Inference of oxygen vacancies in hydrothermal Na0.5Bi0.5TiO3

Abstract

A high-resolution x-ray powder diffraction study has been made of pseudo-rhombohedral and tetragonal phases in Na 0.5 Bi 0.5 TiO 3 (NBT), produced via hydrothermal and conventional solid-state methods. Hydrothermal NBT exhibits significantly greater structural distortion at room temperature than solid-state NBT. Peak widths and superstructure peak intensities show a phase transition at ~ 305 °C, with trends suggesting that the structure tends towards cubic symmetry at this temperature. Structural refinements indicate that the transition occurs via a phase coexistence region with no clear intermediate phase. Piezoelectric data show evidence of polarisation pinning in hydrothermal NBT, interpreted as a high proportion of oxygen vacancies. TextSodium bismuth titanate, Na 0.5 Bi 0.5 TiO 3 (NBT) is a widely-studied relaxor perovskite of potential use as a lead-free piezoelectric, particularly as the end-member of a range of solid solutions including NBT-BaTiO 3 and NBT-K 0.5 Bi 0.5 TiO 3 . 1 At room temperature, the structure has long been designated as rhombohedral R3c, 2 although recent studies have suggested monoclinic Cc symmetry for both single crystals 3 and sintered ceramics. 4 However, the deviation from rhombohedral symmetry is very small and the room-temperature structure may be considered pseudo-rhombohedral. On heating, a phase transition to tetragonal symmetry is known to exist, but has been reported to take place over a range of temperatures: 278-291 °C, 5 240-270 °C 6 and 300-320 °C. 7 At the boundary between pseudorhombohedral and tetragonal structures, some studies 8 have shown the existence of a two-phase region, while others 9 have proposed the presence of an intermediate phase. The phase transition to the cubic structure takes place at 500-550 °C 8,10 although some studies 11,12 refer to the pseudorhombohedral -tetragonal transition as the ferroelectric Curie temperature.In this study, the structures, phase transitions and piezoelectric properties of polycrystalline NBT produced by a low-temperature hydrothermal method are investigated and compared with those of a sample made by conventional solid-state synthesis. This study is motivated by the observation over many years that the structures and physical properties of NBT can vary significantly depending on the synthetic method adopted and the thermal history of the material. 4 Limited previous studies of hydrothermal NBT have been undertaken 12,13 but the structural phase transitions and piezoelectric properties of hydrothermal NBT have not been investigated before.NBT was synthesised in an autoclave with a polytetrafluoroethane liner using a 1:4 molar ratio of Bi 2 O 3 :TiO 2 (99.9%, Janssen Chimica, -Bi 2 O 3 , and >98%, BDH, anatase polymorph, respectively). Typically ~4 g Bi 2 O 3 and the corresponding amount of TiO 2 were reacted in a 125 ml autoclave. 60 ml of 10 M NaOH solution was added and the suspension stirred for 2 hours. The autoclave was placed in an oven at 240 °C and left at autogenous pressure for 5 days before cooling. ...

Additively‐manufactured piezoelectric devices

“…The d 33 when fully poled is 636 (26) pC N À1 , but when reversed, it is À618(34) pC N À1 . Similar behaviour has been observed in Pb(Zr,Ti)O 3 and Na1 /2 Bi1 /2 TiO 3 ceramics and assigned to polarisation pinning by defects [23,24]. In the case of the PMNT ceramics presented here, the effect is relatively slight and may simply reflect the slightly higher porosity of the MSL ceramics.…”

Hydrothermal epitaxy and resultant properties of EuTiO3 films on SrTiO3(001) substrate

“…Such issues have been reported in various perovskites prepared hydrothermally [30-32]. These impurities can contribute to charge balance in the as-prepared perovskites and be removed by annealing to produce defects, which when coupled with a metal can account for charge compensation [30,31].…”

Investigation of the depolarisation transition in Bi-based relaxor ferroelectrics

“…This was also observed for several NBT-based materials 9 and for NBT-based materials under an applied electric field, 13,14 leading to the proposal of a transition consisting of two processes; (1) domain randomisation, causing loss of macroscopic polarisation, followed by (2) the break-up of ferroelectric domains into PNRs, causing a change in dielectric behaviour and associated with the transition from field-induced ferroelectric to ergodic relaxor. 15 Neutron 16 and X-ray 17,18 diffraction (XRD) experiments performed on unpoled NBT-based materials do not give unambiguous evidence of a long-range phase transition at temperatures close to T d and neither have recent birefringence experiments. 19 Subtle changes in XRD peak profiles and the central frequencies of Raman peaks have been observed close to T d , 18 as have the appearance of nanodomains in electron microscopy 20 that point to a phase transition that takes place without long-range order.…”