Final answer:
Solid solution can take place when substituting ions have a similar charge and size, as this allows for the maintenance of the crystal lattice structure by ensuring that each ion is properly surrounded and in contact with ions of the opposite charge without causing instability.
Step-by-step explanation:
When solid solutions form, it is important for the substituting ions to have a similar charge and size to fit into the crystal lattice without disrupting the structure. This is because ionic compounds are stable when each ion is surrounded by as many ions as possible of the opposite charge and when cations and anions are in contact with each other, which requires similar ion sizes and charges.
Structures of ionic compounds are largely determined by the relative sizes of the ions and the ratio of positive to negative ions in the compound. Therefore, when different types of ionic compounds crystallize in the same structure, it is often due to their ions having similar relative sizes and stoichiometries. If the ions are too different in size or charge, the structure may not be stable because the layers of like-charged ions may come into contact without sufficient attraction, or the ions simply may not fit within the interstitial spaces.