Realization of a classical Ruddlesden Popper type bilayer nickelate in Sr$_3$Ni$_{2-x}$Al$_{x}$O$_{7-\delta}$ with unusual Ni$^{4+}$

TL;DR

This study reports the synthesis and characterization of Sr$_3$Ni$_{2-x}$Al$_{x}$O$_{7-}$ crystals, revealing a high oxidation state of Ni$^{4+}$ and magnetic properties, contributing to the understanding of bilayer nickelates related to high-temperature superconductors.

Contribution

We successfully stabilized the Ruddlesden-Popper $n=2$ phase in Sr$_3$Ni$_{2-x}$Al$_{x}$O$_{7-}$ and characterized its structure, oxidation state, and magnetic behavior, expanding the family of bilayer nickelates.

Findings

High oxidation state of Ni$^{4+}$ in the compound

Insulating behavior observed in transport measurements

Magnetic transition around 12 K

Abstract

The discovery of 80 K superconductivity in bilayer La$_3$Ni$_2$O$_7$ at pressures greater than 14 GPa presents a unique opportunity to study a novel class of high-temperature superconductors. Therefore, other bilayer nickelate following the classical ($T^{4+}$) Ruddlesden-Popper (RP) series of Sr$_3$Ni$_2$O$_7$ would present an interesting new candidate. In this work, we study the stabilization of RP $n=2$ phase in Sr$_3$Ni$_{2-x}$Al$_{x}$O$_7$, via floating zone growth of crystals. With powder and single crystal XRD, we study the stability range of the RP-type phase. Our Thermogravimetric Analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas extraction studies reveal a remarkably high oxidation state of Ni$^{4+}$. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K.