Chemical Origins of Optically Addressable Spin States in Eu₂(P₂S₆) and Eu₂(P₂Se₆)

Lanthanide materials with a 4f⁷ electron configuration (⁸S_7/2) offer an exciting system for realizing multiple addressable spin states for qubit design. While the ⁸S_7/2 ground state of 4f⁷ free ions displays an isotropic character, breaking degeneracy of this ground state and excited states can be achieved through local symmetry of the lanthanide and the choice of ligands. This makes Eu²⁺ attractive as it mirrors Gd³⁺ in exhibiting the ⁸S_7/2 ground state, capable of seven spin-allowed transitions. In this work, we identify Eu₂(P₂S₆) and Eu₂(P₂Se₆) as viable candidates for optically addressable spin states. The materials feature paramagnetic behavior at 2.0 ≤ T ≤ 400 K and μ₀ H = 0.01 and 7 T. The field-dependent magnetization M(H) curve reveals a single-ion spin with effective magnetic moments comparable to the expected magnetic moment of Eu²⁺. Seven well-defined narrow peaks in the excitation and emission spectra of Eu²⁺ are resolved. Phonon contributions to the Eu²⁺ spin environment are evaluated through heat capacity measurements. Insights into how the spin-polarized band structure and density of states of the materials influence the physical properties are described by using density functional theory calculations. These results present a foundational study of Eu₂(P₂S₆) and Eu₂(P₂Se₆) as a feasible platform for harnessing the spin, charge, orbital, and lattice degrees of freedom of Eu²⁺ for qubit design.