Dear Mulwa Winfred,
Substituting cations such as Nd3+ on anion site (O-site) in TiO2 is not
expected to be favorable energetically. One good reason comes from the simple
ionic picture. Nd3+ will repel the surrounding Ti4+.
However, in spite of this I doubt the value of 634 eV. Recall that calculating
the formation energy of Nd defects in TiO2 requires a reference state for Nd;
for example Nd2O3.
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