Phase transition behavior of heterostructural alloys: Effects of size mismatch and site preference

Unlike isostructural alloys, phase transitions of heterostructural alloys between two different lattice types involve more complexity and are less explored. In this work by analyzing the local environment around each atom, we study the distinct features of phase transitions in three prototypical alloy systems (Cd,Pb)Te, (Cd,Ge)Te and (Zn,Mg)O with different size and chemical mismatches. We show that for the (Zn,Mg)O alloy with small size mismatch and low site preference energy between the Zn and Mg compounds, the phase transition occurs abruptly from the zinc-blende to rock-salt structures, showing the feature of a first-order transition. On the contrary, many distorted local configurations unlike either tetrahedron or octahedron ones in the zinc-blende or rock-salt structures, respectively, exist near the phase transition in (Cd,Ge)Te and (Cd,Pb)Te alloys, which have low formation energies. This is attributed to the large size mismatch between CdTe and PbTe, and stronger preference of Cd occupying the tetrahedral sites, whereas Pb and Ge occupying the octahedral sites in (Cd,Pb)Te and (Cd,Ge)Te alloys. Consequently, the phase transitions in (Cd,Pb)Te and (Cd,Ge)Te alloys happen gradually in a wide composition range, showing the feature of a second-order transition. Our study, therefore, shows that phase transition behaviors of heterostructural alloys can be rather complex, either abrupt or gradual, largely depending on the size mismatch and site preference of the constituents. This provides deep understanding on the phase transition behavior of these types of heterostructural alloys.