Raman spectroscopy of multiferroic trigonal boracite Co$_3$B$_7$O$_{13}$Cl

Raman microscopy was applied to study the polarized Raman scattering from untwinned domains of Co$_3$B$_7$O$_{13}$Cl (Co-Cl) in the trigonal, $R3c$, ferroelectric phase. The symmetry ($A_1$ or $E$) and mode polarization (LO or TO) for all observed Raman lines were determined from comparison of the spectra measured in several scattering configurations. It was found that as a rule the TO-LO splitting is small or negligible. A group of $A_1$ modes, characterized by a quasi-one-component Raman tensor, was tentatively assigned to stretching vibrations of Cl, O, or B along the trigonal axis, which in this material is parallel to the ferroelectric polarization direction.