Broadening of Paramagnetic-Resonance Lines by Internal Electric Fields

The paramagnetic-resonance linewidth for ions in axial sites in CaW${\mathrm{O}}_4$ has been measured with H in the azimuthal plane. Large variations are observed, the maximum widths occurring at orientations where the shift of the resonance in an applied electric field is greatest. It is concluded that electric fields of the order of 50 kV/cm are seen at the paramagnetic centers, and that these fields are caused by randomly distributed defects in the crystal lattice. The line shape has been calculated and compared with experiment. Agreement is satisfactory and suggests that the measurement of resonance linewidths might be used to obtain accurate measurements of defect concentration in suitable ionic crystals. The effects of mechanical strains, and the role of internal electric fields in determining optical properties are briefly discussed.