Quantitative EPR spectroscopy: Comparison between primary standards and application to MgO-MnO and alpha-Al2O3-Cr2O3 solid solutions

To study their reliability as primary standards in the quantitative EPR spectroscopy, a large series of pure paramagnetic compounds with known spin concentrations, whose spectra vary considerably in intensity, shape, structure and overall width are compared. The paramagnetic species examined as pure solid compounds and solutions, were free radicals (DPPH and TEMPO), vanadyl and Cu2+ ions (S = 112), Cr3+ (S = 3/2) and Mn2+ (S = 5/2) ions. The quantitative EPR findings suggest that all the S = 1/2 paramagnetic compounds investigated and MnSO4. H2O (S = 5/2) are reliable primary standards. By contrast, none of the pure Cr3+ compounds proved useful as primary standards because of their large fine-structure terms or high Neel temperature that invalidated the simple Curie law. Application of quantitative EPR in the study of dilute MgO-MnO and alpha-Al2O3-Cr2O3 solid solutions, focussing on the circumstances making paramagnetic species undetectable, is reported. In MgO-MnO solid solutions of high surface area, detection problems arising from the variation of local site symmetry can be circumvented and almost all Mn2+ are detected only by reducing the surface area. In concentrated alpha-Al2O3-Cr2O3 solid solutions, magnetic interactions lead to paramagnetic species being undetectable.