A transmission electron microscopy study of the A-site disordered perovskite Na0.5Bi0.5TiO3

doi:10.1016/j.actamat.2007.12.027

Acta Materialia

Volume 56, Issue 8, May 2008, Pages 1753-1761

https://doi.org/10.1016/j.actamat.2007.12.027 Get rights and content

Abstract

A transmission electron microscopy study of Na_0.5Bi_0.5TiO₃ (NBT) crystals shows two types of ferroelectric domains characterized by interface boundaries lying in the {1 1 0}_C and {1 0 0}_C planes. A one-dimensional {1 0 0}_C modulated texture is also observed locally. The electron diffraction study reveals the presence of (0 0 1)_T tetragonal platelets a few cells thick within the R3c matrix. They develop within the three basal {1 0 0}_C planes of the prototype phase and represent relics of the high-temperature tetragonal phase. The loss of symmetry compared to the average structure of NBT can thus clearly be attributed to the existence of these tetragonal platelets. The {1 0 0}_C modulated texture corresponds to a modulation of strain induced by the coexistence of two types of octahedra tilting systems: a⁻a⁻a⁻ for the rhombohedral matrix and a⁰a⁰c⁺ for the tetragonal platelets. These (0 0 1)_T platelets indirectly intervene in the relaxation process classically encountered at about 230 °C, in marked contrast to the behaviour of Pb(Mg_1/3Nb_2/3)O₃.

Introduction

During the past 50 years, the perovskite compounds (ABO₃), and especially those exhibiting piezoelectricity and ferroelectricity, have attracted much interest for use in electronic devices such as actuators, sound transducers, sensors, etc. Some of these perovskites belong to the category known as relaxors (or relaxor ferroelectrics), which are typically characterized by cationic disorder. Most relaxors, typified by Pb(Mg_1/3Nb_2/3)O₃ (PMN), are lead-bearing compounds exhibiting B-site disorder. Relaxor behaviour is now quite well understood [1] and is generally attributed to the presence of polar nano regions (PNRs) characterized by local ordering [2]. Evidence for the existence of PNRs was first produced by Burns et al. on PMN, Pb(Zn_1/3Nb_2/3)O₃ (PZN) [3] and (Pb,La)(Zr,Ti)O₃ (PLZT) [4], [5]. These regions have already been directly observed by transmission electron microscopy (TEM) in many relaxors such as PMN, Pb(Sc_1/2 Ta_1/2)O₃, Pb(In_1/2 Nb_1/2)O₃ and PZN, Pb(Ni_1/3 Nb_2/3)O₃ [6], [7], [8], [9], [10].

Na_0.5Bi_0.5TiO₃ (NBT) is one of the rare lead-free perovskites with A-site disorder. This compound is also a well-known example of a relaxor which also exhibits a long-range ferroelectric order, whereas classical relaxors do not [1]. The structural study of NBT was performed by Rietveld refinements of powder neutron diffraction [11]. This study shows that NBT belongs to the polar R3c space group. Within NBT, the oxygen octahedra are tilted according to the tilt system a⁻a⁻a⁻ (Glazer notation [12]) and both A and B cations are mainly displaced along the 3-fold axis [11].

However, various experiments have shown deviation from the ideal R3c structure. The tetragonal to rhombohedral (T → R) phase transition was very recently studied by means of in situ neutron scattering of NBT single crystal [13]. According to the authors, the rhombohedral phase presents a one-dimensional incommensurate modulation linked to an A-site ordering that took place within the high-temperature tetragonal phase along the [0 0 1]_T crystallographic direction. Na–Bi ordering has also been invoked by Petzelt et al. on the basis of IR and Raman spectroscopy studies of NBT [14].

In addition, X-ray patterns of NBT obtained by X-ray diffuse scattering (XRDS) experiments provide evidence of asymmetric sharp streaks emanating from Bragg peaks, together with broad diffuse rods of scattering [15]. Their presence here is, however, attributed to the occurrence of Guinier Preston zones (GPZs) displaying monoclinic symmetry. They are orientated perpendicular to all 〈1 0 0〉_C pseudo-cubic directions of the rhombohedral cell. The nature of the nanoscale structural features that give rise to the observed diffraction features was ascertained by Monte Carlo simulations based on the hypothesis of Na–Bi disorder. The assumption of monoclinic GPZ is also confirmed by a recent study of NBT by X-ray absorption fine structure (XAFS) spectrometry which has shown an additional displacement of Bi cations perpendicular to the 〈1 1 1〉_C polar axis [16].

