Low temperature luminescence spectra of the 3T2g --> 3A2g near infrared (NIR) transition of Ni2+:MgO are investigated for two different Ni2+ concentrations. In dilute crystals, with a 0.2% nominal concentration, the luminescence consists of a single sharp zero phonon band accompanied by a broad Stokes-shifted vibronic sideband. At higher concentrations (2% nominal), additional sharp bands are observed associated with the with the formation of antiferromagnetically coupled (2J = -35.5 cm-1) next nearest neighbor (nnn) and ferromagnetically coupled (2J = 2.5 cm-1) nearest neighbor (nn) Ni2+-Ni2+ pairs. A fast migration model is used to analyze energy transfer from single ions to pairs. Single ions that transfer energy are treated collectively and it is assumed that the donor decay rate is a single exponential. With this model, it is possible to make a quantitative estimate of the relevant transfer rates.
