Relative Orientations of the Electric Field Gradient and Susceptibility Tensors in Monoclinic Symmetry

Using crystalline fields with local monoclinic symmetry and the spin-orbit interaction acting on those paramagnetic ions whose orbital state may be described by a linear combination of $d_{\epsilon }$ (or $T_{2g}$) orbitals, it is shown that the electric field gradient (EFG) and susceptibility ($\chi$) tensors have only one common axis for their respective principal-axis systems. Expressions for the EFG and the magnetic hyperfine interactions referred to the magnetic axes are presented to include the effects of monoclinic symmetry. The reorientation of the magnetic and EFG principal axes may have marked effects on the Mössbauer and NMR spectra where the nuclear probe is sensitive to both magnetic and electric hyperfine interactions. Some examples are presented and Mössbauer spectra calculated to show the effects of monoclinic symmetry.