Co+z-α-Al2O3: cobalt and proton ENDOR, infrared and optical spectra, the nature of the charge compensator and the structure of the defect (Co+2, H+): α-Al2O3

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Abstract
The redox process of Co+z ions (z=2,3) substituted into the sapphire alpha -Al2O3 structure produced by heat treatment in H2 and O2 atmosphere is correlated with ESR and ENDOR spectra of (Co+2, H)) and with UV-VIS and IR spectra of Co+3: alpha -Al2O3 and (Co+2, H+): alpha -Al2O3 respectively. From ENDOR spectra the charge compensator H+ for Co+2(c): alpha -Al2O3 is uniquely identified and the reduction of the crystal-field symmetry by it from D3d to C1 is demonstrated. Location of the charge compensator and structural parameters of the defect (Co+2,H+): alpha -Al2O3 are determined. Furthermore, it is shown that the proton-ENDOR spectra of (Co+2, H+): alpha -Al2O3 are described by a hyperfine interaction term characterised by an asymmetric coupling tensor originating from dipole-dipole interaction.