Influence of Impurities on Electrical Conductivity of Rutile

Electrical conductivity can be produced in rutile (1) by replacing an occasional Ti 4+ ion with another cation having a positive charge higher than four or (2) by incorporating small cations, such as the Be 2+ ion, into interstitial positions in the crystal lattice. This excess positive charge must be balanced by “available electrons,” which then form a constituent of such an impure rutile crystal. The insulating properties of such a contaminated rutile crystal can be partly restored by incorporating into its lattice cations of a lower valency, such as trivalent aluminum and yttrium. Under these conditions a couple, Nb 5+ Al 3+ , takes the place of two Ti 4+ ions and the crystal does not need to retain extra electrons that are available for conductivity.