Compounds with the general formula
[MM‘(C3H2O4)2(H2O)4]
(M = Ba, Sr; M‘ = Cu, Mn;
C3H2O4 = malonate)
have been synthesized and characterized. Single-crystal X-ray
diffraction study on the
[SrCu(C3H2O4)2(H2O)4]
compound indicates that it crystallizes in the orthorhombic space
group, Pccn, Z = 4, with unit cell
parameters
a = 6.719(2), b = 18.513(7), and
c = 9.266(4) Å. The structure consists of
distorted octahedral copper(II)
species which are extended along the ac plane forming a
two-dimensional structure. The geometry of the
alkaline-earth ions resembles a distorted antiprism. The other compounds are
isostructural. The EPR spectra of the [MCu(C3H2O4)2(H2O)4]
(M = Ba, Sr) compounds show an orthorhombic g tensor as
consequence of a linear combination
of the axial symmetry and the exchange interactions between
magnetically different centers, but crystallographically
equivalent. For the manganese compounds, the EPR spectra of
polycrystalline samples show that the intensity
of the signal increases with decreasing temperature down to 20 K, and
at lower temperatures the intensity decreases,
becoming silent below 7 K. Magnetic measurements show
two-dimensional (2D) ferromagnetic and antiferromagnetic interactions for the copper and manganese phases, respectively.
In all cases, the susceptibility data
were fitted by the expression for a Heisenberg square-planar system.
The obtained J/k values are 1.44 and 1.15
K, for the SrCu and BaCu compounds, respectively, and −0.65 and
−0.59 K for the SrMn and BaMn compounds,
respectively. For the manganese compounds, magnetic measurements
show a magnetic ordering below 5 K which
confirms the presence of a weak ferromagnetism. Thermal analyses
of the phases show three different
decomposition steps: dehydration, ligand pyrolysis, and evolution of
the inorganic residue for all compounds.
Taking these results into account, we performed further thermal
treatments to obtain mixed oxides. These were
obtained at short reaction times and at temperatures lower than those
of the conventional ceramic method.