Near-infrared luminescence of isolated and exchange-coupled Ni2+ ions in NicMg1−cO solid solutions

NicMg1−cO (0.01 ⩽ c ⩽ 1) solid solutions were studied by near-infrared luminescence, optical absorption and X-ray absorption spectroscopies. It was found that Ni2+ ions form at c < 0.2 the exchange-coupled pairs, strongly bound via 90 ° super-exchange interactions, and are displaced at c ⩽ 0.6 to the ‘off-center’ positions. This explains the origin of the zero-phonon line splitting observed in the optical absorption and luminescence spectra. It was also found that the effective energy transfer from the single Ni2+ ions to the exchange-coupled Ni2+-Ni2+ pairs occurs at temperatures below 40 K.