High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R4Ni3O10 ( R=La

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) were successfully grown using an optical-image floating zone furnace under oxygen pressure $(p{\mathrm{O}}_{2})$ of 20 bar for ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ and 140 bar for ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$. 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 ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) 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 postgrowth rapid cooling. Both ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ and ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, the MMT is found at 157.6 K. For ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, 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, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.