Absence of hydrogen insertion into highly crystalline superconducting infinite layer nickelates

Abstract

The discovery of superconductivity in the infinite layer nickelates introduced a materials system analogous to the cuprates for the study of unconventional superconductivity. The synthesis of infinite layer nickelates, (${\mathrm{RNiO}}_2$, R = lanthanide) often uses calcium hydride (${\mathrm{CaH}}_2$) to facilitate the deintercalation of apical site oxygen atoms from a precursor perovskite (${\mathrm{RNiO}}_3$) phase via topotactic reduction. However, it remains uncertain whether the use of ${\mathrm{CaH}}_2$ results in the insertion of hydrogen into the infinite layer structure, and if it does, what the implications are for superconductivity. To quantify the hydrogen composition of highly crystalline infinite layer nickelates, we synthesized ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films on LSAT substrates and conducted time-of-flight secondary ion mass spectroscopy measurements to generate hydrogen depth profiles. We compare the hydrogen density of nickelates prepared with and without a ${\mathrm{SrTiO}}_3$ capping layer. Additionally, we measure the hydrogen content in nickelate samples at various doping levels spanning the superconducting phase space, including the underdoped, optimally doped, and overdoped regime. We report no significant increase in hydrogen density between the perovskite and infinite layer phases in any of the measured samples. Furthermore, we put an upperbound on the hydrogen concentration of our nickelate samples to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2{\mathrm{H}}_{0.05}$. Our results imply that hydrogen is not responsible for the emergence of superconductivity in the infinite layer nickelates.