Ab-initio description of heterostructural alloys: Thermodynamic and structural properties of Mg_x Zn_{1-x} O and Cd_x Zn_{1-x} O

摘要

Pseudobinary heterostructural alloys of ZnO with MgO or CdO are studied bycomposing the system locally of clusters with varying ratio of cations. Weinvestigate fourfold (wurtzite structure) and sixfold (rocksalt structure)coordination of the atoms. By means of density functional theory we study atotal number of 256 16-atom clusters divided into 22 classes for the wurtzitestructure and 16 classes for the rocksalt structure for each of the alloysystems. The fraction with which each cluster contributes to the alloy isdetermined for a given temperature T and composition x within (i) thegeneralized quasi-chemical approximation, (ii) the model of a strict-regularsolution, and (iii) the model of microscopic decomposition. From the clusterfractions we derive conclusions about the miscibility and the criticalcompositions at which the average crystal structure changes. Thermodynamicproperties such as the mixing free energy and the mixing entropy areinvestigated for the three different statistics. We discuss the consequences ofthe two different local lattice structures for characteristic atomic distances,cohesive energies, and the alloys' elasticities. The differences in theproperties of Mg_x Zn_{1-x} O and Cd_x Zn_{1-x} O are explained and discussed.