In the Mgx
Cu1 - x
solid solution, progressive substitution of the Jahn-Teller and d9
cation with the spherical and closed-shell Mg2+
cation results in a gradual reduction of the splitting of a
unit-cell parameters, until transformation occurs from the tetragonal I
to the cubic
archetype spinel structure. Tetragonal members of the series transform to cubic at high temperature, and the transition temperature decreases with increasing Mg content. The phase transition is first order in character for Cu-rich samples and evolves towards second-order character at intermediate compositions.