The discovery of superconductivity in square-planar nickelates has uncovered a versatile materials platform for studying cuprate-like superconductivity [1]. While most studies have focused on chemically doped infinite-layer nickelates, R1-xAxNiO2 (R = 3+ rare-earth cation; A = 2+ alkaline metal), these compounds represent only the  n = ∞ member of a broader homologous series: the multi-layer square-planar nickelates, Rn+1NinO2n+2 [2]. These materials comprise n RNiO2 layers separated by RO2 spacer layers. The structural layering, n, can be leveraged to tune the dimensionality and nickel 3d electron count (d9-1/n). Indeed, superconductivity was observed in five-layer Nd6Ni5O12, which shares the same formal d8.8 electron count as chemically doped Sr0.2Nd0.8NiO2 [3]. In this talk, I will first present the correlated phase diagram of multi-layer square-planar Ndn+1NinO2n+2 thin films (n = 3 – 10) [4]. We observe signatures of superconductivity in n = 4 – 8 compounds, with a maximum Tc,onset of 12.8 K for n = 6. These results establish a superconducting dome determined purely by structural layering, demonstrating that hole doping can be achieved without chemical substitution in square-planar nickelates. In the second part of the talk, I will introduce a new family of layered oxides — oxygen-intercalated layered perovskites — realized via topochemical oxidation of Ruddlesden–Popper nickelates [5]. Ozone annealing Ruddlesden–Popper nickelate thin films induces a remarkable 2.1Å c-axis expansion, driven by the intercalation of approximately δ~0.7 oxygen atoms in the rock salt spacer layer — far exceeding the previous record (δ~0.3). I will discuss the structural, electronic and transport properties of these novel compounds.

References:

[1] Danfeng Li. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019).

[2] Lacorre. Passage from T-type to T′-type arrangement by reducing R4Ni3O10 to R4Ni3O8 (R = La, Pr, Nd). J Solid State Chem 97, 495–500 (1992).

[3] Grace A. Pan, Dan Ferenc Segedin, et al. Superconductivity in a quintuple-layer square-planar nickelate. Nat. Mater. 21, 160–164 (2022).

[4] Grace A. Pan*, Dan Ferenc Segedin*, et al. Superconducting phase diagram of multi-layer square-planar nickelates. In review.

[5] Dan Ferenc Segedin, et al. Topotactic oxidation of Ruddlesden-Popper nickelates reveals new structural family: oxygen-intercalated layered perovskites. arXiv:2506.10262 (2025).

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https://us06web.zoom.us/j/5081440931

Meeting ID: 508 144 0931

Seminar hosts: Neven Šantić i Matija Čulo