Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Abstract

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQISwidth) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in 23Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.

Acoustic Anomalies and Phase Transition Behaviors of Lead-Free Piezoelectric (Na1/2Bi1/2)TiO3-xBaTiO3 Single Crystals as Revealed by Brillouin Light Scattering

“…This indicates that, without any prepoling, NBT- x BT shows typical relaxor behaviors in the BT composition of 5~8% near the MPB. This result is consistent with a recent dielectric study, which determined that relaxor state is maintained over a specific composition range near the MPB [16]. …”

“…The broad minimum of ν B and the concomitant broad maximum of Γ B without any sharp, discontinuous changes indicate that there is no long-range ferroelectric order at low temperatures. This is consistent with the recent 23 Na NMR results, which showed that NBT- x BTs near the MPB exhibit substantial octahedral tilting disorder and relaxor states [16]. The microscopic origin of the gradual mode softening of the LA mode is attributed to the coupling between the strains caused by acoustic waves and local polarizations of the dynamic PNRs [34].…”

“…The relation between the local structure and the relaxor behavior, as well as the relation between the local structure and the average structure, has been considered as an important subject and investigated intensively [13,14,15]. A more recent NMR study has suggested that the octahedral tilting disorder is responsible for the relaxor behaviors of NBT- x BT [16]. …”

Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] They found a sequence of phase transitions from the low temperature rhombohedral via a tetragonal to a high temperature cubic phase. These findings were confirmed by X-ray diffraction studies, 7,12,13,[18][19][20][21][22][23][24][25] other characterization techniques 13,18,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] and theoretical ab initio studies. [41][42][43][44][45][46][47][48] However, some of the transmission electron microscopy (TEM) studies reported an intermediate orthorhombic phase.…”

Local disorder in Na0.5Bi0.5TiO3-piezoceramic determined by 3D electron diffuse scattering

“…The difficulty in assigning a definitive average structure for NBT-based materials and the mismatch with the macroscopic properties stems from the short range order/disorder present in these types of materials. Several studies have tried to tackle this problem using EXAFS 23,24 , NMR 25 , X-ray diffuse scattering 26–31 , TEM 32–35 , pair-distribution function (PDF) 5,36 or DFT calculations 37–41 . EXAFS measurements 23 suggest a highly distorted local coordination for Bi with a shorter minimum Bi-O bond of 2.2 Å than the one proposed by diffraction experiments, which is 2.5 Å. RMC refinement of PDF data reveals an anisotropic Bi-O bonding 5,36 which suggest that Bi is strongly displaced off-center in the oxygen polyhedron.…”