Pressure-induced commensurate phase with potential giant polarization in YMn$_2$O$_5$

Abstract : We have studied multiferroic YMn$_2$O$_5$ by high-pressure neutron diffraction in a large pressure range from 0.5 to 6.3 GPa. We observe a pressure-induced commensurate (PCM) phase with a propagation vector ($1\over 2$0$1\over 2$) that is different from those observed at ambient pressure. It coexists with the ambient pressure phases up to the highest pressure, with an increased contribution as the pressure increases. The PCM phase, which is likely generic in the RMn$_2$O$_5$ family, should be taken into account to understand the strong variation of the electric polarization under pressure.