  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.
