What are the charged vacancies

[marie2010]

what are the "charged" vacancies

hi,
I was trying to read some theoretical articles for my research project in solid state physics.
Can someone tell me what are the "charged" vacancies. For example, some articles talk about, let's say, oxygen +2 versus oxygen +1 vacancies. What does it mean physically? Can we measure vacancies (i.e. their charges) experimentally. Why only consider 0, +1, +2 charge, and not +3, +4...etc?
Thank you.

 

[johng23]

In most crystals, oxygen has oxidation state -2, so if you remove an oxygen atom from its site taking all its electrons with it, that vacancy that was created would be a +2 vacancy. The charge of the vacancy is defined relative to the charge that would be on that site. The -2 charge of the oxygen ion is just part of the overall, neutral crystal so in that sense the vacancy acts like a +2 charge even though it is just an empty spot in the lattice.

Since the vacancy acts as a positive charge, it can attract a conduction-band electron in the structure and become a +1 vacancy. The probability of the vacancy combining with two electrons is a lot smaller and may be negligible in many cases.

So you see you generally won't get a +3 vacancy because the +2 vacancy will repel holes. If the vacancy was on the cation sublattice, and had a -1 charge (for example), then it could combine with a valence band hole, but not with an electron. So you could get neutral but never -2.

Let me just stress that I'm not sure if there are mechanisms for a +3 oxygen vacancy or a -2 cation vacancy in this case, there very well may be, I'm not an expert on the subject.

If it is weird to think about a vacancy combining with a hole, just remember that what we're really talking about is an atom adjacent to the vacancy which is missing one of its bonding electrons, which acts as a local region of positive charge, and by sitting adjacent to the negative vacancy, the overall energy of the system will be lowered.

 

[marie2010]

Great and succinct explanation! Thank you.

Can someone (experimentalists!) comment on how vacancies' charges can be verified/measured experimentally?
Thanks in advance.

 

[marie2010]

Hello,
revising the vacancy question, I have another one to ask: if we remove an O atom from its site taking all its electrons with it, the vacancy that was created would be a +2 vacancy. Does it mean that the system is charged now or it just "acts" like the charged one?
Thanks.

 

[Useful nucleus]

I assume that you consider here an oxide.
If you remove a neutral oxygen atom from the oxide, then remaining system will be neutral. While if you remove an oxygen anion (-2), the remaining system will be charged.

A good example for the first scenario is reducing CeO2. The process proceeds by:
1- The oxygen anion on the lattice returns two electrons to two Cerium cations (+4).
2- Each of the Two Cerium cations will become (+3).
3- The oxygen "atom" departs the lattice.

You may consult : A. Migani et. al, Chem. Commun., 2010, 46, 5936.
for more theoretical details.On the other hand, some cations do not favor changing their valence. Zirconium (+4) is a good example for that. In that case , usually the vacancy is created by removing an oxygen anion (-2). The remaining system is charged. This is of course very unfavorable energetically, and such a system will try to achieve charge neutrality. One way to do this, is replacing two Zirconium cations (+4), by two Yttrium cations (+3) .

 

[abhi2005singh]

I am not sure how an O^2- ion can be removed from the solid. As far as my understanding goes, when oxygen vacancies are created, it generally leaves behind two electrons in the conduction band. This does not lead to a charged material.

 

[marie2010]

Thank you all for replying. Reading the last post, I am confused about the following: So when you remove O from its site, do you take all its electrons with it (first reply to my original question), or leave them in the conduction band (last reply)? Or are we taking about the same thing? By "taking all its electrons" do we mean we take them from the valence (it that where they are initially?) to the conduction band?
I appreciate your help.
Thanks.

 

[Astronuc]

See this paper as one example.
http://www.ims.uconn.edu/~rampi/publications/Ms_69_190.pdf

One can search on "charged vacancies" for other examples.

The oxygen anion can displace 2 electrons from the lattice site, so locally there is an electrical field established between the cations (+ ions) remaining in their lattice sites and the two electrons with the oxygen. Actually it's not so simple as that.

 

[Stanley514]

The remaining system is charged. This is of course very unfavorable energetically, and such a system will try to achieve charge neutrality.

1)How exactly will it try to achieve it?If we will create material with many cation vacancies does it mean that material will gain macroscopic charge and will ``suck`` electrons from the surrounding environment?Or that arching could occur?
2)Is it possible to use cation vacancies for practical energy storage?In one article researchers
claim they found material that is able to store electrons in cation vacancies of matrix.
http://findarticles.com/p/articles/mi_m0KJI/is_5_114/ai_86431059/"
Could it be used in some kind of supercaps?