High-temperature phonon spectra of multiferroic BiFeO₃ from inelastic neutron spectroscopy

Abstract

We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO₃. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Néel (T_N ∼ 650 K) and Curie (T_C ∼ 1100 K) temperatures. Experimental results are accompanied by ab initio lattice dynamical simulations of phonon density of states to enable microscopic interpretations of the observed data. The calculations reproduce well the observed vibrational features and provide the partial atomic vibrational components. Our results reveal clearly the signature of three different phase transitions both in the diffraction patterns and phonon spectra. The phonon modes are found to be most affected by the transition at the T_C. The spectroscopic evidence for the existence of a different structural modification just below the decomposition limit (T_D ∼ 1240 K) is unambiguous indicating strong structural changes that may be related to oxygen vacancies and concomitant Fe³⁺ → Fe²⁺ reduction and spin transition.