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We have carried out temperature-dependent inelastic neutron scatteringmeasurements of YMnO3 over the temperature range 50 - 1303 K, covering both theantiferromagnetic to paramagnetic transition (70 K), as well as theferroelectric to paraelectric transition (1258 K). Measurements are accompaniedby first principles calculations of phonon spectra for the sake ofinterpretation and analysis of the measured phonon spectra in the roomtemperature ferroelectric (P63cm) and high temperature paraelectric (P63/mmc)hexagonal phases of YMnO3. The comparison of the experimental andfirst-principles calculated phonon spectra highlight unambiguously aspin-phonon coupling character in YMnO3. This is further supported by thepronounced differences in the magnetic and non-magnetic phonon calculations.The calculated atomistic partial phonon contributions of the Y and Mn atoms arenot affected by inclusion of magnetic interactions, whereas the dynamicalcontribution of the O atoms is found tochange. This highlights the role of thesuper-exchange interactions between the magnetic Mn cations, mediated by Obridges. Phonon dispersion relations have also been calculated, in the entireBrillouin zone, for both the hexagonal phases. In the high-temperature phase,unstable phonon mode at the K point is highlighted. The displacement pattern atthe K-point indicates that the freezing of this mode along with the stable modeat the {\Gamma}-point may lead to a stabilization of the low-temperature(P63cm) phase, and inducing ferroelectricity. Further, we have also estimatedthe mode Gr\"uneisen parameter and volume thermal expansion behavior. Thelatter is found to agree with the available experimental data.

Spin-Phonon Coupling and High-Temperature Phase Transition in Multiferroic Material YMnO3