Near-infrared luminescence of isolated and exchange-coupled Ni²⁺ ions in Ni_cMg_1-cO solid solutions

Ni_cMg_1-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.