Developments in Ceramic Materials Research

138
José M. Rojo, José L. Mesa and Teófilo Rojo
The diffuse reflectance espectra allowed the assignation of the all electronic bands for every
metallic cation. The Dq , B and C parameters were calculated. The ESR spectra were
collected at room temperature and at 4.2 K. For the M(PO3)3 (M= Cr and Mo) the results have
been explained on the basis of the existence of more than one different environment for the
metallic cations, in accordance with the nuclear crystal structure of this phase. The doped
aluminium metaphosphates with Cr(III) and Mo(III) ions indicate the presence of one center
with |D|> hν and two centers with |D|< hν. The ESR spectrum of titanium compound has been
discussed on the basis of a effective trigonal symmetry, being the g-value 1.97. This one of
the vanadium compound exhibits a weak signal attributed to V(IIII) ions, with a zero-field
splitting parameter D estimated between 2 and 8 cm -1 . For the iron(III) metaphosphate the
ESR spectra remains isotropic form RT to 4.2 K, with g= 2.0. Magnetic measurements of the
chromium, molybdenum and iron compounds show antiferromagnetic interactions. The J/k
exchange parameter has been evaluated for the Cr(III) and Fe(III) metaphosphates using a 3D
antiferromagnetic isotropic Heisenberg model. The values obtained are, approximately, 0.3 K.
For the Fe(PO3)3 and Cr2(P6O18) besides has been found a weak ferromagnetic contribution
intrinsic to the sample at low temperature. The Ti(III) is the only metaphosphate for which
has been found a ferromagnetic behavior. The magnetic structures of the of the B and C type-
Cr(PO3)3 and M(PO3)3 (M= Fe, Mo) have been resolved, indicating the existence of
predominant antiferromagnetic superexchange couplings propagated via (PO4) tetrahedra. A
phenomenon of magnetocaloric effect has been found in the iron, chromium and molybdenum
metaphosphates, being the stronger one this observed for the chromium phase, which allows
us to consider this phase a good candidate to be used in magnetic refrigeration at low
temperatures.
ACKNOWLEDGEMENTS
This work has been financially supported by the “Universidad del País Vasco”
(UPV/EHU) (9/UPV00169.310-13494/2001).
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