Comprehension of the structural features of NBT in terms of nanostructure and local structure represents a challenging problem both for the understanding of the origin of the relaxor behaviour and for the knowledge of the mechanisms that operate during phase transition. As presented above, up to now the origin of the local departure from the average structure of NBT is still under controversy. Two main points still remain to be discussed: the nanoscale structure and the local structure associated with cationic order–disorder in the A-site of this perovskite. In view of this, a TEM study was undertaken to shed light on the nanostructure of NBT.

Section snippets

Experimental

The compound Na_0.5Bi_0.5TiO₃ was prepared by a classical solid state route. Stoichiometric amounts of highly pure reagents (Na₂CO₃, Bi₂O₃ and TiO₂) were mixed and fired at 900 °C for 4 h. After grinding, the obtained powder was pressed into pellets and then sintered at 1120 °C for 1 h. A JEOL 2010 microscope with a double tilting stage was used for diffraction studies, operating at 200 kV. Samples for TEM were prepared by mechanical polishing followed by Ar-ion beam milling.

Results

NBT crystals show a complex pattern of ferroelectric domains which generally present lamellar (Fig. 1) or needle-shaped (Fig. 2) morphology. There are two types of mesoscopic domains, characterized, in the pseudo-cubic cell of the perovskite, by interface boundaries lying in the {1 1 0}_C (Fig. 1) and {1 0 0}_C planes (Fig. 1, Fig. 2).

When observed along the $[\bar{1} 1 0]_{C}$ orientation (Fig. 1), the NBT crystals show (1 1 0)_C interface boundaries that are viewed edge-on. Three of them are indicated by white

Discussion

This study has shown for the first time some results obtained by TEM on the mesostructure and nanostructure of NBT. The first part of the discussion will focus on a comparison of the domain structure observed in NBT with those found in closely related pseudo-cubic perovskites. The second part of the discussion will be devoted to the understanding of the local departure from the average structure so often invoked in recent structural studies of NBT [13], [14], [15]. This point is still a matter

Conclusion

The present study shows the following results:

•
NBT crystals show a complex pattern of ferroelectric domains which generally present lamellar or needle-shaped morphologies. There are two types of domains characterized by interface boundaries lying in the {1 1 0}_C and {1 0 0}_C planes. They are associated with pseudo-merohedral twinning laws. In addition, a {1 0 0}_C one-dimensional modulated texture is observed.
•
For the first time, an electron diffraction study has revealed that at ambient temperature NBT

Acknowledgements

The authors would like to thank D. Mercurio and P. Boullay for valuable and fruitful discussions and for their interest in this work. We also wish to thank the referees for their pertinent and useful review and are grateful to B. Soulestin for careful preparation of TEM specimens.

References (25)

G.A. Samara
Solid State Phys
(2001)
G. Burns et al.
Solid State Commun
(1983)
V. Dorcet et al.
J Eur Ceram Soc
(2007)
C.A. Randall et al.
J Mater Res
(1990)
G. Burns et al.
Phys Rev B
(1983)
G. Burns
Phase Trans
(1985)
M. Yoshida et al.
Ferroelectrics
(1998)
K.Z. Baba-Kishi et al.
Philos Mag
(2006)
C. Caranoni et al.
Ferroelectrics
(2000)
X. Pan et al.
J Am Ceram Soc
(1998)

M. Furuya et al.

J Appl Phys

(1994)

G.O. Jones et al.

Acta Cryst B

(2002)

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A transmission electron microscopy study of the A-site disordered perovskite Na0.5Bi0.5TiO3

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusion

Acknowledgements

Solid State Phys

Solid State Commun

J Eur Ceram Soc

J Mater Res

Phys Rev B

Phase Trans

Ferroelectrics

Philos Mag

Ferroelectrics

J Am Ceram Soc

J Appl Phys

Acta Cryst B

A transmission electron microscopy study of the A-site disordered perovskite Na_0.5Bi_0.5TiO₃