Electron paramagnetic resonance of some 3d ions in magnesium oxide
Electron paramagnetic resonance (E.P.R.) and reflection high energy electron diffraction (RHEED) techniques have been used to study the distribution of iron, chromium and manganese in lightly doped magnesium oxide (MgO) single crystals and powders. E.P.R. single crystal spectra were recorded at room temperature for all three systems (with the dopants at various concentrations). From these spectra, which agreed with previously published data, the valency states (i.e. Fe(^3+), Cr(^3+) and Mn(^2+)) of the isolated ions and the symmetry of the sites they occupy were identified. The single crystal Spin-Hamiltonian parameters were used in computer simulations developed to predict the powder spectra and a comparison with the corresponding experimentally observed powder spectra allowed the features due to Fe(^3+), Cr(^3+) or Mn(^2+) in powdered MgO to be identified. The distribution of the dopant ions within the MgO lattice has been investigated in some detail. The magnitudes of the isolated ion spectral linewidths and their dependence upon dopant concentration were compared with the predictions of dipolar broadening theory and this showed that the manganese dopant is homogeneously distributed in MgO, (the range of the exchange interaction for Mn(^2+) being at the most 3.65Å) whereas iron and chromium are not. In Cr/MgO, at the higher dopant concentrations (9,500 p.p.m. and 15,100 p.p.m.), the diffraction patterns obtained from RHEED showed the presence of a separate phase which was identified as the spinel magnesiochromite, MgCr(_2)O(_4).In the same samples there was some evidence in the E.P.R spectra of the presence of a line formerly attributed in the literature to precipitates of MgCr(_2)O(_4) in MgO. The discrepancy between the observed cubic Cr(^3+) isolated ion central transition linewidths and those measured by de Biasi and Fernandes (which were consistent with the predictions of their dipolar broadening theory for n = 5 i.e. the range of the exchange interaction for Cr(^3+) ions in MgO was found by de Biasi and Fernandes to be 5.95Å)allowed the spinel and isolated ion concentrations in each of the chromium doped samples to be calculated. In Fe/MgO the cubic Fe(^3+) central transition linewidth remained approximately constant over the whole range of dopant concentrations examined indicating a constant isolated Fe(^3+) ion concentration which must therefore, at least in the more heavily doped samples, be lower than the total nominal iron concentration. The remaining iron is assumed to exist in clusters and an intense broad line, seen in all the spectra, is attributed to this clustered material. The clusters must be relatively disordered because RHEED showed no evidence of a separate phase with a regular crystal structure. The E.P.R. line broadening of the fine structure transitions in the cubic Fe(^3+) and Mn(^2+) spectra and the variation of the peak height of the central transition in the cubic Cr(^3) spectrum implied that strain is present in the regions of the MgO lattice surrounding all three dopants when they exist as Isolated ions. Annealing heat treatments were performed for the iron and chromium doped samples and further linewidth analysis (after heat treatment) supported the above interpretation.