What is the cause of a substitute defect in a crystal lattice?

A substitute defect in a crystal lattice occurs when an atom in the lattice is replaced or substituted by an atom of a different element. This typically happens when the substituting atom differs in size from the atom it replaces, which can lead to distortions and changes in the properties of the material. The size difference can create stress and affect the overall stability and behavior of the crystal structure.

In the context of the other options, while having an atom occupy space (as mentioned in the first choice) is related to interstitial defects and missing atoms (as in the second choice) refer to vacancies, neither of these situations accurately describes a substitute defect. The fourth option, dealing with an atom that has rearranged, is irrelevant to the definition of substitution, as it focuses on atom movement rather than replacement. Therefore, the presence of an atom from a different element that differs in size is what accurately defines a substitute defect in a crystal lattice.