High temperature transitions in Ruddlesden-Popper nickelates La${}_{n+1}$Ni${}_n$O${}_{3n+1}$

The discovery of superconductivity at $15\mathrm{K}$ in the infinite-layer nickelate $(\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{r}){\mathrm{N}\mathrm{i}\mathrm{O}}_2$, followed by superconductivity at $80\mathrm{K}$ in the Ruddlesden–Popper phase ${\mathrm{L}\mathrm{a}}_3{\mathrm{N}\mathrm{i}}_2{\mathrm{O}}_7$, has ushered in a new era of nickelate research. Despite this progress, large discrepancies between reports exist. Here, we investigate the complete series of bulk-stable ${\mathrm{L}\mathrm{a}}_{n+1}{\mathrm{N}\mathrm{i}}_n{\mathrm{O}}_{3n+1}$ compounds using a comprehensive set of experimental techniques, including PXRD, single-crystal XRD, electron microscopy, heat capacity, differential scanning calorimetry, magnetic susceptibility, and transport measurements, over a broad temperature range from $2$ to $1000\mathrm{K}$. By studying high-quality single crystals, we identify a previously underappreciated high-temperature phase transition in Ruddlesden–Popper nickelates ${\mathrm{L}\mathrm{a}}_{n+1}{\mathrm{N}\mathrm{i}}_n{\mathrm{O}}_{3n+1}$ distinct from the one going to a tetragonal phase.