Elasticity theory is applied to calculate the total strain present in a tetragonal symmetry epilayer due to heteroepitaxial misfit. By relating pseudobinary alloy composition to the lattice constants of the epilayer, it is shown that the composition of a pseudobinary epilayer can be determined from measurement of the strained epilayer lattice constants. This is accomplished by expressing the misfit strain in terms of the composition and the individual lattice constants of the component of the epilayer and using Vegard's rule. As a result, determination of the composition of two chalcopyrite symmetry epilayers from x-ray diffraction measurements is demonstrated. The approach presented here is general and can be applied to any substrate-epilayer combination in the elastic limit.
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