Ordered magnetic frustration. IV. The two magnetic structures of the inverse weberite Fe2F5(H2O)2: an example of the thermal evolution of the frustration character

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Abstract
The nuclear and the two magnetic structures of the ferrimagnetic (Tc=48(1)K) inverse weberite Fe2F5(H2O)2 were solved by neutron powder diffraction experiments at 60, 30 and 4.2K respectively. The room temperature structure proposed by Hall et al. (1977) is confirmed at 60K and hydrogen atoms are located (O-H =0.944(8) AA; H-O-H=111.68(9) degrees ). Below Tc, magnetisation data indicate a change in the magnetic structure at 26(1)K. This transition is confirmed by neutronic thermodiffractometry. Above and below this temperature, the magnetic and nuclear cells are identical. At 30K (space group Imma) all the Fe3+ spins ( mu =2.8(1) mu B) are parallel to (010) (Fy mode), whereas Fe2+ moments ( mu =3.8(1) mu B) lie in the (100) plane (FyGz mode). Below 26K, the magnetic symmetry becomes monoclinic (I2/b); at 4.2K mu Fe3+=4.9(3) mu B, mu Fe2+3.8(3) mu B and the canting mode is FxFyGz on each site; the antiferromagnetic mode Gz of Fe3+ becomes predominant. This peculiarity is analysed in terms of the strength of the magnetic interactions and topological frustration. The thermal variation of the orientation of the moments is also described.

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