Electrical properties and charge compensation mechanisms of Cr-doped rutile, TiO2

Cr-doped rutile, Ti1-xCrxO2-x/2-delta, powders and ceramics with 0 <= x <= 0.05 were prepared by solid state reaction and sintered at 1350 degrees C. Cr distribution is homogeneous with no evidence of either segregation or crystallographic shear plane formation. For high x compositions, >similar to 0.01, Cr substitution is charge-compensated ionically by oxygen vacancies with two Cr3+ ions for each vacancy and the materials are electronically insulating. For low x compositions, the materials are semiconducting. This is attributed to a new charge compensation mechanism involving Ti3+ 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, MUCH LESS-THAN0.01, the dopants are well-separated and instead, some Ti3+ ions act as a second dopant to preserve local electroneutrality. For intermediate x compositions, a core-shell structure is proposed consisting of semiconducting grain interiors containing Ti3+ ions surrounded by a more insulating shell with Cr3+ 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 x similar to 0.005, that is attributed to the composition-dependent presence of Ti3+ ions.

Automated summary

Ti1-xCrxO2-x/2-delta powders and ceramics with various Cr doping concentrations were prepared by solid state reaction, showing homogeneous distribution of doping ions and unique charge compensation mechanisms at different doping levels, resulting in electronic insulation, semiconduction, and core-shell structures. The lattice parameters exhibited non-linear Vegard's law behavior with a maximum cell volume at intermediate Cr doping concentrations, attributed to the presence of Ti3+ ions.