Absence of Hydrogen in Highly Crystalline Superconducting Infinite Layer Nickelates

Infinite-layer nickelates are promising candidates for studying unconventional superconductivity because of their electronic and structural comparison with the cuprates [1]. Superconductivity in the nickelates was first realized in strontium-doped neodymium nickelate (Nd,Sr)NiO₂, which has since sparked a plethora of studies investigating Ni-based compounds [2,3]. The synthesis of infinite-layer nickelate thin films (<i>R</i>NiO₂, <i>R</i> = lanthanide), is a two-step process: first involving the growth of the perovskite precursor phase followed by the deintercalation of apical site oxygen atoms via topotactic reduction [4,5]. The topotactic structural transition is commonly achieved using calcium hydride (CaH₂) as a reducing agent [6,7]. It remains debated, however, whether the use of calcium hydride results in the insertion of hydrogen into the infinite layer structure [8–10]. To determine whether hydrogen is present in the infinite layer nickelates, we performed secondary ion mass spectroscopy (SIMS) measurements on optimally doped (Nd,Sr)NiO₂ thin films to quantify the hydrogen content resulting from calcium hydride reduction. We find that hydrogen does not play a critical role for superconductivity in nickelates, and that the presence of hydrogen is a result of poor crystallinity in non-optimally synthesized thin films.<br/><br/><b>References</b><br/>[1] A. S. Botana and M. R. Norman, Phys. Rev. X 10, 011024 (2020).<br/>[2] D. Li <i>et al.</i>, Nature 572, 624 (2019).<br/>[3] X. Zhou <i>et al.</i>, Mater. Today S1369702122000591 (2022).<br/>[4] Z. Yang <i>et al.</i>, Small 2304146 (2023).<br/>[5] W. Wei <i>et al.</i>, Sci. Adv. 9, eadh3327 (2023).<br/>[6] K. Lee <i>et al.</i>, APL Mater. 12 (2020).<br/>[7] M. Osada <i>et al.</i>, Phys. Rev. Mater. 7, L051801 (2023).<br/>[8] X. Ding <i>et al.</i>, Nature 615, 50 (2023).<br/>[9] L. Si <i>et al.</i>, Phys. Rev. Lett. 124, 166402 (2020).<br/>[10] P. Puphal <i>et al.</i>, Front. Phys. 10, 842578 (2022).