Paramagnetic Resonance ofYb3+in CaWO4

Paramagnetic resonance of ${\mathrm{Yb}}^{3+}$ has been observed in single crystals of CaW${\mathrm{O}}_4$ at 20°K and 3 cm. A tetragonal as well as two rhombic spectra were found. The tetragonal spectrum is fitted to an axial spin Hamiltonian with $g_{\mathrm{II}}=1.058\mathrm{}\pm 0.002$, $g_{\perp }=3.920\mathrm{}\pm 0.005$, $\left|{}_{}{}^{173}{}_{}{}^{}A\right|=(77\mathrm{}\pm 1){10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|{}_{}{}^{173}{}_{}{}^{}B\right|=(285\mathrm{}\pm 1){10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|{}_{}{}^{173}{}_{}{}^{}P\right|<\mathrm{}{2.10}^{-3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|{}_{}{}^{171}{}_{}{}^{}A\right|=(279\mathrm{}\pm 3){10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|{}_{}{}^{171}{}_{}{}^{}B\right|=(1034\mathrm{}\pm 3){10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The rhombic spectra have the $y$ and $z$ axes in the {110} planes. One has the $z$ axis at 6°±1° to the crystallographic $c$ axis and $g_x=3.012\mathrm{}\pm 0.005$, $g_y=4.788\mathrm{}\pm 0.005$, $g_z=0.975\mathrm{}\pm 0.003$. The other has the $z$ axis at 7°±1° to the $c$ axis and $g_x=4.155\mathrm{}\pm 0.005$, $g_y=3.704\mathrm{}\pm 0.005$, $g_z=1.155\mathrm{}\pm 0.003$. The spectra arise from ${\mathrm{Yb}}^{3+}$ ions at ${\mathrm{Ca}}^{2+}$ sites and the rhombic symmetry is caused by charge compensation at next-nearest-neighbor ${\mathrm{Ca}}^{2+}$ sites. The possibility of ${\mathrm{W}}^{6+}$ substitution is discussed.