= Solution
Atomic-number contrast can explain why the gold-rich regions are bright and why the phosphorus-rich NiP$_y$ layer should be somewhat darker than the other <nickel>–<phosphorus> layers. However, the calculated alloy yields differ only modestly: the NiP$_y$ and NiP$_z$ yields differ by about $5.5\%$ relative to NiP$_z$. \b[Composition-only backscattering does not explain all of the strong local edge and defect contrast.]
An <Everhart–Thornley detector> normally collects <secondary electrons>, not a pure backscattered-electron signal. Secondary-electron escape and collection depend strongly on local slope and edge geometry. Differential polishing, relief at interfaces, a crack or cavity, surface <oxidation> and local charging can therefore alter brightness without the corresponding change in bulk <atomic number>. <backscattered electrons> can also generate <secondary electrons> elsewhere. Crystallographic channeling and orientation can contribute additional contrast. The observed dark and bright areas should therefore be correlated with topography and composition rather than assigned a phase from intensity alone.
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