Phonons in the multiferroic langasite Ba3NbFe3Si2O14: Evidence for symmetry breaking.

The chiral langasite Ba3NbFe3Si2O14 is a multiferroic compound. While its magnetic order below TN - 27 K is now well characterized, its polar order is still controversial. We thus looked at the phonon spectrum and its temperature dependence to unravel possible crystal symmetry breaking. We combine optical measurements (both infrared and Raman spectroscopy) with ab initio calculations and show that the signatures of a polar state are clearly present in the phonon spectrum even at room temperature. An additional symmetry lowering occurs below 120 K as seen from the emergence of softer phonon modes in the terahertz range. These results confirm the multiferroic nature of this langasite and open new routes to understand the origin of the polar state. [ABSTRACT FROM AUTHOR]