Doping-Dependent Structural Phase Transitions, Magnetic Order, and Excess Oxygen Behavior in Nd_2-xSr_xNiO_4+δ

Chemical doping in transition metal oxides drives a complex interplay among lattice, charge, and spin degrees of freedom. Herein, we investigate the structural and magnetic evolution of Nd_2-xSr_xNiO_4+δ (0 ≤ x ≤ 0.7), a layered nickelate system with mixed-valence Ni²⁺/Ni³⁺ ions and tunable oxygen nonstoichiometry (δ), using high-resolution synchrotron X-ray and neutron diffraction, complemented by macroscopic measurements over a broad temperature range. High-temperature solid-state synthesis in air promotes oxygen interstitial incorporation, yielding hyper-stoichiometric compounds (δ > 0). For Nd₂NiO_4.23 (x = 0), excess oxygen atoms exhibit long-range ordering, giving rise to incommensurate structural modulations and a sequence of order-order transitions persisting up to 800 K. Sr substitution disrupts this ordering via Coulombic effects and suppresses both oxygen uptake and NiO₆ octahedral tilting. Stripe-like spin ordering emerges at x = 0.25, 0.33, and 0.5. The x = 0.5 sample exhibits magnetic incommensurability (ε ≈ 0.44) below 95 K, indicative of charge discommensuration in NiO₂ planes. Unlike the oxygen-doped (x = 0) phase, Sr-doped samples show quasi-two-dimensional magnetism, with in-plane and out-of-plane magnetic correlation lengths of 114(2) Å and 16(3) Å, respectively. These results reveal strong coupling among chemical doping, crystal structure, oxygen ordering, and magnetism in layered nickelates.