Investigation of High Pressure Phase Transition by Means of Infrared Spectroscopy in the Cairo Frustrated Pentagonal Magnet Bi₂Fe₄O₉ ^†

Bi₂Fe₄O₉ is a common by-product in the synthesis of the multiferroic compound BiFeO₃ and has been claimed itself to display multiferroic properties [1]. The lattice formed by the two different sites of four iron Fe³⁺ magnetic atoms is quite remarkable as it materializes the first analogue of a magnetic pentagonal lattice [2]. For its peculiar lattice geometry it has attracted interest in the field of geometrical frustration. At room temperature and atmospheric pressure, the crystal structure is orthorhombic within the Pbam space-group and the compounds undergoes a magnetic phase transition at 238 K from a paramagnetic state toward a non collinear magnetic state characterized by a propagation vector k = (1/2, 1/2, 1/2) and a large degree of frustration (θ_p/T_N ~ 7) [2]. Recently it has been shown that Bi₂Fe₄O₉ undergoes a structural transition under pressure at 6–8 GPa toward the maximal non-isomorphic subgroup Pbnm, with c’ = 2c. The driving force of the phase transition is the displacement of the O1 oxygen atom from fully constrained Wyckoff position 2b to a less-constrained 4c one [3]. Previous studies have reported the investigation of dynamical properties by mean of Raman spectroscopy both at ambient condition and at high pressure in a diamond anvil cell [3,4]. However, the vibrational modes involving O1 oxygen atoms are not Raman active but infrared active.

We will report the first polarized infrared spectroscopy measurement of Bi₂Fe₄O₉ performed in a DAC from 1 to 20 GPa in the far-infrared range [60–800 cm⁻¹] and at low temperature. The measurements have been performed at the AILES beamline of synchrotron SOLEIL exploiting the high-pressure/low-temperature set-up [5] coupled with the high brilliance of the radiation source in a wide spectral range on a very thin sample (~50 µm) placed between diamonds with culets of 500 µm diameter. From our high quality spectra, we are able to identify the B_3u and B_2u modes within the (ab)-plane. Interestingly, while all phonon frequencies increase with pressure, the phonon mode around 200 cm⁻¹ undergoes an anomalous softening with increasing pressure. In order to assign the phonon modes and reveal the microscopic mechanism of the high-pressure transition we also have performed DFT calculation at different pressures. The calculation mostly accounts for the measured phonon modes allowing the assignation of atomic motions.