Raman Imaging Approach to the Study of Ferroelectric Domains and Raman Spectra of Multiferroic Boracites

The boracites with general formula M(3)B(7)O(13)X (M = divalent metal, X = Cl, Br, I), shortly denoted as M-X, axe among the first known multiferroic materials. They exhibit a sequence of transitions from the high temperature paraelectric cubic phase to ferroelectric orthorhombic, monoclinic, trigonal phases, and finally to a monoclinic phase at low temperatures, where both ferroelectric and magnetic orders coexist. The lattice dynamics of boracites has been scarcely studied with the Raman spectroscopy, the main problem with non-cubic phases being the coexistence of twin variants with different crystallographic and polarization orientation. In this work, on the example Of Co(3)B(7)O(13)Cl (CO-Cl), Co(3)B(7)O(13)Br (Co-Br) and Ni(3)B(7)O(13)Br (Ni-Br) we demonstrate that using the Raman microscopy imaging one can visualize the twin variants, follow their transformation through the crystallographic transitions, obtain Raman spectra from untwinned domains in exact scattering configurations, determine the Raman mode symmetries, and assign Raman lines to definite atomic motions. The effects of structural transitions and elemental substitution are discussed in close comparison with results of ab initio calculations of the phonon structure of the cubic phase.