Paramagnetic resonance studies of V2+and Cr3+in orthorhombic symmetry sites in magnesium oxide

Paramagnetic resonance spectra are reported for V2+ ions occupying orthorhombic symmetry sites in the cubic lattice of magnesium oxide. The local distortion from Oh symmetry at the vanadium ion site is attributed to a cation vacancy located in a nearest neighbour position along a (110) direction. The spectra are composed of 'forbidden' Delta Ms=+or-2 and Delta Ms=+or-3 transitions in addition to the 'allowed' Delta Ms=+or-1 transitions. The spectra which were recorded at microwave frequencies near 9.5 GHz and 35 GHz may be described by the spin Hamiltonian, H=g beta B.S+D(Sz2-1/3 S(S+1))+E(Sx2-Sy2)+AI.S with g=1.9910 (isotropic), mod D mod =0.1819 cm-1, mod E mod =0.0419 cm-1 and mod A mod =0.0072 cm-1 (isotropic). The observed hyperfine structure is complicated by the presence of 'forbidden' lines, which become allowed off axis owing to the breakdown of the Delta MI=0 selection rule by the second order mixing of the fine structure and hyperfine structure terms. The spectrum from Cr3+ in a similarly distorted site has been examined at 35 GHz, a frequency higher than those previously used. In contradistinction to X and K band measurements by other workers, all possible Delta Ms=+or-1 transitions are observed.