Interesting question. From electrical point of view, grain boundaries (GBs) act as resistive barriers across the dielectric layer which results in higher resistivity at GB (in comparison with grains): R_{GB}>R_G. Now, turning to the influence of GBs on the distribution of oxygen vacancies, it is important to keep in mind that any redistribution (and/or aggregation) of oxygen vacancies will inevitably result in the formation of the modulated defect structures in solid dielectrics (like BaTiO3). According to the above-mentioned picture, the GBs will substantially slow down the oxygen vacancy transport (via the conventional thermal diffusion), resulting in an asymmetrical distribution of the oxygen content throughout individual grains.

There is however another possibility for oxygen diffusion (channeling) thru GBs based on the rapid movement of the grain boundary dislocations (GBDs). This is the so-called osmotic (or pumping) mechanism. The osmotic force in oxygen-deficient dislocated samples is directly related to the chemical potential of oxygen vacancies. So, when in such a sample there exists a nonequilibrium concentration of vacancies (that is, point defects), the GB dislocation (that is, a linear defect) is moved for atomic distance by adding excess vacancies to the extraplane edge. What is important, the GBDs driven oxygen vacancies channeling thru GBs is much more efficient (more rapid) than the conventional diffusion mechanism.

For further discussion on the influence of GBs on the oxygen vacancies distribution in BaTiO3, see the attached paper.

dear friend, sorry me too HAVE QUTIONS HERE . what is the oxygen vacancies in materials, why we studies this ? can you tell me this qutions answare

Interesting question. From electrical point of view, grain boundaries (GBs) act as resistive barriers across the dielectric layer which results in higher resistivity at GB (in comparison with grains): R_{GB}>R_G. Now, turning to the influence of GBs on the distribution of oxygen vacancies, it is important to keep in mind that any redistribution (and/or aggregation) of oxygen vacancies will inevitably result in the formation of the modulated defect structures in solid dielectrics (like BaTiO3). According to the above-mentioned picture, the GBs will substantially slow down the oxygen vacancy transport (via the conventional thermal diffusion), resulting in an asymmetrical distribution of the oxygen content throughout individual grains.

There is however another possibility for oxygen diffusion (channeling) thru GBs based on the rapid movement of the grain boundary dislocations (GBDs). This is the so-called osmotic (or pumping) mechanism. The osmotic force in oxygen-deficient dislocated samples is directly related to the chemical potential of oxygen vacancies. So, when in such a sample there exists a nonequilibrium concentration of vacancies (that is, point defects), the GB dislocation (that is, a linear defect) is moved for atomic distance by adding excess vacancies to the extraplane edge. What is important, the GBDs driven oxygen vacancies channeling thru GBs is much more efficient (more rapid) than the conventional diffusion mechanism.

For further discussion on the influence of GBs on the oxygen vacancies distribution in BaTiO3, see the attached paper.

Dear prof. Sergeenkov,

Thank you very much for your kind answer, It's very helpful for me to understand this question.

For the first part, you mentioned the accepted fact that the GBs have higher resistance than Gs and this is explained by the sufficient oxidation of GBs during heating or cooling. From this, we can say that the GBs assist the distribution of oxygen (channeling). I think it's same for the oxygen vacancies. Am I right?

The osmotic mechanism is wonderful. I just have a simple question.

The distribution of oxygen vacancies carries out through hopping of the thermal-activated oxygen between neighboring sites. According to the osmotic mechanism, the oxygen distribution is achieved by means of the movement of GBs? So, is it still based on the hopping of oxygen vacancies?

And the movement of the grain boundaries inevitably gives rise to the regrowth of the grain with less oxygen vacancies. Is it possible?

Thank you again for your attention.

Chunchun Li

Dear Sadha Nandam,

Thank you for your following.

Actually, i am also a beginner of the defect chemistry of dielectrics.

From the literature, the oxygen vacancies are very common for the dielectrics, especially for those sintered at elevated temperatures, because of the oxygen loss. And due to the acceptor impurities in the raw materials.

The oxygen vacancies existing in the dielectrics have significant effects on the electrical properties of dielectrics.

Firstly, they can form defect complexes with the negatively charged defects, and these complexes can orientate under the external electrical field, which contributes to the polarisation at low frequencies. Sometimes, they can lead to high-temperature dielectric anomalies. You can find a lot of papers about this. This is the short range movement.

Then, the defect complexes can be dissociated under the external stress. The free oxygen vacancies can move through hopping of oxygen vacancies. This will lead to the deterioration of the resistance. This is the electrical degradation, which determines the lifetime of the dielectrics.

Thank you!

Chunchun Li

Dear Chunchun,

the term "oxygen vacancies" implies that you have "oxygen deficiency" in your sample. It means that there is some optimal value of oxygen content (atomic concentration) at which your sample will exhibit the best possible (optimal) properties (electrical, mechanical, etc). Below this optimal concentration, you will have some free space (or vacancy, like in a hotel) in your sample (both inside and between the grains) which is not occupied by the oxygen atoms. The distribution (and movement) of these vacancies thru the grain boundaries (GBs) will depend on many different factors, including structure of GBs, presence of other defects (especially linear and planar, like dislocations and twin boundaries), temperature, mechanical deformation, electric fields, etc. The conventional transport of oxygen vacancies thru GBs can indeed be understood within thermally activated diffusion (hopping) mechanism. The term "channeling" is usually referred to a more sofisticated athermal mechanism (like, e.g., a quantum tunneling thru the potential barrier instead of termally assisted hopping over it) which can essencially facilitate the transportation problem. The osmotic (pumping) scenario for motion of oxygen vacancies is an example of such a "channeling". Like I said earlier, for its realization, in addition to vacancies (point defects), osmotic mechanism requires the presence of either linear defects (like grain boundary dislocations (GBDs)) or planar defects (like twin boundaries, essencially based on arrays of GBDs). In this scenario, GBDs serve as effective "carriers" for oxygen vacancies. This is because the GBDs themselves are created from the oxygen vacancies and use them (as a construction material) for their own growth. That is why growth of such GBDs looks like channeling (coherent dissipation-free movement of the attached to them vacancies) thru the grain boundaries. Naturally, by removing oxygen vacancies (that is, by adding extra oxygen atoms) you will weaken and eventually destroy GBDs rendering thus the channeling based travel mechanism ineffective. The same is also true about "interactions" between oxygen vacancies and created from them twin boundaries (stacks of GBDs).