Phase equilibria and thermodynamics of the system Dy–Mg–Cl at 1073 K

Abstract

The phase relations in the system Dy–Mg–Cl at 1073 K have been established by isothermal equilibration and chemical analysis of quenched samples. Liquid Mg-rich alloy was found to be in equilibrium with molten DyCl₂. Therefore, DyCl₂ can be synthesized by reduction of MgCl₂ with excess of metallic Dy at 1073 K. The Gibbs energy of formation of DyCl₂ at 1073 K was evaluated by two different methods. From voltammetric determination of decomposition voltage, the upper limit for the standard Gibbs energy of formation of DyCl₂ was estimated to be −505(±20) kJ mol⁻¹. A value of −543(±10) kJ mol⁻¹ was deduced from phase relations using Gibbs–Duhem integration. The value for the standard Gibbs energy of DyCl₂ indicates that the Dy²⁺ ion has a potential capability for reducing TiCl₄ to metal titanium. At the same time, Mg is a reductant for Dy³⁺ produced during the reduction of TiCl₄. Thus, it is thermodynamically confirmed that reduction of TiCl₄ by magnesium using a reaction mediator in the salt phase is feasible.