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

9A/2003G066

Molecular structure of single-molecule magnet Mn10Fe2 and Mn11CrToshihiko Yokoyama*1, Hidekazu Hachisuka2, Kunio Awaga2
1
Institute for Molecular Sciences, Myodaiji-cho, Okazaki 444-8585, Japan
2
Dept. Chem., Grad. School Sci., Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
Introduction
Single-molecular magnet of [Mn12(AcO)16(H2O)4](Ac=CH3COO) has extensively been investigated becauseof its interesting properties such as stepwisemagnetization due to the quantum tunneling effect.
Previously, we have investigated structural and magnetic
properties of Mn11Cr (one Mn is substituted by one Cr) bymeans of several kinds of physical techniques including
XAFS [1]. In this work, we will report further progressconcerning Mn10Fe2 ([Mn10Fe2O12(AcO)16(H2O)4]) and
Mn11Cr- ([(Ph)4P][Mn11CrO12(PhCOO)16(H2O)4]), whichwere successfully synthesized very recently. The single
crystal x-ray diffraction analysis could not determine thepositions of Fe or Cr because of the crystalline disorder.
In such a case, EXAFS is the most suitable technique todetermine the molecular structure since Fe or Cr may
locate at one of the three inequivalent sites of Mn.
Experimental
Cr, Mn and Fe K-edge x-ray absorption spectra ofMn10Fe2/Mn12 (mixed crystal) and Mn11Cr- wererecorded at BL9A with a conventional transmission modeat room temperature.
Results and discussion
Figure 1 depicts the Fourier transforms of the Fe andMn K-edge EXAFS functions k3χ(k) functions. The
employed k range is around 2.5-10.3 Å-1. The Fouriertransform of Fe (red curve) gives two distinct peaks at 1.5
and 3.1 Å, while the Mn one (green) shows threeprominent peaks at 1.3, 2.4, and 3.2 Å. The shortest
distance (1.3-1.5 Å) is assignable to the O shells aroundFe or Mn, and the other two shells at 2.4-3.2 Å are
ascribed to those of Fe-Mn or Mn-Mn.
In order to determine the Fe site, we have performed
theoretical simulations by using FEFF8 assuming themolecular structure determined in the single-crystal x-ray
diffraction analysis. The x-ray molecular structure gives
the average structure where Fe is not distinguished fromMn and is almost identical with the Mn12 one. In one
molecule, there exist three types of Fe/Mn sites: sites a, band c, as shown in the inset in Fig. 1. Site a is occupied
by Mn4+, which gives the simulation results of the blue
curve in Fig. 1. Sites b and c are for Mn3+ in Mn12, whichshow the results of the lighjt-blue and pink curves in Fig.
1, respectively. The experimental Fourier transform ofMn (green) can be well understood as a superposition of
the three theoretical curves. This is consistent with thefact that the Mn ions occupy the three sites, and in otherwords, this finding justifies the present analysis procedure.

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In contrast, the Fourier transform of Fe is similar to thesimulated one of site c. It is thus concluded that, in
Mn10Fe2, Fe3+ exclusively occupies site c, the tilted sitefor Mn3+ in the Mn12 skeleton. This result is essentiallythe same as in Mn11Cr previously investigated [1].
The magnitude of the Fourier transform of theexperimental Fe-O shell (R=2.08±0.01 Å) is muchstronger than that of the theoretical one of site c. This is
however easily understandable. The Mn3+ ion in site cexhibits significantly distorted octahedron due to theJahn-Teller effect, resulting in the suppression of the MnO contribution in the simulation. On the contrary, the Fe3+
ion shows no Jahn-Teller distortion, yielding more
intense Fe-O contribution in the Fourier transform.
The molecular structure of
Mn11Cr- has been
determined in a similar manner. The specimen shows
more complicated structure because of the presence ofone Mn(II) ion, and strictly speaking, all the metal ionsshould be crystallographically inequivalent to each other.
However, the EXAFS results show similar features tothose of Mn10Fe2 and Mn11Cr, and one can conclude that
Cr(III) locates exclusively at site c. The obtained Cr-O
interatomic distance is 1.86±0.01 Å.
In conclusion, the Fe and Cr substituted sites in
Mn10Fe2 and Mn11Cr- were determined as site c in Fig. 1,
this consequence being identical with the Mn11Cr one.
References[1] H. Hachisuka et al. Phys. Rev. B70, 104427 (2004).
* yokoyama@ims.ac.jp

Fig. 1: Fourier transforms of the Fe (red curve) andMn (green) K-edge k3χ(k) functions, together with theFEFF simulation results for sites a (blue), b (lightblue) and c (pink).