High-Pressure Effects on an Octa-Hydrated Curium Complex: An Experimental and Theoretical Investigation

Understanding the evolution of chemical and physical properties across the f-block, particularly under extreme conditions, is crucial for advancing nuclear science and technology. In this study, we report the synthesis and characterization of an octa-hydrated curium compound, [Cm(H₂O)₈](Hdtp)(dtp)∙H₂O (Cm1, H₂dtp = 2,3-di(tetrazol-5-yl)pyrazine) along with its lanthanide analogs [Ln(H₂O)₈](Hdtp)(dtp)∙H₂O (Ln1, Ln³⁺ = La³⁺–Nd³⁺, Sm³⁺–Lu³⁺). This represents the first report of octa-hydrated Ln³⁺ and Cm³⁺ cations in the solid state. A comprehensive investigation combining single-crystal X-ray diffraction, spectroscopic techniques, and computational analysis was conducted to elucidate the structural and spectroscopic modifications occurring under high pressure. Notably, we quantified a monotonic decrease in the computed energy difference between the ground and first excited states, correlating with a gradual reduction in the average Cm(III)–OH₂ bond length and a red shift in the photoluminescence peak. Concurrently, computational results indicate an increased delocalization of spin densities and an intensified interaction involving the 5f orbitals, coupled with geometric distortions. These effects collectively reshape the energy landscape under pressure, leading to peak broadening and quenching of luminescence. Our findings provide critical insights into the pressure-dependent behavior of curium, enhancing the understanding of transplutonium chemistry. These results have significant implications for nuclear fuel cycle advancements and the management of actinide-containing materials.