  Magnetic and crystallographic transitions in the Cairo pentagonal magnet
Bi2Fe4O9 are investigated by means of infrared synchrotron-based spectroscopy
as a function of temperature (20 - 300 K) and pressure (0 - 15.5 GPa). One of
the phonon modes is shown to exhibit an anomalous softening as a function of
temperature in the antiferromagnetic phase below 240 K, highlighting
spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger
softening is observed through the pressure induced structural transition.
Lattice dynamical calculations reveal that this mode is indeed very peculiar as
it involves a minimal bending of the strongest superexchange path in the
pentagonal planes, as well as a decrease of the distances between second
neighbor irons. The latter confirms the hypothesis made by Friedrich et al.,1
about an increase in the oxygen coordination of irons being at the origin of
the pressure-induced structural transition. As a consequence, one expects a new
magnetic superexchange path that may alter the magnetic structure under
pressure.
