Crystal structure, phase transitions, and thermodynamic properties of magnesium metavanadate (MgV2O6)
Guishang Pei, Cheng Pan, Dapeng Zhong, Junyi Xiang, Xuewei Lv
Abstract
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.