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

TL;DR

This study uses infrared spectroscopy to explore how pressure and temperature affect the phonon modes and structural transitions in the Cairo pentagonal antiferromagnet Bi2Fe4O9, revealing spin-lattice coupling and pressure-induced structural changes.

Contribution

It provides new insights into the pressure-induced structural transition and spin-lattice coupling in Bi2Fe4O9 through combined experimental and lattice dynamical calculations.

Findings

Anomalous phonon softening below 240 K indicating spin-lattice coupling

Large phonon softening under pressure at 40 K during structural transition

Identification of a unique phonon mode involving superexchange path changes

Abstract

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.