The lattice dynamics of the $rm YMnO_3$ magneto-electric compound has been investigated using density functional calculations, both in the ferroelectric and the paraelectric phases. The coherence between the computed and experimental data is very good in the low temperature phase. Using group theory, modes continuity and our calculations we were able to show that the phonons modes observed by Raman scattering at 1200K are only compatible with the ferroelectric $P6_{3} cm$ space group, thus supporting the idea of a ferroelectric to paraelectric phase transition at higher temperature. Finally we proposed a candidate for the phonon part of the observed electro-magnon. This mode, inactive both in Raman scattering and in Infra-Red, was shown to strongly couple to the Mn-Mn magnetic interactions.
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