We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO3 (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO3. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes wer
e found in incommensurate orthorhombic DyMnO3. These observations directly reveal different electric polarization mechanisms in the E-type and IC-type REMnO3.
