High oxygen pressure floating zone growth and crystal structure of the metallic nickelates $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$)

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 $\left(p{\mathrm{O}}_2\right)$ 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.

Figure 1

Rapid cooling products from a melt of ${\mathrm{La}}_2{\mathrm{O}}_3:\mathrm{NiO}=2:3$ at various oxygen pressures. (a) Powder x-ray diffraction patterns vs $p{\mathrm{O}}_2$ with standard patterns of ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ ($n=1$, 2, 3, and ∞) from the ICDD database shown as tick marks below the data. (b) Schematic drawing of empirical phase predominance as a function of $p{\mathrm{O}}_2$.

Figure 2

High-energy synchrotron x-ray single-crystal diffraction from an as-grown ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ crystal. (a) Photograph of as-grown boule with crystal growth direction parallel to the $ab$ plane. (b) Cleaved ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ crystals from (a). (c) a ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ crystal ∼4.5 mm in length attached on Kapton tape and its diffraction patterns at various positions (d)–(n) ($\lambda =0.117\AA$, 11-ID-C, APS; beam size $0.8\times 0.8\mathrm{m}{\mathrm{m}}^2$, scan step size 0.5 mm). Note that diffuse scattering is observed in the $hk0$ plane, reflecting some short-range deviation from the average structure. The vertical lines in (e)–(l) are artifacts caused by overexposure. The observation of multiple spots close together signifies twinning.

Figure 3

Physical properties of $P{2}_1/a$, Bmab ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, and ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (a), (d), (g) Resistivity, magnetic susceptibility, and heat capacity of $P{2}_1/a {\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (b), (e), (h) Resistivity, magnetic susceptibility, and heat capacity of Bmab ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (c), (f), (i) Resistivity, magnetic susceptibility, and heat capacity of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$.

Figure 4

Crystal structures and coordination environments of Ni atoms in $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La}$, Pr) from single-crystal diffraction. (a)–(c) Bmab ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$; (d)–(f) $P{2}_1/a {\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$.

Figure 5

Heat capacity of two specimens of as-grown biphasic ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ before and after annealing. See text for details.

Figure 6

High-resolution synchrotron x-ray powder diffraction pattern and Rietveld refinement of biphasic ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. Insets show the quality of fit in the $Q$ range $2.255–2.277{\AA }^{-1}$.

Figure 7

Temperature dependence of lattice parameters of ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ from 100 to 923 K obtained from Rietveld refinement of high-resolution synchrotron powder x-ray diffraction patterns. (a) $a$ and $b$ axes of $P{2}_1/a$ and Bmab ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (b) $c$ axis. (c) β of $P{2}_1/a$. (d) Volume. (e) Thermal expansion coefficients obtained from refined lattice parameters. Note the lattice parameters were obtained from two samples, one measured from 100 to 300 K, and the other measured from 321 to 923 K.

Figure 8

Annealing effect on biphasic ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (a) High-resolution synchrotron x-ray powder diffraction pattern of as-grown ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. (b) Annealed ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ at 1 bar ${\mathrm{O}}_2$. Insets show the quality of fit in $Q$ range $2.255–2.277{\AA }^{-1}$. (c) Comparison of the peaks before and after annealing in the range of $2.25\le Q\le 2.285{\AA }^{–1}$ by normalizing their heights.