  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.
