Ferroelectric Dynamics in the Perovskite Relaxor PMN

We review results obtained from recent neutron scattering studies of the lead-oxide class of perovskite relaxors PMN and PZN. A ferroelectric soft mode has been identified in PMN at 1100 K that becomes overdamped near 620 K. This is the same temperature at which polar nanoregions (PNR) begin to form, denoted by Td , and suggests that a direct connection exists between the soft mode and the PNR. The appearance of diffuse scattering intensity at Td reported by Naberezhnov et al. lends further support to this picture. At lower temperature the soft mode in PMN reappears close to Tc = 213 K (defined only for E > Ec). These results are provocative because the dynamics below Tc are characteristic of an ordered ferroelectric state, yet they occur in a system that remains cubic on average at all temperatures. We discuss a coupled-mode model that successfully describes these data as well as those from earlier lattice dynamical studies of other perovskites such as BaTiO3. INTRODUCTION SOFT MODES AND PNR Recent neutron scattering studies of the perovskite (ABO3) relaxors Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) have focused on the low-energy (≤ 20 meV) lattice dynamics to determine whether or not a soft mode picture is relevant to these chemically-disordered systems. [1, 2, 3] The answer to this question is not obvious in spite of the similarities between PMN and PZN, and the displacive ferroelectric PbTiO3, which exhibits a phase transition to a tetragonal structure at Tc = 763 K. The occupation of the B-site by either Mg2+ or Zn2+, and Nb5+ cations creates a local charge imbalance, and gives rise to rapidly varying random fields. [4] Therefore anomalies in the lattice dynamics of both PMN and PZN are to be expected. Of particular interest is the role played by the polar nanoregions (PNR), which for PMN form between 600 and 650 K. [5] Given their polar nature they ought to couple strongly to polar phonon modes, including any soft modes if present, in the relaxors PMN and PZN. The existence of a soft transverse optic (TO) phonon mode has been documented in numerous perovskites including PbTiO3, BaTiO3, SrTiO3, and KTaO3. In the case of PbTiO3, the frequency of the lowest-lying TO mode drops when cooled from high temperature and condenses at the (first-order) transition temperature Tc = 763 K, transforming the system into a tetragonal state. [6] Unlike those in other perovskites, the soft mode in PbTiO3 is well defined except for a limited range of reduced wave vectors