Floating zone growth at high oxygen pressures in Ruddlesden-Popper bilayer nickelate Y$_{y}$Sr$_{3-y}$Ni$_{2-x}$Al$_{x}$O$_{7-δ}$

TL;DR

This work demonstrates that Y-doping in Sr-based RP nickelates, grown by optical floating-zone methods under elevated O2, stabilizes RP n=2 phases and yields large, high-quality crystals suitable for neutron studies. DFT indicates a metallic, Sr-doped Sr_3Ni_2O_7-like electronic structure but experimental transport shows semiconducting behavior that is dramatically enhanced by Y incorporation, with resistivity dropping by up to ~10^6 and band gaps narrowing from ~1250 K to ~103 K depending on composition. Comprehensive structural, chemical, magnetic, and spectroscopic characterization confirms phase-pure RP n=2 crystals, high Ni oxidation states, AFM order below ~11–15 K, and orthorhombic Immm reductions upon topochemical treatment, highlighting the role of oxygen content and cation ordering in these systems. The results establish a dopant-tuned route to ambient-pressure RP nickelates and point to a feasible path toward superconductivity with minimized Ni-site disorder, while large crystals enable future neutron scattering and ligand-hole investigations.

Abstract

With the discovery of superconductivity under pressure in the Ruddlesden-Popper (RP) bilayer La$_3$Ni$^{2.5+}_2$O$_7$ and trilayer La$_4$Ni$^{2.66+}_3$O$_{10}$, a new field of nickelate superconductors opened up. In this respect, Sr-Ni-O RP-type phases represent alternative systems that exist with partial cation substitution. We demonstrate that by Y-doping in Sr$_{3}$Ni$_{2-x}$Al$_{x}$O$_7$ (SNAO), as Y$_{y}$Sr$_{3-y}$Ni$_{2-x}$Al$_{x}$O$_7$ (YSNAO), the drawback of an insulating ground state is overcome, and a significant decrease in resistivity is achieved with crystals exhibiting semiconducting behavior. We employ optical floating zone (OFZ) growth at 10 bar oxygen partial pressure to explore the phase formation in a narrow region of Y-Sr-Ni-Al-O and investigate via DFT the general stability of the pure Sr-Ni-O scenario. Using extensive diffraction and spectroscopy, as well as transport and magnetization measurements, the structural, chemical, electrical, and magnetic properties of the as-grown and reduced compounds were investigated. The optimal growth of YSNAO allows for large high quality crystals suitable for neutron studies. In the Al-free growth, a known $n=1$ RP system with Sr$_{1.66}$Y$_{0.33}$NiO$_{4-δ}$, from which the first single crystals were obtained, was further confirmed, opening the door for future exploration of \textit{A}-site substituted RP-type phases without Ni-site disorder.

Figures (10)

• Figure 1: (a) Orbital-projected band structure for Ni $3d$- and O $2p$-orbitals calculated by DFT. (b) High-symmetry path in the Brillouin zone of $\mathrm{Sr_3Ni_2O_7}$. (c) Ternary phase diagram of Sr-Ni-O$_2$ based on stability calculations. Stable (unstable) phases are marked as green (red) points.
• Figure 2: Rietveld fit of the room-temperature PXRD pattern of $\mathrm{Y}_{y}\mathrm{Sr}_{3-y}\mathrm{Ni}_{2-x}\mathrm{Al}_{x}\mathrm{O}_{7-\delta}$. For each plot, the solid black line corresponds to the calculated intensity from the Rietveld refinement, the solid gray line is the difference between the experimental and calculated intensities, and the vertical blue/green bars are the calculated Bragg peak positions.
• Figure 3: Chemical stability of the Y$_{y}$Sr$_{3-y}$Ni$_{2-x}$Al$_{x}$O$_7$ series, with the Al (Y) content $x$ ($y$) as the horizontal (vertical) axis. At the axis, i.e. with finite $y$ at $x = 0$, the defect Ruddlesden-Popper $n = 1$ type-phase Y$_{0.33}$Sr$_{1.66}$NiO$_{3.75}$ (or YSr$_5$Ni$_3$O$_{11}$) and NiO are formed. For $0 < x < 0.166$ and $0.5 < y \leq 1$ (grey region), a mixture of the two phases is found. Notably, the narrow green stripe marks the Y-free area, where Sr$_9$Ni$_7$O$_{21}$ is the phase mixing with $n=2$Yilmaz2024, while in the blue range, phase pure $n = 2$ RP-type Y$_{0.5}$Sr$_{2.5}$Ni$_{2-x}$Al$_{x}$O$_7$ is obtained (see Figure \ref{['XRD']}).
• Figure 4: (a) Picture of the OFZ grown boule of $\mathrm{Y}_{0.5}\mathrm{Sr}_{2.5}\mathrm{Ni}_{1.75}\mathrm{Al}_{0.25}\mathrm{O}_{7-\delta}$, and its single crystal XRD zonal maps along (b) $0kl$, (c) $h0l$ and (d) $hk0$. (e) Overview STEM-HAADF image of a region of $64\times64$ nm from the same crystal shows a defect-free sample. (f) High-magnification image showing the presence of a single stacking order, consisting of bilayer perovskite slabs. The inset corresponds to the iFFT of the image in (e), analogous to (b). (g) Enlarged region within (f) shows the clear RP-type $n=2$ stacking, where the larger (smaller) intensity spots correspond to atomic columns of Sr/Y (Ni/Al). The structural model is overlaid for reference.
• Figure 5: Orientation and size of the obtained single crystals. (a) Image of broken single crystals with the typical orientation indexed on one. The corresponding Laue XRD images of cleaved (b) 001 direction (c) 110 direction and (d) the high symmetry but not visible 100 direction.