EN
The X-band EPR study of Gd^{3+}-doped La_{0.9}Nd_{0.1}F_{3} single crystal in the temperature range 4.2-295 K is carried out in order to investigate crystal field effects, the Gd^{3+} spin-phonon interactions, as well as Gd^{3+} and Nd^{3+} spin-lattice relaxation times. The local distortions at 4.2 K of the positions of the eight F^{-} ions surrounding the Gd^{3+} ion in the La_{0.9}Nd_{0.1}F_{3} crystal were determined from comparison of the theoretical with the experimental spin-Hamiltonian parameters. The spin-phonon interactions can be described by the Einstein model, which better characterizes the behavior of paramagnetic centers in LaF_{3} and La_{0.9}Nd_{0.1}F_{3} crystals than the Debye model. It is suggested, from the rotational invariance mechanism for phonon-induced contributions to spin-Hamiltonian parameters, that the rotational contributions are much smaller than those from the strain.