Localized 4f Magnetism and Low-Lying 5d-Mediated Photoresponses in Layered TmI2

Divalent rare-earth solids with a 4fn+1 configuration provide access to electronic structures in which localized 4f states coexist with low-lying, structurally responsive 5d states, yet the combined magnetic, electrical, and optical manifestations of these states remain underexplored in extended Tm2+ compounds. Here, we investigate layered TmI2 as an extended Tm2+ solid and employ Cs4TmI6 with isolated TmI6 units and dilute SrI2:Tm2+ as targeted reference systems. Structural, spectroscopic, and X-ray photoelectron spectroscopic measurements support Tm2+ as the dominant oxidation state, while electronic-structure calculations reveal narrow, localized, occupied Tm 4f-derived bands and low-lying Tm 5d-derived unoccupied states, with substantially greater 5d dispersion in TmI2 than in Cs4TmI6. Magnetic measurements establish localized 4f13 Kramers-ion behavior on the triangular Tm2+ lattice of TmI2, together with a small negative low-temperature Weiss temperature and no evidence of long-range magnetic ordering down to 2 K. Comparative luminescence, photothermal, and photocurrent measurements reveal distinct photoresponses across the three systems within a shared electronic-level framework encompassing 4f–5d and 4f–4f transitions. Relative to Cs4TmI6 and SrI2:Tm2+, TmI2 exhibits a lower-energy optical absorption onset, weaker photoluminescence, and a more pronounced light-induced current response despite its high electrical resistivity. Its wavelength-dependent photothermal response broadly follows the 4f–5d absorption profile. Together, these results establish layered TmI2 as a distinctive rare-earth platform that combines the 4f13 magnetic characteristics familiar from Yb3+ systems with low-energy 4f–5d excitations, expanding the scope for investigating rare-earth magnetic solids with optical excitation channels beyond 4f–4f transitions.