  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.
