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

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates R₄Ni₃O₁₀ (R = La, Pr) were successfully grown using an optical-image floating zone furnace under oxygen pressure (pO₂) of 20 bar for La₄Ni₃O₁₀ and 140 bar for Pr₄Ni₃O₁₀. 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 R₄Ni₃O₁₀ (R = La, Pr) crystallizes in the monoclinic P2₁/a (Z = 2) space group at room temperature, and that a metastable orthorhombic phase (Bmab) can be trapped by postgrowth rapid cooling. Both La₄Ni₃O₁₀ and Pr₄Ni₃O₁₀ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of Pr₄Ni₃O₁₀, the MMT is found at 157.6 K. For La₄Ni₃O₁₀, the MMT depends on the lattice symmetry: 147.5 K for Bmab vs 138.6 K for P2₁/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 P2₁/a phases, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.