Paramagnetic Resonance of Impurities in CaWO4. I. TwoS-State Ions

Paramagnetic resonance measurements have been made on ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Gd}}^{3+}$ ions in single crystals of CaW${\mathrm{O}}_4$ grown from the melt. In both cases the four different possible substitutional sites in the unit cell lead to identical resonance spectra with tetragonal symmetry. The splitting of the electronic levels in zero magnetic field is much greater for ${\mathrm{Gd}}^{3+}$ than for ${\mathrm{Mn}}^{2+}$, e.g., ${b_2}^0$ is 6.7 times greater and ${b_4}^4$ is 13 times greater. (${b_n}^m$ is the coefficient of an operator function having the same transformation properties as the corresponding spherical harmonic ${Y_n}^m$ used in expanding the crystalline electric field.) The large value of ${b_2}^0$ (-0.0917 ${\mathrm{cm}}^{-1\mathrm{}}$) for ${\mathrm{Gd}}^{3+}$ makes it a potentially useful material for three-level masers. The lines are narrow, and the hfs due to ${\mathrm{Gd}}^{155,157}$ is well resolved: the ratio $\frac{A^{155}}{A^{157}}$ of the hyperfine splitting constants for the two isotopes was determined as 0.763±0.006.