Infrared phonon spectroscopy on the Cairo pentagonal antiferromagnet Bi2Fe4O9 : A study through the pressure-induced structural transition

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 in the distances between second-neighbor irons. The latter confirms the hypothesis made by Friedrich et al., [J. Phys.: Condens. Matter 24, 145401 (2012), 10.1088/0953-8984/24/14/145401] 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.