Soft phonon modes and diffuse scattering in Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ relaxor ferroelectrics

0.29Pb(In_1/2Nb_1/2)O₃-0.45Pb(Mg_1/3Nb_2/3)O₃-0.26PbTiO₃ single crystals have been studied using triple-axis based elastic and inelastic neutron scattering. Elastic diffuse scattering reveals the presence of polar nano-regions (PNR's) in this system, which emerge at the Burns temperature (T_B ∼630 K) and then grow continuously in population and correlation size down to 100 K. At 300 K, characteristic “butterfly” and ellipsoid shaped diffuse scattering patterns are observed in the HK0 scattering plane. Electrical poling along the [110] direction produces a marked asymmetry in the diffuse scattering patterns, with the parallel-to-the-field components enhanced while the perpendicular-to-the-field components suppressed. Several low-energy phonon branches along the [100] and [110] directions have been measured. Most significantly, the PNR-acoustic phonon coupling is confirmed for the [110] transverse acoustic (TA) phonons polarized along the [1-10] real space direction and the [100] TA phonons. This coupling appears to be anisotropic and correlated with the PNRs’ distribution, and also affected by the relative length scales of the PNRs and phonon wave vectors. The well-known “waterfall” phenomenon is observed on the [100] and [110] transverse optical (TO) branches, near the zone center. The optical phonons exhibit a lowest-energy, zone center soft TO mode, whose squared energy increases linearly with decreasing temperature below T_B.