The electrical conductivity of reduced TiO2 was measured over a wide range of temperature and oxygen pressure and was found to be proportional to PO2-1.5. It was concluded that the dominant defects in this material were interstitial titanium ions and not oxygen vacancies

Experimental Evidence from Conductivity Measurements for Interstitial Titanium in Reduced TiO2. Tannhauser, D.S.: Solid State Communications, 1963, 1[7], 223-5