RE: Imperfections in Solids

[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.

Edited September 10, 2017 by curiousitygene

[studiot]

1 hour ago, curiousitygene said:

My thought is that for introducing anionic vacancies, we use lower positive charge and for introducing cationic vacancies, we use lower negative charge

What charge does an anion and a cation have?

Surely you will get

An anionic vacancy if you lower the negative charge (eg by removing an anion)

A cationic vacancy if you lower the positive charge (eg by removing a cation)

You also need to consider the lattice structure. MgO has an NaCl structure with 6:6 coordination.

Edited September 10, 2017 by studiot