Lead-free and lead-based ABO3 perovskite relaxors with mixed-valence A-site and B-site disorder: Comparative neutron scattering structural study of (Na1/2Bi1/2)TiO3 and Pb(Mg1/3Nb2/3)O3

Abstract

We report the results of neutron elastic-scattering measurements made between -250â€... C and 620â€... C on the lead-free relaxor (Na1/2Bi1/2)TiO3 (NBT). Strong, anisotropic, elastic diffuse scattering intensity decorates the (100), (110), (111), (200), (210), and (220) Bragg peaks at room temperature. The wave-vector dependence of this diffuse scattering is compared to that in the lead-based relaxor Pb(Mg1/3Nb2/3)O3 (PMN) to determine if any features might be common to relaxors. Prominent ridges in the elastic diffuse scattering intensity contours that extend along âŸ̈ 110⟩ are seen that exhibit the same zone dependence as those observed in PMN and other lead-based relaxors. These ridges disappear gradually on heating above the cubic-to-tetragonal phase transition temperature T CT = 523â€... C, which is also near the temperature at which the dielectric permittivity begins to deviate from Curie-Weiss behavior. We thus identify the âŸ̈110⟩-oriented ridges as a relaxor-specific property. The diffuse scattering contours also display narrower ridges oriented along âŸ̈100⟩ that are consistent with the x-ray results of Kreisel; these vanish near 320â€... C, indicating that they have a different physical origin. The âŸ̈100⟩-oriented ridges are not observed in PMN. We observe no equivalent relaxor-specific elastic diffuse scattering from the homovalent relaxor analogues K0.95Li0.05TiO 3 (A-site disordered) and KTa0.95Nb0.05O 3 (B-site disordered). This suggests that the âŸ̈ 110⟩-oriented diffuse scattering ridges are correlated with the presence of strong random electric fields and invites a reassessment of what defines the relaxor phase. We find that doping NBT with 5.6% BaTiO3, a composition close to the morphotropic phase boundary with enhanced piezoelectric properties, increases the room-temperature correlation length along [11̄0] from 40 to 60 Å while doubling the associated integrated diffuse scattering. Similar behavior was reported by Matsuura for morphotropic compositions of PMN doped with PbTiO3. Finally, we comment on the recent observation of monoclinicity in NBT at room temperature by placing a strict bound on the strength of the (1/21/21/2) superlattice reflection associated with the Cc space group based on the atomic coordinates published in the x-ray study by Aksel for NBT. We show that a skin effect, analogous to that reported in the relaxors PZN-xPT and PMN-xPT, can reconcile our neutron single-crystal data with the x-ray powder data of Aksel. Our finding of a skin effect in a lead-free, A-site disordered, heterovalent relaxor supports the idea that it arises in the presence of strong random electric fields. © 2013 American Physical Society.