High‐Temperature Defect Structure of Cobalt‐Doped α‐Alumina

Abstract

Room‐temperature optical absorption spectra, electron spin resonance spectra at 15° to 18°K, electrical conductivity, and emf measurements on concentration cells at 1620°C are analyzed and used to determine the defect structure of Codoped α‐Al 2 O 3 . The crystals are mixed ionic and electronic conductors at 1620°C: ionic conduction occurs at 10 ‐8 atm< p O 2 < 10 ‐3 atm, with triply charged interstitial Al ions as the major charge carriers, and electronic conduction occurs at 10 ‐3 atm < p O 2 < 1 atm, with holes as the major charge carriers. A defect model based on the charge compensation of divalent Co at Al sites by triply charged Al interstitials is proposed. The mobilities and activation energy of ions and holes, the oscillator strengths of Co 2+ and Co 3+ absorption bands, parameters for electron spin resonance spectra, level positions of substitutional Co 2+ and an unknown donor, and equilibrium constants for defect formation reactions are determined.