Effect of host paramagnetic ions on the Gd 3+ EPR linewidth in diluted Van-Vleck paramagnets Tm x Lu 1-x PO 4 and Ho x Y 1-x VO 4 and EPR spectra of Er 3+ in Ho x Y 1-x VO 4

${\mathrm{Gd}}^{3+}$ EPR linewidths in diluted Van-Vleck paramagnets ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ (x=0.0, 0.02, 0.05, 0.1, 0.15,0.25, 0.3, 0.5, 0.9, 1.0) and ${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Lu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{PO}}_{4}$ (x=0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were measured at X band (8.7--9.6 GHz) in the temperature range 4.2--300 K as functions of temperature and concentration (x) of the host paramagnetic ions, ${\mathrm{Ho}}^{3+}$ and ${\mathrm{Tm}}^{3+}$, respectively. It was found that the dipole-dipole and exchange interactions significantly influenced the measured linewidths in ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ and ${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Lu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{PO}}_{4}$ samples for the concentrations x\ensuremath{\gtrsim}0.1 and x\ensuremath{\gtrsim}0.6, respectively. Further, in ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ samples with x\ensuremath{\gtrsim}0.3 the EPR lines disappeared at temperatures above 10 K due to the formation of percolation clusters by ${\mathrm{Ho}}^{3+}$ ions; the disorder was deduced to have a negligible effect in these hosts. Interaction of the lattice with the electronic quadrupole moment of ${\mathrm{Ho}}^{3+}$ ions created different structural sites of ${\mathrm{Gd}}^{3+}$ in ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ samples with x\ensuremath{\gtrsim}0.1 depending on the number of ${\mathrm{Ho}}^{3+}$ ions in the immediate neighborhood of a ${\mathrm{Gd}}^{3+}$ ion, resulting in the observation of up to three different sets of ${\mathrm{Gd}}^{3+}$ EPR spectra. As for ${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Lu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{PO}}_{4}$ samples with 0.1x0.8, it was the disorder, rather than a percolation cluster, that dominated the ${\mathrm{Gd}}^{3+}$ EPR linewidths due to the rather large difference in the ionic radii of ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ ions, as well as in the unit-cell parameters of ${\mathrm{TmPO}}_{4}$ and ${\mathrm{LuPO}}_{4}$. The temperature variation of the linewidths were found to be in accordance with those of the average magnetic moments of ${\mathrm{Ho}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions in ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ and ${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Lu}}_{1\mathit{x}}$${\mathrm{PO}}_{4}$, respectively. ${\mathrm{Gd}}^{3+}$ spin-Hamiltonian parameters were estimated at different temperatures for the two families of hosts. EPR spectra of the ${\mathrm{Er}}^{3+}$ ion, present as a chemical impurity, have also been observed in ${\mathrm{Ho}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{VO}}_{4}$ samples at liquid-helium temperatures. \textcopyright{} 1996 The American Physical Society.