Hi Guilhem!

This is a very interesting question! However I believe it is not quite entirely correct. I mean it is impossible to reduce some particular selected element while leaving everything around it intact. Reduction of an oxide leads to the formation or annihilation of some lattice sites or point defects, charge redistribution etc. In this respect the concept of oxidation states of elements which we extensively use to characterize the red-ox behavior of each particular element is a very formal instrument which just helps us to distinguish and to identify different behavior of different species. This also helps us to count electrons and charges. But the real truth is such that the oxidation states have almost no relation to the real physical nature of things.

Rather we should question what determines the reducibility of a given compound (oxide): nature of constituent elements, crystal structure, CNs, elasticity etc. I believe that all mentioned factors are playing the role to some extent. For example, in KNbO3 with perovskite-type structure Nb has octahedral coordination. The oxide itself is probably almost oxygen stoichiometric. So, there is a principal possibility to reduce this material. Indeed, here it is possible to remove some oxygen from the lattice and reduce a bit Nb coordination, say, from 6 to 5. This is OK for perovskite-type structure at least to some extent. However in the case of LaNbO4 niobium already has tetrahedral coordination. So, its reduction would lead to CN=3 this is a way too low, so, it is quite expected that this process would be unfavorable. We observed a similar situation in YBaCo4O7-d oxide where cobalt (+2 and +3) has only tetrahedral coordination. This compound is much-much less reducible as compared to perovskite-type cobaltites. Even a tiny reduction leads to YBaCo4O7 decomposition!

In the case of Re3NbO7 compound it is already a defective fluorite-type compound containing a significant amount of 'intrinsic (or structural)' oxygen vacancies. So, its reduction is again unfavorable here. 

Hope it will help you and sorry for quite long answer!

Best wishes,

Dmitry

Hi Guilhem!

This is a very interesting question! However I believe it is not quite entirely correct. I mean it is impossible to reduce some particular selected element while leaving everything around it intact. Reduction of an oxide leads to the formation or annihilation of some lattice sites or point defects, charge redistribution etc. In this respect the concept of oxidation states of elements which we extensively use to characterize the red-ox behavior of each particular element is a very formal instrument which just helps us to distinguish and to identify different behavior of different species. This also helps us to count electrons and charges. But the real truth is such that the oxidation states have almost no relation to the real physical nature of things.

Rather we should question what determines the reducibility of a given compound (oxide): nature of constituent elements, crystal structure, CNs, elasticity etc. I believe that all mentioned factors are playing the role to some extent. For example, in KNbO3 with perovskite-type structure Nb has octahedral coordination. The oxide itself is probably almost oxygen stoichiometric. So, there is a principal possibility to reduce this material. Indeed, here it is possible to remove some oxygen from the lattice and reduce a bit Nb coordination, say, from 6 to 5. This is OK for perovskite-type structure at least to some extent. However in the case of LaNbO4 niobium already has tetrahedral coordination. So, its reduction would lead to CN=3 this is a way too low, so, it is quite expected that this process would be unfavorable. We observed a similar situation in YBaCo4O7-d oxide where cobalt (+2 and +3) has only tetrahedral coordination. This compound is much-much less reducible as compared to perovskite-type cobaltites. Even a tiny reduction leads to YBaCo4O7 decomposition!

In the case of Re3NbO7 compound it is already a defective fluorite-type compound containing a significant amount of 'intrinsic (or structural)' oxygen vacancies. So, its reduction is again unfavorable here. 

Hope it will help you and sorry for quite long answer!

Best wishes,

Dmitry

Hi Dmitry,

Thanks for these inputs. I agree with you on all the aspects you mention. My question (maybe written in a too short form) was indeed to make in collaboration with others an exhaustive list of the factors affecting the reducibility of some cations, this mainly for an educational purpose, 

In other word, if i want to make an electrolyte material, one often tries to avoid transition metal elements (for obvious reasons of reducibility) but this diminishes a lot the possibilities of playing with compositions. But it is important to mention that sometimes, we can also play with transition metal elements, even for electrolytes, from the moment some conditions are fulfilled.

You mention some of them and i would add:

- cation intrinsical reducibility

- local already present and induced strain

-  local (or extended ?) induced charge

- energetical cost of oxygen vacancy formation

- local environment, potentially different from site mean environment

...

The topic is complex in the sense that the arguments you mention are of course true but sometimes difficult to anticipate. The point is that to have a significant electron conductivity in reducing atmosphere i.e. to have a partial reduction of the cation, you need a small amount of oxygen vacancies to be created..

Other inputs are welcome.

Guilhem

Hi all

The question is a quite broad one.   However, in general, the thermodynamic stability of solids in the surrounding environment is always what decided the reducibility of ions in solids.   In other words, the higher the activation energy of the reduction reaction of ions in solids the faster the susceptibility of the reduction of ions in solids.   The activation energy of the reduction reaction of ions in solids is dependent on many Physical/chemical factors.   Among those factors are the followings:

1-The chemistry of the surrounding environment, including the number of the adjacent neighboring ions, pH, and so on.

2-The physical chemistry condition of the solids, including weather or not the present of points ,lines,or planar defect in solids.

3-The physical chemistry properties of the solids in the surrounding environment.