Pressure Dependence of the Phonon Modes of Hexagonal-RMnO3*

We present high pressure IR measurements of the phonon spectra of HoMnO3 and YMnO3. Measurements were conducted over the pressure range ambient to $\sim $20 GPa. No phase changes were observed over this broad range of hydrostatic pressures. A strong non-linear variation of frequency with pressure is observed suggesting saturation at higher pressures. A discussion of the effect of hydrostatic pressure on the ferroelectric properties of these systems will be given based on comparisons with density functional calculations.