EPR of Gd/sup 3+/-doped single crystals of LiY/sub 1-//sub x/Yb/sub x/F4

Extensive X-band EPR measurements have been carried out on Gd/sup 3+/-doped single crystals of LiY/sub 1-//sub x/Yb/sub x/F4 (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 Gd/sup 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 Gd/sup 3+/-Yb/sup 3+/ exchange interaction, averaged over the nearest and next-nearest neighbors, has been estimated, in LiY/sub 0.9/Yb/sub 0.1/F4, 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, LiYF4, at liquid-helium temperature