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International audience; We report an experimental study of the phonon dispersion in BiFeO(3) single crystals at ambient conditions by inelastic x-ray scattering (IXS). The phonon dispersions were recorded along several symmetry directions up to 35 meV. Our results compare favorably with first-principles calculations performed using density functional theory (DFT) within the local-density approximation (LDA). We resolve a discrepancy concerning the symmetry of the optical phonon branches observed by Raman spectroscopy, determine the energy of the lowest Raman and infrared silent mode, and derive a subset of the elastic moduli of BiFeO(3).

We report on a first-principles study of the structural, electronic, and magnetic properties of multiferroic double perovskite Bi${}_{2}$FeCrO${}_{6}$, using density functional theory within the local spin-density approximation (LSDA), the LSDA+U approximation as well as a hybrid functional scheme. We show that Bi${}_{2}$FeCrO${}_{6}$ presents two competing ferrimagnetic phases, sharing the same total magnetic moment of 2${\ensuremath{\mu}}_{B}$ per unit cell but with a different electronic configuration for the Fe${}^{3+}$ species. The phase with high-spin iron is the ground state at ambient conditions, but we predict that low-spin iron gets stabilized under compression. We also investigate the corresponding ferromagnetic phases, and show that the magnetic couplings sharply decrease when moving form high- to low-spin Fe${}^{3+}$.

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors) properties of bismuth ferrite, a prototypical multiferroic compound. We have not only considered the usual approximations to density-functional theory such as the local-density approximation (LDA), generalized (GGA), and $\text{LDA}+U$, but also hybrid approaches such as B3LYP and B1. The recent B1-WC hybrid functional of Bilc et al. [Phys. Rev. B 77, 165107 (2008)], with the GGA functional of Wu and Cohen and an exact exchange mixing parameter of 0.16, provides very good overall agreement with experiments and can be considered as a valuable alternative to LDA, GGA, and $\text{DFT}+U$ for the study of bismuth ferrite. This does not only allow a reliable interpretation of the physical properties of this specific compound but also opens perspectives for further and more predictive first-principles investigations of multiferroic materials.

The entire zone-center phonon spectrum of the $R3c$ ferroelectric antiferromagnetic phase of bismuth ferrite is computed using a first-principles approach based on density functional theory. Two phonon modes exhibiting eigendisplacement vectors that strongly overlap with the atomic distortions taking place at the ferroelectric structural phase transition are identified and give support to a transition with displacive character. Both Raman and infrared reflectivity spectra are also computed, providing benchmark theoretical results for the assignment of experimental spectra.

In a recent paper, Tutuncu and Srivastava [J. Appl. Phys. 103, 083712 (2008)] report the electronic structure and the assignment of the zone-center phonon modes of the R3c phase of BiFeO3 using the density functional theory. In the present comment, we point out some physical inaccuracies in their mode assignment. We review their interpretation and analyze which conclusions are actually justified.