Emergent Electronic and Structural Landscapes in a Novel Valence Ordered Thin-Film Nickelate prepared by Topochemical Reduction

The on-demand design of transition-metal oxides (TMO) with emerging properties is imparted by the multivalent nature of the transition-metal ions and the accessible complexity of lattice structures [1, 2]. One can access a wide range of electronic landscapes by varying the structure and constituent elements in TMO. The oxygen dynamics in the system determines the resulting structure, and thereby their electronics. The metal hydride based topochemical reduction is one of the oxygen (de-) intercalation methods and it has been employed to synthesize otherwise difficult systems such as the infinite-layer nickelates (IL-nickelates), which was found to be superconducting upon doping [3].<br/>In this path, from topochemical reduction of a perovskite SmNiO₃ thin-film, we obtain a novel valence-ordered and tri-component coordinated nickelate phase [4]. This new phase, with the chemical formula of Sm₉Ni₉O₂₂ (SmNiO_2.44) is formed by intricate planes of {303}_pc(subscript pc refers to pseudocubic) apical oxygen vacancies (Vo) from the parent perovskite as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM). Transport measurements indicated this phase to be highly insulating, over the measured temperature range (30K - 400K). A coherent analysis between ab-initio simulations and Synchrotron based X-ray spectroscopy techniques that evidenced a strong orbital polarization elucidated that this nickelate hosts multi-valent Ni sites that are in NiO₅ pyramidal, NiO₄ square-planar, and NiO₆ octahedral coordinations. The resonant inelastic x-ray scattering (RIXS) measurements at the O-K edge also revealed this system to be having a strong carrier localization, marked by the disappearance of a ligand-hole configuration at low temperature. The periodicity of these polyhedral ordering, that can also be interpreted as valence ordering, follows the periodicity of the apical Vo ordering. The NiO₄ square-planar sites forms at the intersection of the families of {303}_pc apical Vo planes, indicating a route to the infinite-layer phase.<br/>Apart from these, this phase being an intermediate between the perovskite ABO₃ and infinite-layer ABO₂, it could have been present as a defect in many IL-nickelate samples, and caused complexity in identifying their pristine properties. The ongoing debate on the (1/3, 0, 1/3) r.l.u. Charge ordering reported in IL-nickelates is one such prototypical example [5- 10]. Our STEM-EELS analysis found quasi-2D nanodomains of (303)_pc apical Vo ordering in a charge ordered IL-nickelate, and its absence in a sample without charge ordering [8]. Later on, another group also reported similar Vo ordering combining with resonant x-ray scattering that directly corroborated our idea [9]. It is evident from the measurements in those thin-films and with that of Sm₉Ni₉O₂₂ that the nano-domains of (303)_pc ordering there was in fact the presence of nano-domains of Sm₉Ni₉O₂₂ phase as a defect. This directly questions the intrinsic nature of (1/3, 0, 1/3) r.l.u. Charge ordering reported in IL-nickelates.<br/>This new nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents a unique platform where mixed Ni valence can give rise to exotic phenomena.<br/>[1] Dagotto, E. Science 309, 257–262 (2005). [2] Ahn, C. et al. Nature materials 20, 1462–1468 (2021). [3] Li, D. et al. Nature 572, 624–627 (2019). [4] <b>Raji, Aravind</b>, et al. arXiv preprint arXiv:2308.02855 (2023) (Under revision, ACS Nano). [5] Tam, Charles C., et al. Nature Materials 21.10 (2022): 1116-1120. [6] Rossi, Matteo, et al. Nature Physics 18.8 (2022): 869-873. [7] Krieger, G., et al. Physical Review Letters 129.2 (2022): 027002. [8] <b>Raji, Aravind</b>, et al. Small, 2304872. (2023). [9] Parzyck, C. et al. arXiv preprint arXiv:2307.06486 (2023). [10] Pelliciari, Jonathan, et al. arXiv preprint arXiv:2306.15086 (2023).