Ferroelectric Ordering in the Relaxor Pb(Mg1/3Nb2/3)O3 as Evidenced by Low-Temperature Phonon Anomalies

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg1/3Nb2/3)O3 (PMN) are reported for 10<=T<=750 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 hw0^2 increases linearly with decreasing temperature, and is consistent with the behavior of a ferroelectric soft mode. At 10 K, hw0 reaches 11 meV, the same value observed in ferroelectric Pb(Zn1/3Nb2/3)O3 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.