V. V. Kurshev, A. M. Raitsimring, K. M. Salikhov, “Angular dependence of the electron-spin-resonance line dipole broadening in single crystals containing paramagnetic clusters with anisotropic $g$-tensor: $\mathrm{Er}^{3+}$ in $\mathrm{CaWO}_{4}$”, Fizika Tverdogo Tela, 30:2 (1988), 420–426
