The recent observation of superconductivity in (Nd,Sr)NiO₂ thin films [1] has revived the long-standing interest in nickel oxides that has persisted since the discovery of high-temperature superconductivity in copper oxides. The superconducting state of (Nd,Sr)NiO₂ manifests itself when the formal 3d electron count of Ni is 8.8, which is equivalent to that of Cu in copper oxides, suggesting an intimate relation between the electron state and superconductivity. In fact, a subsequent study reported the observation of superconductivity in a Nd₆Ni₅O₁₂ thin film, which has the same formal electron count [2]. Stimulated by these observations, we fabricated fluorinated (Nd,Sr)₂NiO₄ and studied their properties. The formal electron configuration of Ni in the parent compound Nd₂NiO₄ is 3d⁸. That means that if one of the four oxygen atoms is replaced by fluorine, the electron configuration would be 3d⁹ and substitution of Sr should lead to a 3d electron count of 8.8.

The fluorinated samples were fabricated via a three-step process based on a report on La₂NiO₃F [3] but with an improvement to reduce the amounts of impurity. Both bulk and thin film samples were studied, starting with the fabrication of (Nd,Sr)₂NiO₄ phase samples. The bulk samples were synthesized via solid-state reaction, while the thin films by pulsed laser deposition (PLD) on SrTiO₃ or LaAlO₃ substrates. These samples were fluorinated to yield (Nd,Sr)₂NiO₃F₂ using ZnF₂ as a fluorinating agent. (Nd,Sr)₂NiO₃F was then obtained by a topochemical reduction of (Nd,Sr)₂NiO₃F₂ using CaH₂ as a reducing agent. The obtained samples were single-phase without apparent impurity phases as was confirmed by X-ray diffraction. The temperature dependence of the magnetic susceptibility of bulk Nd₂NiO₃F revealed a magnetic phase transition at around 160 K, closely resembling the behavior observed in La₂CuO₃F₂, which possesses the same 3d⁹ electron configuration [4]. Therefore, it is plausible to assume that the electronic structure of Nd₂NiO₃F is similar to La₂CuO₃F₂ as expected. However, the resistivity of the Nd₂NiO₃F thin film increased with decreasing the temperature and showed an insulating behavior. Further, as of now, superconductivity has not been observed including in the Sr-doped samples that were fabricated using a similar approach. Further investigation to understand the reason behind the absence of superconductivity in (Nd,Sr)₂NiO₃F is ongoing as clarifying this aspect could provide valuable insights into the mechanism of high temperature superconductivity in copper oxides.

[1] D. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita and H. Y. Hwang, Nature 572, 624 (2019).
[2] G. A. Pan, D. Ferenc Segedin, H. LaBollita, Q. Song, E. M. Nica, B. H. Goodge, A. T. Pierce, S. Doyle, S. Novakov, D. Córdova Carrizales, A. T. N’Diaye, P. Shafer, H. Paik, J. T. Heron, J. A. Mason, A. Yacoby, L. F. Kourkoutis, O. Erten, C. M. Brooks, A. S. Botana and J. A. Mundy, Nat. Mater. 21, 160 (2021).
[3] K. Wissel, A. M. Malik, S. Vasala, S. Plana-Ruiz, U. Kolb, P. R. Slater, I. da Silva, L. Alff, J. Rohrer and O. Clemens, Chem. Mater. 32, 3160 (2020).
[4] J. Jacobs, M. A. L. Marques, H.-C. Wang, E. Dieterich and S. G. Ebbinghaus, Inorg. Chem. 60, 13646 (2021).

Keywords: nickel oxide, fluoridization, topochemical reduction