Some studies of Cr³⁺ions and Mn⁴⁺ions in magnesium oxide

The electron spin resonance spectra of magnesium oxide show that Cr 3+ and Mn 4+ ions occupy substitutional sites in the lattice. Their symmetry will be either octahedral or lower depending on whether the necessary chargecompensating defect is remote from or near to these ions. A new spectrum due to Cr 3+ in sites with tetragonal symmetry has the following spin-Hamiltonian parameters g = 1 9782 ± 0 0003 and D = 798 ± 3 × 10 -4 cm -1 . This is attributed to a charge neutral configuration in which the nearest- and next-nearest-neighbour cation sites along a [100] axis are respectively vacant and occupied by an Al 3+ ion. In powder samples Mn 4+ may be found in both octahedral and axially symmetric sites The spin-Hamiltonian parameters are respectively g = 1 9938 ± 0 0003 and g = 1 995 ± 0 001, D = 280 ± 20 × 10 -4 cm -1 . In both cases A 55 = 70 8 ± 0 5 × 10 -4 cm -1 . A sharp fluorescence line at 15 279 cm -1 with associated vibronic side bands has been identified with the Mn 4+ ion in sites with octahedral symmetry.