Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution

Abstract

Extended x-ray absorption fine-structure studies have been performed at the Zn K and Cd K edges for a series of solid solutions of wurtzite Zn 1−x Cd x S samples with x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0, where the lattice parameter as a function of x evolves according to the well-known Vegard's law. In conjunction with extensive, large-scale first-principles electronic structure calculations with full geometry optimizations, these results establish that the percentage variation in the nearest-neighbor bond distances are lower by nearly an order of magnitude compared to what would be expected on the basis of lattice parameter variation, seriously undermining the chemical pressure concept. With experimental results that allow us to probe up to the third coordination shell distances, we provide a direct description of how the local structure, apparently inconsistent with the global structure, evolves very rapidly with interatomic distances to become consistent with it. We show that the basic features of this structural evolution with the composition can be visualized with nearly invariant Zn-S 4 and Cd-S 4 tetrahedral units retaining their structural integrity, while the tilts between these tetrahedral building blocks change with composition to conform to the changing lattice parameters according to the Vegard's law within a relatively short length scale. These results underline the limits of applicability of the chemical pressure concept that has been a favored tool of experimentalists to control physical properties of a large variety of condensed matter systems.

Local structural evolution in the anionic solid solution ZnSexS1−x

“…Similar thinking has often led the community to equate changing lattice parameters under the application of a physical pressure to changing lattice parameters via alloying, labeling the latter process as chemical pressure. Intriguingly, however, over the years several experiments [8,[22][23][24] on the extended x-ray absorption fine structure (EXAFS) have established a relative invariance of individual bond lengths in alloy systems. This has cast severe doubts on the extensively used concept of chemical pressure [23].…”

“…Intriguingly, however, over the years several experiments [8,[22][23][24] on the extended x-ray absorption fine structure (EXAFS) have established a relative invariance of individual bond lengths in alloy systems. This has cast severe doubts on the extensively used concept of chemical pressure [23].…”

“…In Ref. [23], the Zn x Cd 1−x S alloy with 0 x 1 was investigated to understand the relation between Vegard's law and bond lengths. While the nearest neighbors (1st coordination shell) were found to be relatively invariant, the interatomic distances of the Cd-Cd and Zn-Zn pairs (2nd coordination shell) were shown to change much more rapidly, but still falling short of explaining the observed changes in the lattice parameters with respect to the alloy composition.…”

“…Interestingly, EXAFS studies on such solid solutions in inorganic compounds have been exclusively focused on cationic substitutions, for example Zn x Cd 1−x S [23], Ga x In 1−x As [22], Mn x Cd 1−x Te [8], or YIn x Mn 1−x O 3 [24]. While anionic substitution is also a well-established route to tune material properties, there is no information available in the literature on the microscopic evolution of various atom-pair distances in an anionic-substituted inorganic system despite the known validity of Vegard's law for such systems.…”

Large scale synthesis of copper nickel alloy nanoparticles with reduced compressibility using arc thermal plasma process

“…However, EXAFS model results indicate that local Cu–Ni bonds are only slightly different than the Cu–Cu or Ni–Ni bonds. Such a deviation of the local bond distances from what is expected from Vegard’s law has been observed in several systems 38 – 40 . This is also reflected in the extra broadening of the XRPD pattern, modelled by the microstrain term added to the profile shape.…”