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We report on polarized infrared reflectivity measurements of multiferroic,monoclinic MnWO4 between 10K and 295K. The full dielectric tensor and thefrequency dependence of the orientation of the principal axes have beendetermined in the frequency range of the phonons. All infrared-active phononmodes (7 A_u modes and 8 B_u modes) are unambiguously identified. In particularthe strongest B_u modes have been overlooked in previous studies, in which themonoclinic symmetry was neglected in the analysis. Using a generalizedDrude-Lorentz model, we determine the temperature dependence of the phononparameters, including the orientation of the B_u modes within the ac plane. Thephonon parameters and their temperature dependence have been discussedcontroversially in previous studies, which do not include a full polarizationanalysis. Our data does not confirm any of the anomalies reported above 20K.However, in the paramagnetic phase we find a drastic reduction of the spectralweights of the weakest A_u mode and of the weakest B_u mode with increasingtemperature. Below 20K, the parameters of the A_u phonon modes for E||b showonly subtle changes, which demonstrate a finite but weak coupling betweenlattice dynamics and magnetism in MnWO4. A quantitative comparison of ourinfrared data with the quasi-static dielectric constant epsilon_b indicatesthat the spectral weight of the electromagnon is tiny for E||b, in particularmuch smaller than the weak changes of the spectral weight of the phonons.

Infrared-active phonon modes in monoclinic multiferroic MnWO4