Transition metal impurities in wide gap materials: are the electronic properties well described through the ligand field theory?

Abstract

In the traditional ligand field theory the electronic properties due to a transition metal (TM) impurity, M, in an insulator are explained only in terms of the MX N complex formed with the N nearest anions. This work is aimed at emphasizing the role played by the electrostatic potential, V R (r), exerted by the rest of lattice ions upon the localized electrons of the TM complex. This potential, neglected in the traditional ligand field theory, is shown to play a key role when comparing the electronic properties of the same TM complex but embedded in two lattices which are not isomorphous even if both are cubic. As a relevant example it is shown that the different 10 Dq values exhibited by $${\rm CrF}_6^{3-}$$ in the normal perovskite KMgF3 (10 Dq = 14,100 cm−1) and in the inverted perovskite BaLiF3 (10 Dq = 16,720 cm−1) can hardly be understood only through a distinct Cr3+–F− distance. In contrast such a difference is shown to come mainly from the different shape of V R (r) in these two cubic lattices. the importance of this internal electric field is pointed out to grow when comparing two host lattices with the same ligand and coordination number but very different symmetry. This situation is found in the case of ruby (Al2O3 : Cr3+) and emerald (Be3Si6Al2O18 : Cr3+) where V R (r) is behind the different colours exhibited by such gemstones. For the sake of clarity a brief discussion on the foundations of the ligand field theory is reported in the present work as well.