Ce₃T₄P₄O₂ (T = Cu, Ni): chemical tuning of the Ce valence state

Heavy-fermion compounds are intriguing physical systems due to the strong electronic correlations arising from the presence of localized states near the Fermi energy level. We synthesized the layered oxypnictides Ce₃Cu₄P₄O₂ and Ce₃Ni₄P₄O₂ in the polycrystalline form, and report a transition from localized-moment magnetism in Ce₃Cu₄P₄O₂ to mixed-valent behavior in Ce₃Ni₄P₄O₂.

Magnetic measurements on Ce₃Cu₄P₄O₂ yield an effective magnetic moment of μ_eff = 2.32 μ_B/Ce, consistent with a fully Ce³⁺ state of Ce. At low temperatures, the magnetic susceptibility saturates and exhibits a ZFC/FC splitting, the resistivity shows a field-suppressed knee, and the heat capacity displays an upturn, all suggesting a magnetic transition below 2 K. In contrast, the magnetic susceptibility of Ce₃Ni₄P₄O₂ follows the interconfiguration fluctuation (ICF) model, with no signatures of magnetic ordering in resistivity or heat capacity measurements. The Sommerfeld coefficient decreases from γ = 206(3) mJ mol^-1 K^-2 in Ce₃Cu₄P₄O₂ to γ = 40(2) mJ mol^-1 K^-2 in Ce₃Ni₄P₄O₂, consistent with the transition from a local-moment to a mixed-valent state. Such transitions are commonly induced by external pressure in heavy-fermion systems. Here, we attribute its origin to chemical pressure arising from the different oxidation tendencies: while Cu has two easily attainable oxidation states (Cu⁺ and Cu²⁺), oxidation of Ni⁺ to Ni²⁺ would depopulate the Ni-3d states and push the system into the region of high density of states at the Fermi energy level. Because Ce-4f states are close to the Fermi energy level, oxidation of Ce³⁺ to Ce⁴⁺ provides an alternative, energetically favorable mechanism to obtain charge neutrality in the structure.