Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 133
(b)
Figure 29. Thermal evolution of (a) neutron diffraction patterns, and (b) the ordered magnetic moments
[M, Mx and Mz] for the Fe(PO3) 3compound.
On the other hand, the neutron diffraction patterns do not show any atypical thermal
evolution at temperatures less than TN = 8 K. Consequently, the magnetic anomalies observed
in the specific heat study can not be attributed to the presence of magnetic transitions.
8. MAGNETO-STRUCTURAL CORRELATIONS
The magnetic interactions in the B and C chromium metaphosphates and iron and
molybdenum ones are only propagated via (PO4) tetrahedra (Cr-O-P-O-Cr) and can be
described as super-superexchange [58] interactions owing to the exchange M-O-M
interactions are absent in all the compounds. The super-superexchange magnetic couplings
are always antiferromagnetic and are characterized by both the O-P-O angles and the M-O
bond lengths. The shorter metal-metal distances range between 5.0 and 6.2 Å (see Table 4)
and consequently direct interactions are negligible. The magnetic exchanges are more
effective for the shorter M-O distances and for an angle near 109° (tetrahedral angle) [51].
For these compounds, the metal-oxygen distances are approximately 2.0 Å and the O-P-O
angles are quite different (110-120°) from those of tetrahedral geometry. In this sense, the
weak antiferromagnetic interactions observed in both chromium compounds are a
consequence of the large distances between the M(III) ions and the open angles in the M-O-P-
O-M pathways.
Schematic views of the most important exchange pathways in the B and C chromium
types metaphosphates are given in Figure 30. For Cr(PO3)3, [see Figure 30(a)] the chromiumchromium
interactions principally occur through one and two (PO4) groups. Nine exchange