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

The redox process of Co +z ions (z=2,3) substituted into the sapphire alpha -Al 2 O 3 structure produced by heat treatment in H 2 and O 2 atmosphere is correlated with ESR and ENDOR spectra of (Co +2 , H ) ) and with UV-VIS and IR spectra of Co +3 : alpha -Al 2 O 3 and (Co +2 , H + ): alpha -Al 2 O 3 respectively. From ENDOR spectra the charge compensator H + for Co +2 (c): alpha -Al 2 O 3 is uniquely identified and the reduction of the crystal-field symmetry by it from D 3d to C 1 is demonstrated. Location of the charge compensator and structural parameters of the defect (Co +2 ,H + ): alpha -Al 2 O 3 are determined. Furthermore, it is shown that the proton-ENDOR spectra of (Co +2 , H + ): alpha -Al 2 O 3 are described by a hyperfine interaction term characterised by an asymmetric coupling tensor originating from dipole-dipole interaction.