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

Abstract

The boracites with general formula M3B7O13X (M = divalent metal, X = Cl, Br, I), shortly denoted as M{X, are among the flrst known multiferroic materials. They exhibit a sequence of transitions from the high temperature paraelectric cubic phase to ferroelectric orthorhombic, monoclinic, trigonal phases, and flnally 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 difierent crystallographic and polarization orientation. In this work, on the example of Co3B7O13Cl (Co{Cl), Co3B7O13Br (Co{Br) and Ni3B7O13Br (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 conflgurations, determine the Raman mode symmetries, and assign Raman lines to deflnite atomic motions. The efiects 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.