UNUSUAL VALENCE STATES OF IMPURITIES IN THE DIRAC SEMIMETAL

The valence states of transition and rare-earth metal impurities in the Dirac semimetal α-Cd3As2, which is a three-dimensional analog of graphene in terms of the electronic zone structure, are studied by EPR. Evidence is presented that doping with transition and rare-earth metals can increase the number of free charge carriers by changing the valence state of impurity ions. The valence state of the bulk of impurity ions corresponds to the valence state of the substituted ion in the matrix compound. But, apparently, there is always a small fraction of impurity ions, in a different valence state from the bulk. This fraction of impurity ions, which are in an unusual valence state, is determined by various factors: the degree of chemical compression in different crystallographic positions, the density of dislocations, growth defects and other factors that change the coordination number of the impurity ion and, accordingly, the number of chemical bonds formed by it with neighboring atoms. As a consequence, the question of whether the impurity is donor or acceptor, in contrast to the doping of semiconductors, in the case of semimetals has an ambiguous answer and requires more scrupulous study. It is shown that the EPR method unambiguously reveals unusual, not corresponding to stoichiometry valence states of magnetic impurities, which are stabilized by certain features of the crystal lattice, changing the coordination number of the ion, and features of the electronic zone structure of semimetals, associated inversion or partial overlap of the valence band and conduction band.