Thermodynamic evidence for phase transition in MoO_2-δ

Abstract

The standard Gibbs free energy of formation of MoO_2-δ, Δ_fG(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|Mo + MoO_2-δ||(Y₂O₃)ThO₂||(CaO)ZrO₂||O₂(0.1MPa)|Pt The 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_fG°(MoO_2-δ)±570(Jmol^-1)=-579,821+170.003(T/K) in the temperature range(925 to 1533)K, and Δ_fG°(MoO_2-δ)±510(Jmol^-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(M^±O_2-δ) = (-592.28 ± 0.33) kJ· mol^-1. The new measurements refine thermodynamic data for MoO₂.