Developments in Ceramic Materials Research

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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).
Synthesis, Spectroscopic and Magnetic Studies… 125 Figure 21. Refinement of the Cr(PO 3) 3 (D1B) neutron diffraction profiles at 1.8 K (the intensity is in arbitrary units). The positions of the Bragg reflections for the crystallographic (first row) and magnetic (second row) structures are presented, The fifference curve is plotted at the botton of the figure. c b a Figure 22. Projection of the magnetic structure of Cr(PO 3) 3 onto c. The direction of the electronic spins and the fundamental unit cell are shown.
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DEVELOPMENTS IN CERAMIC MATERIALS R
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Copyright © 2007 by Nova Science P
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PREFACE Ceramics are refractory, in
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Preface ix crystals is discussed. A
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Preface xi spectra and the directio
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2 Leslie G. Cecil comprehensive dat
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4 Leslie G. Cecil Ringle et al. (19
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6 Leslie G. Cecil The main architec
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8 Leslie G. Cecil can study “the
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10 Leslie G. Cecil Middleton et al.
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12 Leslie G. Cecil The laser ablate
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14 Leslie G. Cecil There are two ge
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16 Leslie G. Cecil distinct recipes
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18 Leslie G. Cecil second group (Fi
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20 Leslie G. Cecil The Vitzil-Orang
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22 Leslie G. Cecil Table 3. Mahalan
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24 Leslie G. Cecil Ixlú and Ch’i
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26 Leslie G. Cecil structures (D. R
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28 Leslie G. Cecil paste and Macanc
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32 Leslie G. Cecil Kepecs, S. M., a
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34 Leslie G. Cecil Schele, L., Grub
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36 Z. C. Li, Z. J. Pei and C. Tread
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70 Fluorescence, a.u. 1 0.1 0.01 0.
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Modeling of Thermal Transport in Ce
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212 R. Ramesh, H. Kara, Ron Stevens
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242 Development of Display Technolo
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244 Li Chen technology moved to the
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246 Li Chen The continuing evolutio
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248 Li Chen the back contact cathod
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250 Li Chen Figure 3. Vertical side
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252 Li Chen Figure 6. Molybdenum mi
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254 Li Chen Figure 8(a). I-V curve
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256 Li Chen Figure 9. Life time tes
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258 Figure 11(a). Plasma generated
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260 Li Chen [13] C.A. Spindt, K.R.
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A absorption spectra, 66, 67, 77, 7
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correlation function, 156 corrosion
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group membership, 13, 20, 21, 22, 2
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microstructure(s), x, xi, 73, 74, 2
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