Electrical Properties of Rutile Type Oxides: Doped TiO₂ and RuO₂

Abstract

Conducting oxides have long been used in electrochemical and photoelectrochemical devices because of their stability and catalytic properties. Among them, the rutile structure oxides constitute a family of compounds with a wide spectrum of electronic properties. For example pure TiO₂ is insulating but becomes a good semiconductor when doped with donors such as Nobium, Tantalum or Lithium, or when non-stoichiometric (oxygen vacancies and interstitial donor Ti³⁺ ions). Pure RuO₂ by contrast shows metallic conductivity of around 10⁵ Ω^-1 cm^-1 at room temperature. In spite of the fact that these two oxides have the same crystal structure and similar lattice parameters, the solid solubility limits are only around 5% of each oxide in the other. The behaviour of Ruthenium as an impurity in TiO₂ is surprising. Although in RuO₂ the four Ru 4d electrons are delocalised in the conduction band, the Ru impurity in TiO₂ retains its electrons and is present as the Ru⁴⁺ ion.