Formation of MgSO₄-Al₂(SO₄)₃ Solid Solution and Thermal Decomposition into the Spinel Solid Solution

Abstract

Solid solution of MgSO₄ and Al₂(SO₄)₃ was prepared from mixed aqueous solutions by rapid dry using micro-wave heating. The Mg²⁺ ion is soluble into the anhydrous aluminum sulfate crystal lattice over a wide range up to near the equimolar composition of MgSO₄ and Al₂(SO₄)₃. The anhydrous aluminum sulfate solid solution, at first, thermally decompose into the mixture of η-Al₂O₃ and MgSO₄. The crystalline spinel phase appears at the second step of the thermal decomposition. The lattice parameters of the spinel changes linearly with the Mg content ranging from 0.5 to 0.8 at the fraction of MgAl₂O₄/(MgAl₂O₄+Al₂O₃). The produced nonstoichiometric spinel solid solutions must be metastable because the stable region of the composition of the spinel solid solutions is very narrow at the heating temperature employed to the thermal decomposition.