Ordering tendencies in octahedral MgO-ZnO alloys

Abstract

Isostructural II-VI alloys whose components are either rocksalt stable ͑e.g., CaO-MgO͒ or zincblende stable ͑e.g., ZnS-ZnSe͒ are known to be thermodynamically unstable at low temperatures, showing a miscibility gap and no bulk ordering. In contrast, we show that heterostructural MgO-ZnO is stable, under certain conditions, in the sixfold-coordinated structure for Zn concentrations below 67%, giving rise to spontaneously ordered alloys. Using first-principles calculations, we explain the origin of this stability, the structures of their lowtemperature ordered phases, short-range-order patterns, and their optical band-gap properties.

Structural phase transitions and fundamental band gaps ofMgxZn1−xOalloys from first principles

“…37 The formation enthalpy is positive throughout, as seen in Fig. 2, in agreement with previous theoretical studies.…”

“…This finding implies a tendency towards phase separation if the integration of different constituents into the medium cannot be maintained. 37 In the following, five special cases are discussed in more detail, namely x = 0.0, 0.33, 0.52, 0.71, and 1.0. With low Mg concentration, the WZ phase is favorable in any case (Fig.…”

“…2, in agreement with previous theoretical studies. 37,41 There are two global minima (i. e. the most stable phases; black, dark regions), for pure ZnO in WZ (x = 0) and pure MgO in RS phase (x = 1). This finding implies a tendency towards phase separation if the integration of different constituents into the medium cannot be maintained.…”

“…46,47 The validity of the present approach, especially the use of the CPA, is discussed by comparing our results with those of previous studies. 37,40,41 In summary, our study addresses important issues in Mg x Zn 1−x O alloys which, with respect to magnetic tunnel junctions, might also be relevant for magnetoelectronics.…”

“…Recently, thermodynamical stability and ordering tendencies of the alloys have been carefully investigated using the clusterexpansion method. 37 Based on the parameterization of total energies for various alloy configurations, the latter allows to study accurately structural properties, including short-range order (SRO) effects. 38,39 It is found that the RS ↔ WZ transition occurs at x ≈ 0.33, which is consistent with experiment (x = 0.33, Ref.…”