Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 121
1.78 μB at 2K for the Mo phase and from 5.58 μB to 2.52 μB for the Fe compound, is
observed. The estimated Debye (θD) temperature for the chromium phase is 436 K which is
higher than those of M(PO3)3 (M= Mo, Fe) phases due to the smaller atomic weight. These
results are in good agreement with those obtained from susceptibility measurements of
Cr(PO3)3 where a weak decrease in the χmT vs. T curve was observed (3.83 μB at 300 K and
1.55 μB at 2 K).
Table 3. Summary of specific-heat data of the M(PO3)3 (M III = Cr, Mo and Fe)
metaphosphates
a The experimental data can not be fitted to a logarithmic expression for T < TN.
As can be see in Table 3, the fit near the ordering temperature (TN= 4.2 K) for Cr(PO3)3
follows the equation Cm= α –βLn|T – TN|, equation [1], given by Kopinga et al [50] (see inset
in Figure 17) allowed us to obtain α= 4.16JK -1 and β= 2.12 JK -1 mol -1 . Considering the
similar λ-type maximum observed in the heat capacity measurements, especially for
Mo(PO3)3, a fit near the ordering temperatures following the same equation [1], above
indicated, was also carried out. The magnetic specific-heat data for Mo metaphosphate plotted
versus Ln|T – TN| for TN= 4 K show linear behavior below and above the ordering
temperature. The values near the transition point, TN, satisfy the equation Cm= 4.37 -2.81Ln|T
– TN| and Cm= 5.01 – 1.54Ln|T –TN| for T < TN and T> TN, respectively. These equations are
described by the solid lines given in the inset of Figure 18(a). In the case of Fe(PO3)3 only the
region above the ordering temperature TN= 7.8 K, can be fitted following the logarithmic
expression Cm= 4.77 – 1.84 Ln|T – TN| (see inset in Figure 18(b)). For T < TN, the
experimental data show a shoulder and therefore can no be fitted with a logarithmic
expression. A temperature dependence of the specific-heat below the ordering temperature