Orthorhombic Electron-Spin-Resonance Spectra of Thulium Ions in CaF2-I

A new type of charge-compensation mechanism has been found in Ca${\mathrm{F}}_2$ from a study of the electron-spin-resonance spectrum of the thulium ion. Irradiated crystals of Ca${\mathrm{F}}_2$ which have been grown from a melt containing Tm${\mathrm{F}}_3$ normally exhibit only a single isotropic spectrum which arises from ${\mathrm{Tm}}^{2+}$ ions in sites of cubic symmetry; but when the crystal is grown from a melt containing ${\mathrm{Tm}}_2$ ${\mathrm{O}}_3$, highly anisotropic spectra with orthorhombic symmetry are found after irradiation. These lines can be fitted to a spin Hamiltonian of the form $\mathcal{H}=\beta \mathbf{H}\mathbf{·}\mathbf{g}\mathbf{·}\mathbf{S}+\mathbf{I}\mathbf{·}\mathbf{A}\mathbf{·}\mathbf{S}$, where the $x$, $y$, and $z$ components of the $g$ tensor lie along the mutually orthogonal directions $〈100〉$, $〈\overline{1}10〉$, and $〈001〉$, respectively. The constants in the Hamiltonian are $\{g_{110}=2.51\mathrm{}\pm 0.01,,g_{\overline{1}10}=0.36\mathrm{}\pm 0.01,,g_{001}=5.71\mathrm{}\pm 0.01,\}\{g_{110}=1300\mathrm{}\pm 30 \frac{\mathrm{Mc}}{sec},,g_{\overline{1}10}=1100\mathrm{}\pm 400 \frac{\mathrm{Mc}}{sec},,A_{001}=2570\mathrm{}\pm 30 \frac{\mathrm{Mc}}{sec}.\}$ Various models which might give rise to the observed spectrum are discussed. Two alternative possibilities appear to exist: (1) The thulium ion is in the tetravalent state and is compensated by either a calcium ion vacancy or by additional ${\mathrm{F}}^{-}$ and/or ${\mathrm{O}}^{2-}$ ions, or (2) the resonance arises from a ${\mathrm{Tm}}^{2+}$-${\mathrm{Tm}}^{3+}$ pair, with the two impurity ions occupying adjacent sites and bonded together by two substitutional ${\mathrm{O}}^{2-}$ ions.