Developments in Ceramic Materials Research

130
José M. Rojo, José L. Mesa and Teófilo Rojo
The results confirm the presence of a three-dimensional magnetic ordering in both
phases. Nevertherless, the different positions of the magnetic peaks for both compounds
reveal than the magnetic structures are different. All magnetic peaks were indexed with a
propagation vector k = (0,0,0) referenced to the room temperature unit cells, indicating that
both the magnetic and nuclear lattices are similar.
The possible magnetic structures compatible with the Ia (Mo phase) and Cc (Fe phase)
equivalent space groups have been evaluated with the help of Bertaut’s macroscopic theory
[53] that allows one determine the constraints imposed by the symmetry of the crystal
structure between the orientation of each magnetic moment belonging to the same general
crystallographic position. Taking into account the restraints, the different possible orientation
of the magnetic moments along the three directions x, y and z were calculated. The agreement
between the observed and calculated diffraction patterns for each possible magnetic structure
has been tested. The best agreement for both compound was obtained with the component My
= 0, indicating that the magnetic moments lie in the (010) plane.
The fits of the D2B patterns at 2 K for M(PO3)3 (M= Mo, Fe) are plotted in Figure 27.
For the Mo(PO3)3 phase the refined magnetic components are Mx = 1.30(9) μB and Mz =
1.37(6) μB per ion (referenced to the Ia unit cell), with a resultant magnetic moment of M =
1.76(6) μB per Mo(III) ion, close to that obtained from magnetic measurements (1.78 μB at 2
K). The nuclear and magnetic discrepancy factors in the final Rietveld refinements of
Mo(PO3)3 are Rp= 5.69, Rwp= 7.17, χ 2 = 2.74, RBragg= 4.57 and Rmag= 16.3. For the Fe(PO3)3
compound, the components of the refined magnetic moments are Mx = 5.26(8) μB and Mz =
2.2(1) μB with a resultant magnetic moment per Fe(III) ion of 4.30(6) μB. The discrepancy
factors of the refinement are Rp = 8.21, Rwp = 9.95, χ 2 = 5.56, RBragg= 14.2 and Rmag= 16.2.
The results for Fe(PO3)3 are similar to those obtained by Elbouaanani et al. [15] (M= 4.42(3)
μB). After considering the effects of the covalence in the Fe-O bonds, the ordered moment for
a six-coordinated Fe(III) cation should be expected to lie between 3.9 and 4.5 μB [56] which
is in good agreement with those obtained for other Fe-O-P systems [52,57].
The resulting magnetic structures of M(PO3)3 (M= Mo, Fe) are shown in Figure 28 where
the unit cell is represented in the standard Cc space group for comparison (solid lines). In
both magnetic structures the moments are overall antiferromagnetically ordered with the spins
ferromagnetically disposed along the y direction. The main difference between the magnetic
structures of both compounds is the orientation of the spins in the x, z axes. In molybdenum
phase, the magnetic moments are ferromagnetically coupled along the y and z directions
whereas those corresponding to the x direction are antiparallel aligned [see Figure 28(a)].
However, for Fe(PO3)3 the magnetic model shows a ferromagmetic arrangement along y and x
directions, with an antiferromagnetic coupling in the z axis [see Figure 28(b)]. The
components are compensed leading to a three-dimensional (3D) antiferromagnetic ordering
than can be described as formed by ferromagnetic layers [in the (100) and (001) planes for the
molybdenum and iron metaphosphates, respectively] disposed antiparallel between them.
These arrangements imply dominant antiferromagnetic superexchange interactions between
moments from one metal atom with its neighbors via (PO4) tetrahedra. However, the models
can not be accurate in any details because they do not explain the field dependence of the
magnetic susceptibility below 10 K, observed in the iron phase.