Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 137
hand, for iron and molybdenum metaphosphates, the higher spin number in Fe(III) (S= 5/2)
than in Mo(III) (S= 3/2) and its magnetically isotropic behavior [41] can gives rise to new
competing interactions. So, the differences observed in the magnetic behavior of the
molybdenum and iron metaphosphates can be attributed to both the slight geometrical
differences and the small magnetic frustration in the (MO6) octahedra together with the
electronic configurations in M(III) (d 3 ) ions with respect to the Fe(III) (d 5 ) ones. The
complexity of the three-dimensional arrangements, the presence of distinct exchange
pathways and the existence of nine distinct phosphorous ions make it impossible to find a
simple magnetic model to explain the differences in the magnetic behavior observed in these
systems.
9. MAGNETOCALORIC EFFECT IN
M(PO3)3 (M= FE, CR AND MO) METAPHOSPHATES
The magnetocaloric effect (MCE) is the heating or the cooling of magnetic materials due
to the varying magnetic field. Actually there is a renewed interest in MCE due to its
possibilities of application in magnetic refrigeration [60]. In this way, the research in the area
of magnetic refrigeration has been concentred in found materials with large magnetocaloric
response capable of operating at different temperatures range, depending on the intended
application. Hear, we presented the magnetocaloric properties of the M(PO3)3 metaphosphates
(M= Fe, Cr and Mo) studied by means of heat capacity under fields up to 90 KOe. Previous
studies of the magnetic properties show that this family orders antiferromagnetically at
temperatures closer to the liquefaction of helium one. At zero field, the heat capacity shows a
well-defined λ-type anomaly centered at 7.8, 4.2 and 4.1 K for M= Fe, Cr and Mo
respectively, which is associated to the presence of long-range magnetic order. With
increasing magnetic fields, initially the peak becomes broader and shifts to lower
temperatures; confirming the antiferromagnetic character of the transition. However, in the Cr
and Mo compounds, at higher field a broad maximum emerges above the Néel temperature
indicating a transformation to ferromagnetic behavior. We have found than both the
isothermal magnetic entropy (ΔS) and the adiabatic temperature change (ΔT), display a peak
around the Néel temperature in the three studied materials. At low fields the MCE is negative
(ΔS>0 and ΔT< 0) proving the antiferromagnetic order, however by increasing the field the
MCE become positive. In particular, the stronger MCE has been found in Cr(PO3)3 which
exhibits a maximum value of ΔS= -5.4 J/Kmol and ΔT= 11.5 K at μ0H= 90 KOe. These
values are larger considering the small magnetic moment of the Cr(III) (S= 3/2) and show that
Cr(PO3)3 is a good candidate to be used in magnetic refrigeration at low temperatures.
10. CONLUDING REMARKS
The M(PO3)3 (M= Ti, V, Cr, Mo and Fe) metaphosphates have been synthesized by the
ceramic method at 800 °C in air, except the titanium phase which synthesis required a inert
nitrogen atmosphere. The IR spectra show bands characteristic of metaphosphates groups
with chain or ring structure, this later observed for the chromium(III) hexametaphosphate.