High oxygen pressure floating zone growth and crystal structure of the layered nickelates R$_4$Ni$_3$O$_{10}$ (R=La, Pr)

Abstract: Single crystals of the metallic Ruddlesden-Popper trilayer nickelates R$4$Ni$_3$O${10}$ (R=La, Pr) were successfully grown using an optical-image floating zone furnace under oxygen pressure (pO$2$) of 20 bar for La$_4$Ni$_3$O${10}$ and 140 bar for Pr$4$Ni$_3$O${10}$. A combination of synchrotron and laboratory x-ray single crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that R$4$Ni$_3$O${10}$ (R=La, Pr) crystallizes in the monoclinic $P$2$1$/$a$ (Z=2) space group at room temperature, and that a metastable orthorhombic phase ($Bmab$) can be trapped by post-growth rapid cooling. Both La$_4$Ni$_3$O${10}$ and Pr$4$Ni$_3$O${10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of Pr$4$Ni$_3$O${10}$, the MMT is found at ~157.6 K. For La$4$Ni$_3$O${10}$, the MMT depends on the lattice symmetry: 147.5 K for $Bmab$ vs. 138.6 K for $P$2$_1$/$a$. Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between $Bmab$ and $P$2$_1$/$a$ phases, demonstrates a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.