Several polynuclear iron(III)-o-phthalates precipitated from acid and
alkaline solutions were characterized by chemical and thermogravimetri
c analysis, and infrared, Mossbauer and magnetic measurements. Compoun
ds prepared in acid solutions contain the trimer unit Fe3O and, althou
gh different in chemical composition and in thermal decomposition beha
viour, show very similar IR, Mossbauer and magnetic characteristics. T
he variable temperature magnetic susceptibility measurements of these
compounds were fitted to a theoretical expression derived from a spin
Hamiltonian, taking into account two different exchange pathways along
inequivalent sides of an isosceles triangle, yielding J(12)/cm(-1) =
-35.4 and J(13)/cm(-1) = -21.1. The Mossbauer spectra of this group of
samples could be satisfactorily fitted by considering the superpositi
on of two quadrupole split doublets with different relative areas. Sam
ples prepared in highly alkaline solution show a characteristic band i
n the infrared spectra at 890 cm(-1), assigned to the asymmetric Fe-O-
Fe stretch. Their Mossbauer spectra taken in the temperature region 77
-300 K reveal superparamagnetic behaviour of small particles or cluste
rs. Magnetic susceptibility measurements of these polynuclear iron(III
) species show T-max/K = 10 and 25, respectively. Zero field-cooled, f
ield-cooled and torque measurements were used to prove their spin glas
s behaviour in the low temperature region.