Local structural evolution in the anionic solid solution ZnSex S1-x

The century-old Vegard's law has been remarkably accurate in describing the evolution of the lattice parameters of almost all solid solutions. Contractions or expansions of lattice parameters of such systems depend on the size of the guest atom being smaller or larger than the host atom it replaces to form the solid solution. This has given rise to the concept of "chemical pressure"in analogy to the physical pressure. We have investigated using EXAFS the evolution of the local structure in terms of atom-pair distances extending up to the third-nearest neighbors in the family of compounds, ZnSexS1-x as an example of an anionic solid solution, in contrast to all previous studies focusing on cationic solid solutions. Our results establish several common features between these two types of solid solutions, while strongly suggesting that the concept of a chemical pressure is inaccurate and misleading. Most interestingly, we also find a qualitative difference between the cationic solid solutions, reported earlier, and the anionic solid solution.