Valency and oxidation state in PbTiO3

[adisen]

I have two related questions on PbTiO3 perovskite structure and chemical environment.

The Pb K-egde Xanes data reveals that Pb is in a mixed valence state of Pb4+ and Pb2+. However in literature Pb is claimed to be in Pb2+ state. The Pb 6s2 electrons hybridize with the O 2p electrons to form strong covalent bonding which results in the relative displacement of Pb cage with respect to the O-octahedron. This results in increase ferroelectric properties of PbTiO3.
However the question is that in the covalent state of the Pb2+ will it appear as Pb4+ state in the Pb K-edge? The reason argued here is that the Pb will lose the 6s2 electrons to form the bond and hence appear to be Pb4+. Hence the argument placed by this pool of thought is from the EXAFS data what appears to be Pb4+ is actually the covalently bonded Pb2+ while what appears to be Pb2+ is actually the ionic type Pb2+.
What is the oxidation state of Pb and Ti in PbTiO3 ?

Comments on this issue will be appreciated.

 

[Navin A S]

In PbTiO3, the oxidation state of Pb is typically 2+ and that of Ti is typically 3+. However, due to the hybridization of O 2p and Pb 6s2 orbitals, there is a mixed valence state for Pb. Therefore, in the Pb K-edge XANES data, it is possible to observe both Pb4+ and Pb2+ states. The Ti atoms are typically in a 3+ oxidation state, but under certain conditions, they can also exist in a 4+ oxidation state.

 

[quicknote]

I would like to begin by clarifying that valency and oxidation state are related but distinct concepts. Valency refers to the number of chemical bonds that an atom can form, while oxidation state refers to the number of electrons an atom has gained or lost in a chemical reaction. In the case of PbTiO3, the valency of Pb and Ti is determined by their position in the periodic table and the number of valence electrons they have. Pb has a valency of 2 due to its position in group 14 of the periodic table, while Ti has a valency of 4 as it is in group 4.

Now, onto the issue of the mixed valence state and oxidation state of Pb in PbTiO3. It is not uncommon for elements to have a mixed valence state, where they can exist in different oxidation states depending on their chemical environment. In the case of Pb in PbTiO3, it has been reported in literature to exist in both Pb2+ and Pb4+ states. The XANES data mentioned in the question supports this, showing a mixture of both states.

As for the argument that the covalent state of Pb2+ may appear as Pb4+ in the XANES data, this is a valid concern. It is possible that the loss of 6s2 electrons in the covalent bonding could result in the appearance of a higher oxidation state. However, this would also depend on the specific energy range of the XANES data being analyzed and the sensitivity of the technique to distinguish between different oxidation states.

In terms of the oxidation state of Pb and Ti in PbTiO3, it is important to note that the overall compound must have a neutral charge. This means that for every Pb2+ ion, there must be a corresponding Ti4+ ion to balance out the charge. Therefore, in the compound as a whole, the oxidation state of Pb and Ti would be +2 and +4, respectively.

In conclusion, the issue of mixed valence and oxidation states in PbTiO3 is a complex one and requires further investigation and analysis. It is important to consider the specific experimental techniques used and the limitations of those techniques in determining the exact valence and oxidation states of the elements in this compound.