curiousitygene

Hi, My teacher had asked us to solve a question. To introduce cationic vacancies and anionic vacancies in an ionic crystal like MgO. Which would you introduce as substitutional impurities: cations with a higher positive charge or lower positive charge than the cations in the parent structure; or anions with a higher negative charge or lower negative charge than the host anion? Explain your choice. My thought is that for introducing anionic vacancies, we use lower positive charge and for introducing cationic vacancies, we use lower negative charge. However, I do not know the right way to phrase or explain my answer. Can anyone help? Thanks.