A study of the decay curves of the ${}_{}{}^1{}_{}{}^{}$${\mathit{T}}_{2\mathit{g}}$${\to }^3$${\mathit{A}}_{2\mathit{g}}$ luminescence of ${\mathrm{Ni}}^{2+}$:MgO at two different concentrations of ${\mathrm{Ni}}^{2+}$ (0.11 and 0.25 at. %) reveals a single exponential decay for the 0.11 at. % crystal and non-single-exponential decay for the 0.25 at. % crystal. This is interpreted in terms of transfer of excitation energy by cross relaxation. Fits of the nonexponential decay are most consistent with a dipole-quadrupole mechanism being responsible for the energy transfer. Examination of the oscillator strengths of the relevant ${\mathrm{Ni}}^{2+}$ transitions appears to support this notion, although the presence of an exchange interaction cannot be ruled out. Separate measurements on the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_{2\mathit{g}}$${\to }^3$${\mathit{A}}_{2\mathit{g}}$ luminescence show that energy transfer from single ions to pairs also takes place. Two different types of ${\mathrm{Ni}}^{2+\mathrm{-}}$${\mathrm{Ni}}^{2+}$ pairs are identified, nearest-neighbor ferromagnetic pairs with 2J=2.5 ${\mathrm{cm}}^{\mathrm{-}1}$ and next-nearest-neighbor antiferromagnetic pairs with 2J=-35 ${\mathrm{cm}}^{\mathrm{-}1}$.

• Received 1 October 1990

DOI:https://doi.org/10.1103/PhysRevB.43.7377