Raman- and infrared-active phonons in hexagonalYMnO3: Experiment and lattice-dynamical calculations

Raman-scattering and far-infrared-transmission spectra of hexagonal ${\mathrm{YMnO}}_3$ (space group ${P6\mathrm{}}_{3}cm)$ are reported and discussed. In the Raman spectra measured in several exact scattering configurations, lines of $A_1$ (at 148, 190, 257, $\sim 300$, 433, 459, and 681 cm${}^{-1})$, $E_1$ (at 376, 408, and 632 cm${}^{-1})$, and $E_2$ (at 135, and $\sim 215$ cm${}^{-1})$, symmetries have been observed. Maxima in the infrared absorption have been found at 211, 238, 265, 281, 308, 398, 428, 457, 491, 596, and 612 cm${}^{-1}$. An assignment of the phonon lines to definite atomic vibrations has been made on the basis of their symmetries in close comparison with results of lattice-dynamical calculations for ${\mathrm{YMnO}}_3$ in its ferroelectric ${(P6\mathrm{}}_{3}cm,\hspace{0.5em}Z=6)$ and high-temperature paraelectric ${(P6\mathrm{}}_3/mmc,\hspace{0.5em}Z=2)$ phases.