  The results of the synthesis and characterization of the optimally doped
(La)1.4(Sr1-yCay)1.6Mn2O7 solid solution with y=0, 0.25 and 0.5 are reported.
By progressively replacing the Sr with the smaller Ca, while keeping fixed the
hole-concentration due to the divalent dopant, the 'size effect' of the cation
itself on the structural, transport and magnetic properties of the bilayered
manganite has been analysed. Two different annealing treatments of the solid
solution, in pure oxygen and in pure argon, allowed also to study the effect of
the oxygen content variation. Structure and electronic properties of the
samples have been investigated by means of X-ray powder diffraction and X-ray
absorption spectroscopy measurements. Magnetoresistivity and static
magnetization measurements have been carried out to complete the samples
characterization. Oxygen annealing of the solid solution, that showed a limit
for about y=0.5, induces an increase of the Mn average valence state and a
transition of the crystal structure from tetragonal to orthorhombic while the
argon annealing induces an oxygen under-stoichiometry and, in turn, a reduction
of the Mn average valence state. Along with the Ca substitution, the
Jahn-Teller distortion of the MnO6 octahedra is reduced. This has been directly
connected to a general enhancement of the transport properties induced by the
Ca-doping. For the same cation composition, oxygen over-stoichiometry leads to
higher metal-insulator transition temperatures and lower resistivity values.
Curie temperatures (TC) reduce by increasing the Ca-doping. The lower TC for
all the annealed samples with respect to the 'as prepared' ones are connected
to the strong influence on the magnetic interaction of the point defects due to
the oxygen content variation.
