Ferroelectric ordering in the relaxor Pb(Mg1/3Nb2/3)O-3 as evidenced by low-temperature phonon anomalies

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg1/3Nb2/3)O-3 (PMN) are reported for 10less than or equal toTless than or equal to750 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T-d=620 K, surprisingly recovers below 220 K. The square of the soft-mode energy ((h) over bar omega(0))(2) increases linearly with decreasing temperature and is consistent with the behavior of a ferroelectric soft mode. At 10 K, (h) over bar omega(0) reaches 11 meV, the same value observed in ferroelectric Pb(Zn1/3Nb2/3)O-3 at low T. An unusual broadening of the TA phonon starts at T-d and disappears at 220 K, coincident with the recovery of the TO mode. These dynamics suggest that a well-developed ferroelectric state is established below 220 K.