Developments in Ceramic Materials Research

124
José M. Rojo, José L. Mesa and Teófilo Rojo
7. LOW-TEMPERATURE NEUTRON DIFFRACTION
7.1. B and C type Cr(PO3)3 Metaphosphates
The neutron diffraction petterns at 1.8 K for B and C chromium metaphosphates were
collected in the D2B powder diffractometer. The patterns clearly exhibit extra magnetic peaks
indicating that both compounds are magnetically ordered at 1.8 K. The thermal evolution for
Cr(PO3)3 was followed in the D1B instrument form 1.8 to 10 K. The results confirm the
presence of a three-dimensional order. The D1B diffraction pattern at 1.8 K shows a better
resolution of the magnetic reflections. The magnetic reflections exhibit low intensities due to
the low ordering magnetic temperatures observed in these compounds. All magnetic peaks
were indexed with a propagation vector k = (0,0,0) referenced to the RT unit cells, indicating
that both the magnetic and nuclear unit cells are similar. The possible magnetic structures
compatible with Cc and P21/a crystal symmetries for Cr(PO3)3 and Cr2(P6O18), respectively,
have been deduced with the help of Bertaut’s macroscopic theory [53], which allows one to
determine the symmetry constraints between each magnetic moment of the Cr(III) ions
belonging to the same general crystallographic position. The ordering of the magnetic
moments compatible with the symmetry operations was determined and the agreement
between the observed and calculated diffraction patterns for each possible magnetic structure
was evaluated.
7.1.1. C-Type Cr(PO3)3
The magnetic reflections observed for Cr(PO3)3 can be indexed on the basis of the crystal
unit cell and obey the rules (hkl) wit h+k = 2n (i.e., C-centered mode remains) and (h0l) with l
= 2n+1 (i.e., forbidden by the c glide plane). It is worth mentioning than some of the nuclear
reflections could have magnetic contributions to their intensities. The magnetic atoms occupy
three general 4a different crystallographic sites. The positions in the cell of the different
atoms belonging to the Cr(1), Cr(2) and Cr(3) sublattices are numbered 1a, 1b, 1c, 1d; 2a, 2b,
2c 2d and 3a, 3b, 3c, 3d, respectively.
Using the method developed by Bertaut and taking into account the restraints imposed by
symmetry elements of the space group Cc along the three directions x, y and z, the different
possible orientations of the magnetic moments were evaluated. The refinement of the
components of the magnetic moments gives My = 0, indicating than they lie in the (010)
plane. The best fit of the D1B experimental patterns for Cr(PO3)3 at 1.8 K is plotted in Figure
21. The components of the refined magnetic moments of Cr(1), Cr(2) and Cr(3) are,
respectively, Mx = 1.7(2) μB and Mz = 1.1(1) μB per chromium ion with a resultant magnetic
moment of 2.58(9) μB per Cr(III) ion. The nuclear and magnetic discrepancy factors are Rp =
3.22, Rwp = 4.58, χ 2 = 18.3, RBragg = 4.35 and Rmag = 23.3. The magnetic structure of Cr(PO3)3
is shown in Figure 22. The results are similar to those observed for the Mo(PO3)3 phase,
which was resolved in the Ia unit cell, and some chromium phosphates in which the magnetic
moments are near 2.4 μB [54,55].
The magnetic structure of the C-type Cr(PO3)3 metaphosphate is consistent with the
presence of (CrO6) octahedra where the magnetic moments are antiferromagnetically ordered
in the x direction whereas those along the y and z directions show parallel alignment (see
Figure 22).