Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

3A/2005G139

Crystal structure of ferrimagnetic phase of spinel compound NiCr2O4
Hiroki ISHIBASHI*, Tomonori YASUMIOsaka Prefecture Univ, Sakai, Osaka 599-8531, Japan
Introduction
NiCr2O4 has a tetragonally distorted spinel structure atroom temperature and shows a unique ferrimagnetic
ordering at TC ~ 70 K. Recently, we reported that thiscompound exhibits a structural transition accompanied by
the magnetic ordering, i.e., the crystal symmetry becomes
orthorhombic below TC [1]. However, the detail of thecrystal structure of the ferrimagnetic phase has not been
determined. The aim of this study is to determine thecrystal structure of the ferrimagnetic phase of NiCr2O4 andto discuss the correlation between magnetic and structuralproperties.
Experimental
The NiCr2O4 powder sample was prepared by heating
the mixture of high purity NiO and Cr2O3 in air. The
powder diffraction experiments were performed using ahigh-resolution diffractometer installed at a beam line ofBL-3A station. A wavelength of incident beam used was0.900 Å. A flat Si(111) crystal analyzer was used in orderto improve the angular resolution. The whole powder
patterns were measured at 15, 40 and 100 K using 2θ/θscanning mode.
Results and discussion
Figure 1(a) and 1(b) show the powder patterns of
NiCr2O4 at 100 K and 15 K, respectively. The results of
the Rietveld analysis using the data at 100 K, at which thecrystal structure is known to be tetragonal with a spacegroup I41/amd, are shown in Fig. 1(a) and Table 1(a). The
overall quality of the fitting is fairly good. Next, in order
to construct the structural model at 15 K, the crystal
symmetry and the lattice constants are determined fromthe peak positions of Bragg reflections. All reflections canbe indexed by the orthorhombic structure with the lattice
constants of a ~ 8.171 Å, b ~ 8.184 Å and c ~ 8.563 Å.
From the extinction rule, a space group Fddd, which is asubgroup of I41/amd, was assumed for a structuralanalysis. The results of the structural refinement areshown in Fig. 1(b) and Table 1(b), which show
satisfactory results. The whole pattern at 40 K was alsoanalyzed using the space group Fddd. The result is similar
to that at 15 K except for the slight difference of thelattice constants.
From the present results, it is found that there are threekinds of bond distances between the neighbor Cr-Cr
bonds for the ferrimagnetic phase. Compared to themagnetic structure obtained by Tomiyasu et al [2], theshortest Cr-Cr bond is corresponding to antiferromagnetic
coupling, while the longest one is corresponding toferromagnetic one. The distortion in the ferrimagnetic

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Users' ReportFig. 1. X-ray diffraction patterns of NiCr2O4 at(a) 100 K and (b) 15 K and the Rietveld fit.
Table 1. Structural parameters obtained by Rietveld
analysis at (a) 100 K and (b) 15 K.
(a) 100 K
Space group I 41/amd (Origin choice 2)
a = 5.78951(3) Å, c = 8.54666(5) ÅR wp = 10.51 %, R B = 2.42 %, S = 1.31xyz
atom site
B (Å2)
Ni 4a
1/8
1/8
1/8
0.040
1/2
1/2
1/2
0.062
Cr 8dO 16h
00.0146(6) 0.2649(4) 0.052(b) 15 KSpace group Fddd (Origin choice 2)
a = 8.17161(6) Å, b = 8.18427(6) Å, c = 8.56287(5) ÅR wp = 10.42 %, R B = 2.94 %, S = 1.27
2xyzatom site
B (Å )
Ni 8a
1/8
1/8
1/8
0.017
1/2
1/2
1/2
0.055
Cr 16d
O 32h0.2576(4) 0.2554(4) 0.2669(2) 0.015

phase exhibits long range ordering, therefore there exists
strong coupling between lattice and magnetism in
NiCr2O4.
References[1] H. Ishibashi et al., PF Activity Report 2004 #22 PARTB, 156 (2005).
[2] K. Tomiyasu et al., J. Phys. Soc. Jpn. 73, 2539 (2004).
* hiroki@p.s.osakafu-u.ac.jp