Lead-free and Lead-Based ABO3 Perovskite Relaxors with Mixed-Valence A-Site and B-Site Disorder: A Comparative Neutron Scattering Structural Study of (NaYd1/2^Bi1/2)TiOd3^ and Pb(MgYd1/3^Nb2/3)O3

Published

November 27, 2013

Author(s)

Wenwei Ge, Christopher P. Devreugd, Daniel Phelan, Qinhui Zhang, Muhtar Ahart, Jiefang Li, Haosu Luo, Lynn A. Boatner, Dwight Viehland, Peter M. Gehring

Abstract

We report the results of neutron elastic scattering measurements between -250°C and 620°C on the lead-free relaxor Na_1/2Bi_1/2TiO₃ (NBT). Strong, anisotropic, elastic diffuse scattering intensity decorates the (100), (110), (111), (200), (220), and (210) Bragg peaks at room temperature. The wavevector dependence of this diffuse scattering is compared to that in the lead-based relaxor PbMg_1/3Nb_2/3O₃ (PMN) to determine if any features might be common to relaxors. Prominent ridges in the elastic diffuse scattering intensity contours that extend along <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 = 523°C, which is also near the temperature at which the dielectric permittivity begins to deviate from Curie-Weiss behavior. We thus identify the <110>-oriented ridges as a relaxor-specific property. The diffuse scattering contours also display narrower ridges oriented along <100> that are consistent with the x-ray results of Kreisel et al. (2003); these vanish below 320°C indicating that they have a different physical origin. The <100>-oriented ridges are not observed in PMN. We observe no equivalent relaxor-specific elastic diffuse scattering from the homovalent relaxor analogues K_0.95Li_0.05TiO₃ (A-site disordered) and KTa_0.95Nb_0.05O₃ (B-site disordered). This suggests that the <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 from 40 Å to 60 Å while doubling the associated integrated diffuse scattering. Similar behavior was reported by Matsuura et al. (2006) for compositions of PMN doped with PbTi_O3. 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 et al. (2011) for NBT. We argue that a skin effect, analogous to that reported in the relaxors PZN and PMN-10%PT, can reconcile our single-crystal data with the powder data of Aksel et al. We believe this represents the first evidence of the relaxor skin effect in a lead-free relaxor.

Citation

Physical Review B

Volume

88

Issue

17

Pub Type

Journals

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Keywords

Relaxors, Lead-free, NBT, PMN, Polar nano-regions, Diffuse scattering, Neutron scattering

Citation

Ge, W. , Devreugd, C. , Phelan, D. , Zhang, Q. , Ahart, M. , Li, J. , Luo, H. , Boatner, L. , Viehland, D. and Gehring, P. (2013), Lead-free and Lead-Based ABO₃ Perovskite Relaxors with Mixed-Valence A-Site and B-Site Disorder: A Comparative Neutron Scattering Structural Study of (NaYd1/2^Bi_1/2)TiOd3^ and Pb(MgYd1/3^Nb_2/3)O₃, Physical Review B, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=914599 (Accessed September 17, 2024)