The new phases, Na4M3(PO4)2(P2O7) (M = Mn, Co, Ni), have been synthesized by solid-state reactions. Single crystals of Na4M3(PO4)2(P2O7) (M = Mn, Ni) have been isolated and
their structure has been determined by X-ray diffraction techniques using as the starting
model the structure of the isostructural compound Na4Co3(PO4)2(P2O7). These compounds
crystallize in the orthorhombic noncentrosymmetric space group Pn21
a with a = 17.991(3)
Å, b = 6.6483(1), and c = 10.765(2) Å for the manganese compound and a = 17.999(2), b =
6.4986(6) Å, and c = 10.4200(9) Å for the nickel compound, with Z = 4. Magnetic
measurements reveal the existence of antiferromagnetic interactions in the nickel compound.
The manganese and cobalt compounds show canting antiferromagnetic behavior at low
temperatures. Magnetic correlation is also studied from the analysis of possible superexchange pathways in the structure. The ionic conductivity, due to Na+ ions, is measured for
the three compounds. The activation energy is nearly the same (0.81−0.86 eV), but the
conductivity at 300 °C changes, increasing from 2.1 × 10-7 S cm-1 for the Ni compound to
1.3 × 10-6 S cm-1 for the Co one, and then from this value to 2.7 × 10-5 S cm-1 for the Mn
compound.
By using a domestic microwave furnace and depending on the nickel precursor used, either tetrahydrated
nickel acetate or dihydrated nickel formate, different nanosized materials are obtained: Ni/NiO composites,
Ni metal, or NiO. The acetate leads to core−shell composites: Ni on the outside and NiO on the inside,
while the formate behaves oppositely, yielding the metal that is progressively oxidized in air, the shell
being in this case NiO. All these materials show ferromagnetic behavior.
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