Gibbs' free energy of formation of magnesium stannate

Abstract

The thermodynamic properties of the spinel Mg₂SnO₄ have been determined by emf measurements on the solid oxide galvanic cell, Pt,w,Mgo+Mg₂SnO₄+Sn⁄Y₂O₃−ThO₂Sn+SnO₂,w,Pt in the temperature range 600 to 1000°C. The Gibbs' free energy of formation of Mg2SnO4 from the component oxides can be expressed as 2Mgo (r.s) +SnO₂ (rut)→Mg₂SnO₄(sp) ΔG°=1420−2.96T (±100)cal mole−¹ These values are in good agreement with the information obtained by Jackson et al. [Earth Planet. Sci. Lett.24, 203 (1974)] on the high pressure decomposition of magnesium stannate into component oxides at different temperatures. The thermodynamic data suggest that the spinel phase is entropy stabilized, and would be unstable below 207 (±25)°C at atmospheric pressure. Based on the information obtained in this study and trends in the stability of aluminate and chromite spinels, it can be deduced that the stannates of nickel and copper(II) are unstable.