A variety of rutile samples were heat treated in atmospheres of H2, H2O and O2 according to prescriptions which theoretically and experimentally either favor or suppress H interstitial formation. The H concentration and conduction-electron concentrations in the samples were monitored optically. The "A" spectrum, which was universally accepted as being due to Ti3+ interstitials, was found instead to be associated with H.

Reinterpretation of the "Ti3+ Interstitial" Electron-Spin- Resonance Spectrum in Rutile. Shen, L.N., Johnson, O.W., Ohlsen, W.D., DeFord, J.W.: Physical Review B, 1974, 10[5], 1823-5