Local Structural Investigations of Composition-driven Changes in Inorganic Solid Solutions
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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. The drastic modifications of th e MIT on Ti and Cr doping with such dissimilar sized ions have been equated with positive and negative pressure effects, respectively. However, this is surprising, since Ti3+ being a larger ion and Cr3+ a smaller one compared to V3+ ions, one would expect Ti doping to correspond to a dilation of the lattice and hence correspond to a negative pressure, with Cr doping leading to a contraction of the lattice parameters to represent a positive pressure, exactly opposite of what has been claimed in the literature. We use local structural investigations to establish that the doping of Ti leads to an electronic reconstruction, leading to the formation of Ti4+ with the additional electron being donated to the V states and driving the system towards metallicity. Doping of Cr on the other hand conforms to a more conventional view of doping with local structural adjustments due to the presence of the smaller sized Cr3+ at the V3+ site without any electronic reconstruction.