Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

3A, 4B2/2003G190

Structural phase transition of spinel NiCr2O4 at low temperatures
Hiroki ISHIBASHI*, Tomonori YASUMI
1Osaka Prefecture Univ, Sakai, Osaka 599-8531, Japan
Introduction
NiCr2O4 with a normal spinel structure is known toshow structural phase transition from cubic phase (spacegroup: Fd 3 m ) to tetragonal one (I41/amd) at ~310 K
2+
because of Jahn-Teller effect on Ni site. This compoundshows magnetic ordering to ferromagnetism below TC ~65 K. Very recently, Klemme et al reported anothertransition at ~30 K by heat capacity measurement [1].
After that, Tomiyasu et al reported that this transition isdue to the change in magnetic structure by neutron andmagnetization measurements [2].
In this study, we investigate the precise crystal structureof NiCr2O4 by high resolution x-ray powder diffraction
measurement and discuss the correlation between
magnetic and structural properties.
Experimental
The NiCr2O4 powder sample was prepared by heating
the mixture of high purity NiO and Cr2O3 in air. The
powder diffraction experiment was performed using highresolution diffractometer installed at a beam line of BL3A station. A wavelength of incident beam used was1.600 Å. A flat Si(111) crystal analyzer was used in orderto obtain the data with high angular resolution. The
temperature dependence of the diffraction profiles of
specific reflections were measured at temperatures
between 12 K and 100 K using 2θ/θ scanning mode.
Results and discussion
Figure 1 shows the temperature dependence of powder
diffraction profile in the 2θ range around 400 reflection,
where the indices of Bragg reflections are based on thecubic structure. Above 70 K, the tetragonal structure isconfirmed as previously reported [2]. However, the line
width of 400 reflection becomes larger below 60 K andfinally 400 reflection clearly splits into two peaks at thelower temperatures. We carefully checked the peak
splitting for the other reflections, it is concluded that thecrystal symmetry of NiCr2O4 below 60 K is orthorhombic.
Next, we determined the lattice constants for eachtemperature using a profile fitting technique. Figure 2shows the temperature dependence of the lattice constants.
The structural transition at ~ 60 K seems to be of second
order because the lattice constants a and b change
continuously. The transition temperature is almost thesame as the magnetic ordering one, TC, so that the crystal
lattice of NiCr2O4 is strongly coupled with the magnetic
ordering. On the other hand, the lattice constants show noevident anomaly at ~ 30 K.
The magnetic properties below TC of NiCr2O4 has been
discussed based on an assumption that the crystal

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Users' Reportstructure is tetragonal so far. In the present study, we
found that NCr2O4 has an orthorhombic structure below TC,
therefore it is necessary to reconstruct the magneticstructure and to reconsider the magnetic properties.

Fig. 1. Temperature dependence of powder X-ray
diffraction patterns of NiCr2O4 on heating inthe 2θ range around 400 reflection.

Fig. 2. Temperature dependence of lattice constantsof NiCr2O4.
References[1] S. Klemme et al., Phys. Chem. Miner. 29, 663 (2002).
[2] K. Tomiyasu et al., J. Phys. Soc. Jpn. 73, 2539 (2004).
* hiroki@p.s.osakafu-u.ac.jp