Local Structural Investigations of Composition driven Changes in Inorganic Solid Solutions

Abstract

A smaller sized dopant replacing a larger sized element in a compound creates chemical pressure, resulting in a contraction of the lattice parameter and likewise a negative pressure is created by substitution of larger ion. Since the chemical pressure concept implicitly presupposes a contraction of the bond-lengths, as under the application of a physical pressure, and therefore, leading to the changes in the lattice parameters, we critically evaluate these concepts of Vegard s Law and chemical pressure by investigating local bond-lengths at the atomic scale to see how these evolve with the composition in a solid solution. We have investigated three related series of solid solutions, namely ZnSexS1-x, CdSexS1-x (anion substitution) and ZnxCd1-xSe (cation substitution). We find that the nearest neighbour bond-lengths, namely Zn-Se and Zn-S in ZnSexS1-x, Cd-Se and Cd-S in CdSexS1-x, and Zn-Se and Cd-Se in ZnxCd1-xSe, are insensitive to the changing composition and remains within 1% of the original bond-length of the parent compound; this is only about one-fourth of what would be expected on the basis of the changes in the lattice parameters across the given solid-solution. This apparent disagreement with the Vegard s Law, requiring a smooth evolution of the lattice parameters with the composition, is resolved by our observation that the third nearest neighbour bond-distances in all cases vary in conformity with the Vegard s Law. Our results for the second nearest neighbour bond-lengths provide us with the answer to this question, indicating that the bond angles (MX4/ XM4 angle, where, M/X = cation/anion) change with the composition while the bond-lengths remain relatively invariant, leading to the observed changes. We extend this study to investigate the remarkable influence doping of Ti and Cr at the V site has on the well-known metal-insulator transition (MIT) in V2O3...