The observation of a Mott transition from a metal to an insulator ... This theory relies on the notion of a perfect crystal in which the valence electrons interact with the periodic potential ...

In the more recent past, however, transition metal oxides with complex lattice structures have become the stage on which collective quantum electrons display a plethora of unprecedented ...

We find that it is the 4s orbital electrons that are ionised first before the 3d orbitals. We have to conclude that, ironically for the d-block elements, the 4s orbitals are in fact higher in energy ...