“…In addi-tion, the Al3þ and Co2þ ions were also used to substitute Mg2þ ions and to form the solid solution [11,16e18]. It shall be noted that ionic substitution is related to an ionic radius and electronegativity [19,20]. In comparison to Al3þ (r 0.535 Å) and Co2þ ions (r 0.745 Å), the ionic radius of Cu2þ ions (r 0.73 Å) is closer to that of Mg2þ (r 0.72 Å).…”