Electrical properties and charge compensation mechanisms of Cr-doped rutile, TiO.

Cr-doped rutile, TiCrO, powders and ceramics with 0 ≤ ≤ 0.05 were prepared by solid state reaction and sintered at 1350 °C. Cr distribution is homogeneous with no evidence of either segregation or crystallographic shear plane formation. For high compositions, >∼0.01, Cr substitution is charge-compensated ionically by oxygen vacancies with two Cr ions for each vacancy and the materials are electronically insulating. For low compositions, the materials are semiconducting. This is attributed to a new charge compensation mechanism involving Ti ions created in response to the local electroneutrality requirement for two trivalent cations to be in close proximity to each oxygen vacancy. At very low dopant concentrations, ≪0.01, the dopants are well-separated and instead, some Ti ions act as a second dopant to preserve local electroneutrality. For intermediate compositions, a core-shell structure is proposed consisting of semiconducting grain interiors containing Ti ions surrounded by a more insulating shell with Cr ions as the only acceptor dopant. Lattice parameters show unusual, non-linear Vegard's law behaviour characterised by a maximum in cell volume at intermediate ∼ 0.005, that is attributed to the composition-dependent presence of Ti ions.