| Summary: | Binary zinc(II) oxide (ZnO) and copper(II) oxide (CuO) are used in a number of applications, including optoelectronic and semiconductor applications. However, no crystal structures have been reported for ternary Cu-Zn-O oxides. In that context, we investigated the structural characteristics and thermodynamics of Cu<i><sub>x</sub></i>Zn<i><sub>y</sub></i>O<i><sub>z</sub></i> ternary oxides to map their experimental feasibility. We combined evolutionary crystal structure prediction and quantum chemical methods to investigate potential Cu<i><sub>x</sub></i>Zn<i><sub>y</sub></i>O<i><sub>z</sub></i> ternary oxides. The USPEX algorithm and density functional theory were used to screen over 4000 crystal structures with different stoichiometries. When comparing compositions with non-magnetic Cu<sup>I</sup> ions, magnetic Cu<sup>II</sup> ions, and mixed Cu<sup>I</sup>-Cu<sup>II</sup> compositions, the magnetic Cu<sub>2</sub>Zn<sub>2</sub>O<sub>4</sub> system is thermodynamically the most favorable. At ambient pressures, the thermodynamically most favorable ternary crystal structure is still 2.8 kJ/mol per atom higher in Gibbs free energy compared to experimentally known binary phases. The results suggest that thermodynamics of the hypothetical Cu<i><sub>x</sub></i>Zn<i><sub>y</sub></i>O<i><sub>z</sub></i> ternary oxides should also be evaluated at high pressures. The predicted ternary materials are indirect band gap semiconductors.
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