Non-stoichiometric Compound - Non-stoichiometry Vs. Inhomogeneity

Non-stoichiometry Vs. Inhomogeneity

Non-stoichiometry, which is change in the composition at the atomic scale, should be distinguished from macroscopic sample inhomogeneity and associated measurement artifacts. The chemical composition of materials is often measured using high-energy particles (electrons, ions, X-rays, etc.) as the probes. Those particles have different penetration depths into the studied material and often have limited accuracy (~10 %). When applied to a chemically reactive material, they can falsely evaluate it as non-stoichiometric. Let us consider a metal, such as aluminium. In air, it is covered by nanometer-thick natural oxide Al2O3. Surface sensitive techniques will detect stoichiometric Al2O3. Techniques probing beyond the oxide would detect a non-stoichiometric oxide, and the bulk analysis methods would yield (almost) pure aluminum. Because of such measurement problems, the composition of most thin films is measured as non-stoichiometric.

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