Realization of a classical Ruddlesden Popper type bilayer nickelate in Sr3Ni2−xAlxO7−δ with unusual Ni4+

Abstract

The discovery of 80 K superconductivity in bilayer La₃Ni₂O₇ at pressures greater than 14 GPa presents a unique opportunity to study a novel class of high-temperature superconductors. Therefore, other bilayer nickelates following the classical (T⁴⁺) Ruddlesden-Popper (RP) series of Sr₃Ni₂O₇ would present an interesting new candidate. In this work, we study the stabilization of RP n = 2 phase in Sr₃Ni_2−xAl_xO_7−δ, 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 Ni⁴⁺ stabilized by chemical strain from Al. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K.