Relationship between oxygen stoichiometry and energy of CEF levels in Pr2NiO4+𝛿

Since the discovery of high-temperature superconductivity in La2CuO4+, the impact of oxygen nonstoichiometry on the structure and physical properties of layered oxide compounds was under analysis. Among these, RE2NiO4+ (Re = Pr, La, Nd), with their K2NiF4-type structure have been studied for their structural similarity to the superconducting phase. We are presently conducting a detailed study of Pr2NiO4+δ (δ = 0.25) single crystals, through the analysis of inelastic neutron scattering measurements. CEF calculations point out a complete different energy scheme for the Pr3+ with 10 oxygen environment (excess oxygen) with one very low lying level (presently asked for at ILL @IN5) and one very high energetic level (~100 meV) for which we would need MARI. Our CEF calculations also revealed a strong link between the position of the first energy transition peak and the Pr-Oap atomic distances (and ultimately oxygen stoichiometry), since we observed an exponential correlation between the Pr-Oap distances and the crystal electric field (CEF) energy levels. However, the analysis is restricted so far to the first transition due to the lack of CEF data at higher energies. Accessing this range will help us shedding light on a more general ground on the effect of excess oxygen on the full CEF energy levels of these systems.