Abstract
The high-temperature phase behaviour of the ferroelectric layered perovskite Bi4Ti3O12 has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that Bi4Ti3O12 undergoes two separate structural transitions from the aristotype tetragonal phase (space group I4/mmm) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, B1a1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above T C with tetragonal symmetry, space group P4/mbm. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi3+ cations within the perovskite blocks.
Highlights
Bi4Ti3O12 is an n = 3 member of the Aurivillius family of layered perovskite ferroelectrics (Aurivillius, 1950; Subbarao, 1962), (Bi2O2)AnÀ1BnO3n+1
The high-temperature phase behaviour of the ferroelectric layered perovskite Bi4Ti3O12 has been re-examined by high-resolution powder neutron diffraction
This study confirms that Bi4Ti3O12 undergoes two separate structural transitions from the aristotype tetragonal phase to the ambient-temperature ferroelectric phase
Summary
Variabletemperature crystallographic studies of Bi4Ti3O12 using electron microscopy (Nistor et al, 1996), powder X-ray diffraction (PXRD) (Hirata & Yokokawa, 1997) and high-resolution powder neutron diffraction (PND) (Hervoches & Lightfoot, 1999) concluded that Bi4Ti3O12 did adopt the aristotype I4/mmm structure at temperatures well above the ferroelectric Curie temperature, TC ’ 675C These analyses did not cover the nature of the evolution of the ambient-temperature monoclinic phase towards this aristotype tetragonal phase; for example, the present author’s previous study (Hervoches & Lightfoot, 1999) only reported data at two intermediate temperatures (500 and 650C). The optical microscopy measurements of Iwata et al (2013) suggested a tetragonal phase above TC, but one of lower
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