If the substitute atom bigger than the base atom, the lattice should be stretched but my result shows the opposites no matter how many times I redo the experiments. I've already check XPS to confirm whether the substitution happens too, and it display positive result. In the end I conclude that substitution happen but the lattice change contradict the common rule. I've been thinking about several explanation but in my opinion my reasoning is not convincing enough.
hopefully you could give some opinion regarding this matter.
Thank you,
Akbar Imam
It’s Vegard’s rule or conjecture not a law. There are many examples where it is not obeyed especially when there is significant interaction between the components.
In addition to the interaction between the components, mentioned by Alan, the crystal structure of each component, in their pure form, must be also considered. This is because the empirical Vegard's rule was established for the homogeneous solutions (both components have the same structure in their pure form).
Moreover, the charge of the two components may also affect the parameters of the crystal structure of the solid solution. The charge difference increases the concentration of the vacancies in the structure to maintain the charge balance in the system.
It is also important to note the effect of interfaces in the case of multi-phase system.
As already said, one shouldn't necessarily expect Vegard's-like behavior.
More detail on the material system are needed in order to try answering your question (e.g. are the materials even soluble?)
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