetragonal structure characteristic of rutile with space group P42/mnm. The enthalpy and entropy change for the phase transition are: ΔHtr = (15.19 ± 2.1) kJ · mol−1; ΔStr = (9.91 ± 1.27) J · mol−1 · K−1. The standard enthalpy of formation of MoO2−δ at T = 298.15 K assessed by the third-law method is: ΔfH∘(MoO2−δ) = (−592.28 ± 0.33) kJ · mol−1. The new measurements refine thermodynamic data for MoO2.