Fyzikální ústav Akademie věd ČR

Bismuth ferrite (BiFeO₃) is a prototypical multiferroic system exhibiting extraordinary properties; it is ferroelectric up to TC ~ 1100 K as well as antiferromagnetic up to TN ~ 640 K. Characterisation of polar phonon modes is essential for understanding its dielectric and electromechanical behaviour. Assigning the BiFeO₃ polar modes probed detected in Raman spectroscopy is not straightforward due to the presence of so-called oblique phonon modes that do not match any pure TO or LO mode frequencies; instead, their frequencies continuously vary with the orientation of the phonon propagation vector. Using a monocrystal, mode assignment can be obtained by analysing a set of Raman spectra taken for different values of the angle between the phonon propagation vector and the optical axis (angular dispersion). To achieve the goal in absence of a de-twinned single crystal sample, we have developed an original technique in collaboration of the Institute of Physics AS CR and The University of Sheffild which enabled for the first time assigning these strongly anisotropic crystal properties using a polycrystalline material. We have taken advantage of the efficiency and spatial resolution of up-to-date micro-Raman spectrometers to collect a large enough set of spectra acquired on different randomly oriented grains of coarse-grain bulk ceramics. The pure TO and LO frequencies needed for angular dispersion determination were obtained from limit values of the phonon frequencies within the whole set of data. For each grain, the angle between the phonon propagation vector and optical axis was than determined from measured frequency of a chosen oblique mode. The obtained angular dispersion of the phonon mode parameters served as a basis of a full mode assignment [J. Hlinka et al., Phys. Rev. B 83, 020101 (2011)].

Phonon spectra of multiferroic BiFeO3. (a) Raman spectra from selected ceramic grains. Point symbols indicate positions of the pure modes. (b) Angular phonon dispersion curves. Full lines represent the oblique modes, dashed lines the purely transverse E(TO) branches, open circles are frequencies determined from individual Raman spectra as a function of the angle between the phonon propagation vector and the optical axis.