Temperature-dependent Raman scattering study of multiferroic MnWO₄

We have measured polarized Raman spectra of MnWO₄ single crystals at low temperatures, and studied the temperature dependence of the various phonon modes. From our Raman studies of the MnWO₄, a new transition temperature, ~180 K, was found. We have completely assigned the symmetries of the 18 observed Raman modes of the MnWO₄, as expected from a group theoretical analysis. These Raman modes have been classified into three groups according to weak, intermediate and strong temperature dependence of the modes in each group. Six internal modes have been identified by their weak temperature dependence of the Raman wavenumbers. The temperature dependence of the wavenumbers of the B_g modes in Mg-O bonds, modes of intermediate temperature dependence group, shows an anomalous behavior under 50 K. The phonon modes of strong temperature dependence show an anomalous change at ~180 K in the linewidths. This is believed to be a new transition temperature which involves the changes in the inter-WO₆ octahedra structure.