Evidence for tetragonal distortions from E.S.R. spectra in crystals of calcium oxide

The line width of the cubic E.S.R. spectra of iron group elements in single crystals of calcium oxide depends on the orientation of the symmetry axis with respect to the external magnetic field. It is found that the narrowest line width is along the [111] direction and the widest line along the [100] direction. This has been observed for Cr³⁺, Co²⁺, Fe¹⁺, Ni²⁺, Cu²⁺, Mn²⁺, and Fe³⁺ at various temperatures. In addition, it was found that the various hyperfine components have different line widths, for Co²⁺ the narrowest line being at the high field hyperfine component for Cu²⁺ the opposite. It is suggested that small tetragonal distortions are partially responsible for the line width anisotropy. The small radius of the iron group elements in the large volume of the calcium oxide octahedron may be the reason for the delocalization of the impurity ions in this lattice. The Jahn-Teller effect of Ni¹⁺ and Cu²⁺ is briefly discussed.