Stabilization of Tetragonal Phase and Aluminum-Doping Effect in a Bilayer Nickelate

TL;DR

The study demonstrates ambient-pressure stabilization of the tetragonal $I4/mmm$ phase in the bilayer RP nickelate La$_3$Ni$_{2-x}$Al$_x$O$_{7-\delta}$ by aluminum doping together with post-annealing in moderately high oxygen pressure, guided by the Goldschmidt tolerance factor. Structural refinements show the tetragonal phase emerges for $0.3\le x\le 0.5$, with refined lattice parameters and near-complete occupancy of apical O sites, implying enhanced interlayer coupling and Ni $3d_{x^2- y^2}$–O $2p$ hybridization. Physically, Al doping drives strong carrier localization (well-described by 2D variable-range hopping) and induces local magnetic moments with spin-glass-like freezing, while high-pressure transport reveals that even small Al content ($x\approx0.05$) suppresses superconductivity, and higher Al content leads to semiconducting behavior up to tens of gigapascals. The work highlights the sensitivity of metallicity and superconductivity to nonmagnetic impurities in RP nickelates and provides a route toward exploring ambient-pressure superconductivity by lattice engineering through doping and oxygenation.

Abstract

Recent studies suggest that the tetragonal phase of the Ruddlesden-Popper (RP) bilayer nickelate, La$_3$Ni$_2$O$_7$ or La$_2$PrNi$_2$O$_7$, which is stabilized under high pressures, is responsible for high-temperature superconductivity (HTSC). In this context, realization of the tetragonal phase at ambient pressure could be a rational step to achieve the goal of ambient-pressure HTSC in the nickelate system. By employing the concept of Goldschmidt tolerance factor, we succeed in stabilizing the tetragonal phase by aluminum doping together with post annealing under moderately high oxygen pressure. X-ray and neutron diffractions verify the tetragonal $I4/mmm$ structure for the post-annealed samples La$_3$Ni$_{2-x}$Al$_x$O$_{7-δ}$ (0.3 $\leq x \leq$ 0.5). The Al-doped samples, including the tetragonal ones, show semiconducting properties, carry localized magnetic moments, and exhibit spin-glass-like behaviors at low temperatures, all of which can be explained in terms of charge carrier localization. Furthermore, high-pressure resistance measurements on post-annealed samples reveal that even a low Al doping ($x$ = 0.05) suppresses superconductivity almost completely. This work gives information about the effect of nonmagnetic impurity on metallicity as well as superconductivity in bilayer nickelates, which would contribute to understanding the superconducting mechanism in RP nickelates.

Figures (14)

• Figure 1: Characterization of as-prepared (a-c) and post-annealed (d-f) samples of La$_3$Ni$_{2-x}$Al$_x$O$_{7-\delta}$ ($0\leq x\leq0.5$) by powder X-ray diffractions. In panel (e), the peaks marked by asterisks are attributed to a new phase formed during the annealing (see the text and SI). Plots (c) and (f) show lattice parameters as functions of Al content $x$. The error bars are smaller than the symbols.
• Figure 2: (a) Rietveld refinement profiles of the neutron diffractions for the as-prepared (top) and post-annealed (bottom) samples of La$_3$Ni$_{1.6}$Al$_{0.4}$O$_{7-\delta}$. The insets show bond distances and angles in the NiO$_6$ bilayers. (b) The axial ratios $c/a_\mathrm{p}$ as functions of $c$ for the bilayer and monolayer-trilayer (1313) nickelates. The simple perovskite unit, $a_\mathrm{p}=\frac{1}{2}\sqrt{a^2+b^2}$, is employed to effectively compare the tetragonal phases with orthorhombic ones.
• Figure 3: Temperature dependence of electrical resistivity of the as-prepared (a) and post-annealed (b) samples of La$_3$Ni$_{2-x}$Al$_x$O$_{7-\delta}$. The effect of post annealing is highlighted in panel (c) for $x=$ 0, 0.05 and 0.1.
• Figure 4: Magnetic susceptibility data for the as-prepared (a,b) and post-annealed (c,d) samples of La$_3$Ni$_{2-x}$Al$_x$O$_{7-\delta}$. In panels (a) and (c), a logarithmic scale is used to distinctly show the lower-value data. FC and ZFC denote field cooling and zero field cooling, respectively.
• Figure 5: Temperature dependence of resistance under pressures for La$_3$Ni$_2$O$_{7-\delta}$ (a), La$_3$Ni$_{1.95}$Al$_{0.05}$O$_{7-\delta}$ (b) and La$_3$Ni$_{1.9}$Al$_{0.1}$O$_{7-\delta}$ (c). $T_\mathrm{c}$ is determined as the interception between two linear extrapolations below and above the superconducting transition. The inset in panel (b) shows an enlargement of the $R(T)$ for $x =$ 0.05 sample at 24.8 and 27.9 GPa below 50 K. phases with orthorhombic ones.