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Thermal behaviour of the BaMF4 ferroics

Jose M. Posse 1Andrzej Grzechnik 1Andy Fitch 2Karen Friese 1

1. Dpto. Física Materia Condensada, Universidad del País Vasco (UPV/EHU), Facultad de Ciencia y Tecnología, Apdo. 644, Bilbao 48080, Spain
2. European Synchrotron Radiation Facility (ESRF), Grenoble 38043, France


The isostructural piezoelectrics BaMF4 crystallize in the orthorhombic space group Cmc21 at ambient conditions. The M-cation (M = Zn, Cu, Ni, Co, Fe, Mn, or Mg) is surrounded by six fluorines in an irregular octahedron. Each octahedron shares four fluorines with neighbouring octahedra forming layers perpendicular to the b axis. The barium atoms are accomodated in the interlayer space with coordination number nine. Individual representatives (M=Mn, Fe, Co, and Ni) of the BaMF4 family show a multiferroic behaviour [1].
Based on inelastic-neutron scattering experiments [2], it was concluded that the BaMF4 compounds have a structural instability at low temperatures. Up to now, this has been confirmed only for BaMnF4 - a transition to an incommensurately modulated phase at temperatures below 250K being reported in [3]. BaMnF4 also presents antiferroelectric anomalies in the magnetic susceptibility curve at 45K related to an intermediate magnetic phase [4].
We studied the thermal behavior of three representative members of the family (M = Mg, Zn and Mn) by powder diffraction measurements from 290K to 10K at the beamline ID31 (Grenoble, ESRF). No phase transition in this temperature range was observed for BaMgF4  and BaZnF4. However, refinements reveal a negative thermal expansion of the a lattice parameter and a "bump" of the c parameter below 70K. For BaMnF4, we can confirm the transition to the incommensurate phase. However, our high resolution data cannot be explained with the model given earlier [3]. Instead, we propose a new superspace group X21(½ ½ γ) with X=(½ ½ 0 ½). The γ-component of the q-vector increases continuously with decreasing temperature till at 45K (coinciding with the magnetic anomaly) a constant value of 0.3948Å-1 is reached.


Normalized lattice parameters and γ-component of the q-vector of BaMnF4.

[1]C. Ederer & N.A. Spaldin (2006) Phys. Rev. B74, 024102.

[2] R. Almairac, et al. (1997), Phys. Rev. B55, 8249.
[3] Ph. Sciau, et al. (1988), Acta Cryst. B44, 108.
[4] M. Yoshimura, et al. (2006), J. Magn. Magn. Mat. 299, 404.


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Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Jose M. Posse
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-25 10:08
Revised:   2009-06-07 00:48