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 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 nickelates 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− 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 Ni 4+ stabilized by chemical strain from Al. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K.