Thermodynamic evidence for phase transition in MoO_2-δ

The standard Gibbs free energy of formation of MoO_2-δ, Δ_fG^{{ring operator}}(MoO_2-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{divides}Mo + MoO_2-δ∥(Y₂O₃)ThO₂∥(CaO)ZrO₂∥O₂(0.1 MPa){divides}PtThe Gibbs free energy of formation of MoO_2-δ, which is directly related to the measured cell e.m.f., can be represented by two linear segments:Δ_f G^{{ring operator}} (MoO_{2 - δ}) ± 570 / (J · mol^{- 1}) = - 579, 821 + 170.003 (T / K),in the temperature range (925 to 1533) K, andΔ_f G^{{ring operator}} (MoO_{2 - 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: ΔH_tr = (15.19 ± 2.1) kJ · mol^-1; ΔS_tr = (9.91 ± 1.27) J · mol^-1 · K^-1. The standard enthalpy of formation of MoO_2-δ at T = 298.15 K assessed by the third-law method is: Δ_fH^{{ring operator}}(MoO_2-δ) = (-592.28 ± 0.33) kJ · mol^-1. The new measurements refine thermodynamic data for MoO₂.