  The structural phase transitions and the fundamental band gaps of
Mg(x)Zn(1-x)O alloys are investigated by detailed first-principles calculations
in the entire range of Mg concentrations x, applying a multiple-scattering
theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are
treated within the coherent potential approximation (CPA). The calculations for
various crystal phases have given rise to a phase diagram in good agreement
with experiments and other theoretical approaches. The phase transition from
the wurtzite to the rock-salt structure is predicted at the Mg concentration of
x = 0.33, which is close to the experimental value of 0.33 - 0.40. The size of
the fundamental band gap, typically underestimated by the local density
approximation, is considerably improved by the self-interaction correction. The
increase of the gap upon alloying ZnO with Mg corroborates experimental trends.
Our findings are relevant for applications in optical, electrical, and in
particular in magnetoelectric devices.
