Inversion recovery with electron spin-echo detection has been used to study the electron spin-lattice relaxation rates 1 / T 1 for transition metal impurities in heavily cobalt-doped hydrothermally grown ZnO single crystals. The relaxation dynamics of Co 2 + ions dominates the phonon bottleneck effect in the Orbach-Aminov process, which involves the modulation of the zero-field-splitting tensor. The relaxation mechanism may be treated in terms of phonon heating with fast rate of energy pump from Co 2 + spins into the lattice phonon modes. The measurements reveal the cross-relaxation process in which the single Co 2 + ions cross-relax to exchange-coupled clusters of cobalt ions. The higher temperature relaxation of Co 2 + indicates an additional Orbach-Aminov process via a state of ∼ 226 cm − 1 above the ground state Kramers doublet. It is shown that in this system Co 2 + ions play a role of the rapidly relaxing …