Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 111
In all cases isotropic signals centered around g= 2 were observed. These signals are
characteristic of Fe(III) cations in octahedral environment without large tetragonal distortion.
The room temperature spectrum (g= 1.99) fits exactly the characteristic of a Lorentzian curve
being the same result obtained for all spectra registered down 10 K. Below 10 K the fits are
not considerably good. The temperature dependence of the intensity and linewidth of the
signal calculated by simulation of the experimental spectra to Lorentzian curves is displayed
in Figure 8a. The g values remain apparently unchanged in all the temperature range studied.
The intensity of the signal increases with decreasing temperature, reaches a maximum at
about 12 K and after that rapidly decreases [see Figure 8b]. In fact, at 4.2 K no relevant signal
could be observed. As can be deduced from the straight line that fits the 1/I vs. T curve at
high temperatures the magnetic susceptibility of the compound follows a Curie-Weiss law
and no short order effects are detectable above 30 K.
As long as the high temperature conditions (kT>> Hz, Hex, Hdip,…) is expected to be
satisfied than the line width observed has low temperature dependence. Only a small increase
is observed between room temperature and 50 K probably due to the dipolar homogeneous
broadening. Furthermore, when temperature is decreased the linewidth increases rapidly
reaching a maximum at 8 K. This behavior is characteristic of systems with a threedimensional
order and, in this case, it must be of antiferromagnetic nature considering the
thermal evolution of the intensity of the signal [43,44]. The apparent temperature
independence of the resonance field is also in good agreement with a three-dimensional
behavior with quasi-isotropic interactions. In any case, the presence of three different Fe(III)
sites and the relatively large linewidth (430-1050 Gauss) imply that the conclusions observed
from the powder spectra must be considered with caution. In fact, the observed isotropic
signal is the result of the collapse by exchange of those expected for the three different
Fe(III).
6.1. Magnetic Properties of Ti(PO3)3
6. MAGNETIC PROPERTIES
Variable-temperature magnetic susceptibility measurements, performed on a powder
sample of Ti(PO3)3 from 1.8 to 100 K are shown in Figure 9. The χm -1 vs. T line follows a
Curie-Weiss law in practically all the temperature range studied. The Curie constant value is
Cm= 0.17 cm 3 Kmol -1 , which is notably lower than that expected from the g-values obtained
from the ESR spectra. The calculated value of the Curie-Weiss constant is θ= +0.31 K. The
χmT product at room temperature is 1.17 μB and remains unchanged until 20 K. At lower
temperatures than 20 K a slight increase of the χmT product can be observed. This result
suggests the existence of ferromagnetic interactions in the Ti(PO3)3 compound.
6.2. Magnetic Properties of V(PO3)3
The thermal evolution of the molar magnetic susceptibility for the V(PO3)3 compound is
shown in Figure 10. The susceptibility follows a Curie-Weiss law between 1.8 and 300 K