Crystal structure, phase transitions, and thermodynamic properties of magnesium metavanadate (MgV₂O₆)

As a promising anode material for magnesium ion rechargeable batteries, magnesium metavanadate (MgV₂O₆) has attracted considerable research interest in recent years. A MgV₂O₆ sample was synthesized via a facile solid-state reaction by multistep-firing stoichiometric mixtures of MgO and V₂O₅ powder under an air atmosphere. The solid-state phase transition from α-MgV₂O₆ to β-MgV₂O₆ occurred at 841 K and the enthalpy change was 4.37 ± 0.04 kJ/mol. The endothermic effect at 1014 K and the enthalpy change was 26.54 ± 0.26 kJ/mol, which is related to the incongruent melting of β-MgV₂O₆. In situ XRD was performed to investigate phase transition of the as-prepared MgV₂O₆ at high temperatures. The cell parameters obtained by Rietveld refinement indicated that it crystallizes in a monoclinic system with the C2/m space group, and the lattice parameters of a = 9.280 Å, b = 3.501 Å, c = 6.731 Å, β = 111.76°. The solid-state phase transition from α-MgV₂O₆ to β-MgV₂O₆ was further studied by thermal kinetics, indicating that this process is controlled first by a fibril-like mechanism and then by a spherulitic-type mechanism with an increasing heating rate. Additionally, the enthalpy change of MgV₂O₆ at high temperatures was measured utilizing the drop calorimetry, heat capacity was calculated and given as: Cp = 208.3 + 0.03583T-4809000T⁻² (298 – 923 K) (J mol⁻¹ K⁻¹), the high-temperature heat capacity can be used to calculate Gibbs free energy of MgV₂O₆ at high temperatures.