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

14A/2004G037

Electron density distribution of LiMn2O4 by the single-crystal diffraction study
1Kenji TATEISHI1, Nobuo ISHIZAWA1* and Shuji Oishi2
Ceramics
Research Laboratory, Nagoya Institute of Technology, Tajimi, Gifu 507-0071, Japan
2
Faculty of Engineering, Shinshu University, Wakasato, Nagano 380-8553, Japan
Introduction

Lithium manganese spinels are attractive candidates forcathode materials of rechargeable lithium ion batteries.
Stoichiometric LiMn2O4 is a mixed valence compoundcomprised of distinct Mn(III) and Mn(IV) ions in equal
proportions. Above the transition temperature Tc, Mn(III)and Mn(IV) ions are randomly distributed amongst the16d sites of Fd-3m symmetry, while they are localized on
five independent sites in Fddd symmetry below Tc. The
lithium ion dynamics, which is very important from aviewpoint of practical applications, is also different in thehigh (HT) and low temperature (LT) phases. Structural
information about these two phases is therefore essential
for understanding of the processes involved. Powder Xray and neutron diffraction experiments have beenreported previously by other authors but high precision
single crystal X-ray diffraction experiments in principle
can provide more accurate three dimensional information
about the electron density. Here we report on the LT
structure determined via single-crystal synchrotron X-raydiffraction.
Experimental
Single crystals of LiMn2O4 were synthesized using aLi2MoO4 flux. Reagent grade Li2CO3 and MnCO3 were
weighed and combined together to form a nominal 7
mol% LiMn2O4 mixture. Approximately 31.7 g of themixture was placed in a platinum crucible and heated to-1
1373 K at the rate of 45 K h in an air atmosphere. The
peak temperature was maintained for 10 h, and then-1
reduced to 723 K at 5 K h before discharge. The flux
was rinsed out with warm water. The crystals were blackand octahedrally shaped with a maximum size of about
30.1x0.1x0.1 mm along the edge. Transition temperatures
were revealed by the existence of superlattice reflections
using a Rigaku RAPID imaging plate diffractometer witha cryostat using nitrogen gas flow. This compound
presented a first order phase transition at 294(1) K on
cooling and at 310(1) K on heating.
Single crystal diffraction data were measured using ahorizontal-type four-circle diffractometer at beamline 14Aof the Photon Factory, Tsukuba, Japan. An eight-channel
avalanche photodiode detector was used for photon
counting (Kishimoto et al. 1998). The 0.75044(5) A
wavelength was calibrated using a spherically ground Si
standard reference crystal. Crystals of LiMn2O4 weremounted on tapered glass capillaries and preprocessed at
low temperatures before the synchrotron diffraction

experiment. Cell dimensions were determined using 24
reflections. The crystal has a 3x3x1 superstructure of the3spinel-type, with unit cell volume exceeding 5000 Å . A
half sphere of diffraction data up to 2θ=73° was measured
at 297(1) K using an octahedrally shaped crystal of size
0.05 mm along the edge. Because no reflections violating
the F lattice-centering conditions were observed in thesphere of 2θ<15°, only the F-centred reflections weremeasured up to 2θ=73°. Moreover the measured
reflections also satisfied the conditions h00: h=4n, 0k0:
k=4n and 00l: l=4n which are the conditions for the Fddd
symmetry. The merging R factor reduced from 2.7 to
2.5 % after applying an analytical absorption correction.
The populations of individual chemical elements arerefined with all other parameters fixed at values obtainedin the final cycle. They ranged from 1.12(3) to 1.13(4) for
independent four Li atoms, from 0.991(3) to 0.996(3) for
five Mn atoms, and from 0.992(6) to 1.002(7) for nine Oatoms. Given that the excess values at Li sites was only
0.39(12) electrons at the most, the degree ofcontamination of Mn atoms at Li sites seemed marginal.
Thus a stoichiometric composition was assumed for thepresent crystal. The checkCIF software suggested thepossibility of pseudosymmetry I41/amd for the final
positional parameters. However the merge R factor was0.107 after transforming the unit cell from orthorhombic
to tetragonal. In addition the difference between the a andb cell dimensions was 0.028(2)Å, being 14 times larger
than the esu. The tetragonality of the crystal was thus
discarded in the present study. The final least-squares R
factor reduced to R(F)=0.038. The maximum positive
3residual density of 3.0 e/ Å was found at 0.71 Å fromMn3 and the minimum negative residual density of -0.7
3e/Å at 1.65 Å from Li2.
Part of the initial results have been published in ourpaper (Tateishi et al., 2004). A full discussion, including
new findings about the ordered arrangement of Zenner
polarons in the structure, will be given in our next paper
which is being in progress.

References[1] S. Kishimoto et al., Rev. Sci. Instrum. 69, 384 (1998).
[2] K. Tateishi et al., Acta Cryst. E60, i18 (2004).

* ishizawa@nitech.ac.jp

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