Unveiling the Key Role of Rare-Earth (La Versus Nd) in Ni+-Based Layered Nickelates: Impact on Structures and Physical Properties

We report the synthesis of bulk RE_1–xA_xNiO₃ (RE = La, Nd; A = Sr, Ca) perovskite phases under high oxygen pressure (T = 900 °C, P_O2 = 250 bar) and identify a solubility limit of x = 0.07 for phase-pure samples. This solubility limit, shared by the various RE and A combination investigated, is likely constrained by the Ni⁴⁺ content (t_2g⁶) whose electronic effect limits doping, regardless the size of RE and A atoms. Alkaline earth (hole) doping induces a decrease in the volume of the unit cell of RE_1–xA_xNiO₃, despite the presence of larger alkaline earth atoms, and an increase in the orthorhombic distortion in the case of RE = Nd (Nd_1–xA_xNiO₃). After topotactic reduction, RE_1–xSr_xNiO₂ infinite-layers were obtained by mixing the perovskite phase with 2 mol CaH₂ in a tube sealed under secondary vacuum and treated at low temperature (250 °C). Thermogravimetric analysis was used to determine the oxygen stoichiometries of the compounds with accuracy. We find that while the perovskite parent compound RENiO₃ displays a high crystalline quality, regardless the nature of the rare-earth (Nd or La), marked stacking faults are present in the reduced NdNiO₂ infinite-layer, as determined by X-ray diffraction measurements using synchrotron radiation. The high-pressure diffraction experiment further demonstrates the role of pressure in attenuating the effect of these stacking faults that were also partially simulated and modeled using the FAULTS program. These defects are virtually absent for Sr²⁺ doping at 7% or when Nd is replaced by La. The occurrence of such stacking faults is confirmed by HRTEM analysis in the case of NdNiO₂. These stacking faults are more pronounced in the reduced infinite layer compositions for which the parent perovskite structure is more distorted, as measured by the departure of the Ni–O–Ni angle from 180°, establishing therefore a memory effect between the distortion of the perovskite structure and the occurrence of stacking faults after reduction. The more the perovskite structure is distorted, the greater the stacking fault rate. Finally, we report and discuss the magnetic properties, the electrical resistivity, and the specific heat of La and Nd-based infinite layer compositions with respect to their structural properties. More specifically, we shed light on the contribution of f-electrons of Nd³⁺ to the specific heat and discuss the possible signature of spin glass state in LaNiO₂ through magnetic and specific heat measurements.