EPR study of the low-spin state of Ru³⁺in the YAl₃(BO₃)₄ and EuAl₃(BO₃)₄ aluminum borates

New data on the ground state of impurity Ru³⁺ ions in the crystals of YAl₃(BO₃)₄ and EuAl₃(BO₃)₄ aluminum borates were obtained. It was shown that Ru³⁺ ion replaces trivalent rare-earth ions without breaking the symmetry of the site. The crystal field acting on 4d⁵ ions forms an EPR spectrum, which is described by the spin Hamiltonian with S=1/2. The spin-Hamiltonian parameters determined are equal to g_II=1.963, g_⊥=3.796, A_II=43.03*10⁻⁴ cm⁻¹, A_⊥=84.86*10⁻⁴ cm⁻¹ in the YAl₃(BO₃)₄ crystal (at T=15 K) and g_II=2.016, g_⊥=3.796 in the EuAl₃(BO₃)₄ crystal (at T=15 K). It is found that the value of ∆g=g_II-g_⊥ is an indicator of distortions of nearest environment of Ru³⁺ ion. The angle between the C₃ axis and the direction into nearest oxygen ion was determined. The EPR linewidth of Ru³⁺ ion increases with increasing temperature due to the dipole-dipole and exchange interactions with the excited states of the host lattice Ru³⁺ ion.