<p>Doping erbium ions into europium-based hosts offers a promising platform for quantum repeater nodes, combining europium’s exceptional coherence properties for long-term quantum storage with erbium’s microwave compatibility and direct telecom-band emission for efficient optical interfacing. In this work, we investigated erbium-doped EuCl₃ ⋅ 6H₂O stoichiometric crystals as a candidate for such nodes. We demonstrate that erbium doping shifts the optical transition frequencies of nearby Eu³⁺ ions, producing well resolved satellite lines in the inhomogeneous absorption profile. We experimentally probe the coupling between Er³⁺ and Eu³⁺ ions under varying temperature and magnetic field conditions, quantifying the interaction strength, which ranges from tens to hundreds of kilohertz, depending on field orientation, magnitude, and the lattice position of the Eu³⁺ ions. At 60 mK and a moderate magnetic field of 0.1 T, we observed a strong frozen core effect from Er³⁺ spins, substantially extending the Eu³⁺ optical coherence time from 62 μs to 162 μs, approaching the lifetime limit, and enabling hour-long hyperfine state lifetimes. These results underscore the potential of dual-species rare-earth systems for photonic quantum technologies and highlight their promise for precise quantum control.</p>