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Electron doping of the layered nickelate La$_4$Ni$_3$O$_{10}$ by aluminum substitution: A combined experimental and DFT study

The physical properties of La$_4$Ni$_3$O$_{10}$ with a 2D-like Ruddlesden-Popper-type crystal structure are extraordinarily dependent on temperature and chemical substitution. By introducing Al$^{3+}$ atoms ($x$) randomly at the Ni-sites, the average oxidation state for the two non-equivalent Ni-cations is tuned and adopt values below the average of +2.67 in La$_4$Ni$_3$O$_{10}$. La$_4$Ni$_{3-x}$Al$_x$O$_{10}$ is a solid solution for $x=0.00$ to 1.00, and are prepared by the citric acid method. The samples adopt a slightly distorted monoclinic structure (P21/a), evidenced by peak broadening of the (117) reflection. We report on a remarkable effect on the electronic properties induced by tiny amounts of homogeneously distributed Al-cations, with clear correspondence between resistivity, magnetization, diffraction, and DFT data. DFT shows that electronically there is no significant difference between the non-equivalent Ni atoms and no tendency towards any Ni$^{3+}$/Ni$^{2+}$ charge ordering. The electron doping via Al-substitution has a profound effect on electric and magnetic properties. The resistivity changes from metallic to semiconducting with increasing band-gap at higher Al-levels, consistent with results from DFT. The metal-to-metal transition reported for La$_4$Ni$_3$O$_{10}$, which is often interpreted as a charge density wave, is maintained until $x = 0.15$ Al-level. However, the temperature characteristics of the resistivity change already at very low Al-levels (below 0.03). A coupling of the metal-to-metal transition to the lattice is evidenced by an anomaly in the unit cell dimensions. No long-range magnetic order is detected by powder neutron diffraction. The introduction of the non-magnetic Al$^{3+}$ changes the Ni$^{3+}$/Ni$^{2+}$ ratio and is likely to block double-exchange pathways by means of -Ni-O-Al-O-Ni- fragments into the network of corner shared octahedra with the emergence of possible short-range order in ferromagnetic like islands.