EPR of ${\mathrm{Gd}}^{3+}$-doped single crystals of ${\mathrm{LiY}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{F}}_4$

Extensive X-band EPR measurements have been carried out on ${\mathrm{Gd}}^{3+}$-doped single crystals of ${\mathrm{LiY}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Yb}}_{\mathrm{x}}$ ${\mathrm{F}}_4$ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0), covering the temperature range 4.2–290 K. The spin-Hamiltonian parameters (SHP) are evaluated by the use of a rigorous least-squares fitting procedure. The systematics of the SHP are studied both as functions of x and of temperature. Newman’s superposition model has been applied to the calculation of SHP; it is found that the calculated values agree quite well with the experimental values, provided that one considers minor distortions of the positions of the eight fluorine ions surrounding the ${\mathrm{Gd}}^{3+}$ ion. The temperature variation of SHP has been explained to be predominantly due to spin-phonon interaction and partially due to thermal expansion of the crystal lattice. Finally, the average ${\mathrm{Gd}}^{3+}$-${\mathrm{Yb}}^{3+}$ exchange interaction, averaged over the nearest and next-nearest neighbors, has been estimated, in ${\mathrm{LiY}}_{0.9}$ ${\mathrm{Yb}}_{0.1}$ ${\mathrm{F}}_4$, to be 3.8±2.5 GHz, as found by the application of the molecular-field model using the g shift from the isostructural diamagnetic host, ${\mathrm{LiYF}}_4$, at liquid-helium temperature.