Modelling of Disorder in Lead-Free Sodium Bismuth Titanate Ferroelectric Single Crystals Using Diffuse X-ray Scattering

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Copyright: Tung, Patrick
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Abstract
Lead-free piezoelectric materials are likely to be needed in the near future to replace lead-based materials in applications due to regulatory, health and safety issues. Local atomic structural disorder in these materials is suspected to be partially responsible for the functional properties. Therefore, the local structures in promising materials must first be elucidated in order for the future design of high performance lead-free piezoelectrics. Computer simulations have been carried out to model the crystal structure of lead-free pure Na1/2Bi1/2TiO3 (NBT) crystals to explore the effects of occupational disorder in the A-site of the unit cell. “Big-box” models of 30 x 30 x 30 perovskite unit cells were produced using Monte-Carlo simulations. The relationship between neighbouring atoms that occupy the A-site of the perovskite unit cell, known as the correlation coefficient, was simultaneously investigated along the [100], [110] and [111] directions of the crystal structure. Adjusting the correlation coefficient from negative values (Bi-Na or Na-Bi) to positive values (Bi-Bi or Na-Na) along the [100] direction caused changes to the [110] and [111] correlations. However, alterations of [110] and [111] correlations produced overall correlations of zero in the other two respective directions. The diffuse scattering patterns were calculated for the models and the effects of varying A-site occupancy correlations are evident from the contrasting features within these patterns. Furthermore, the calculated diffuse scattering patterns were compared with patterns from reported models and discrepancies were apparent between the two results. Additionally, diffuse X-ray scattering intensities from a single crystal of 0.96Na1/2Bi1/2TiO3 – 0.04BaTiO3 (NBT-4BT) collected at a temperature of 275 oC were analysed. Two distinct satellite peaks, not previously reported, suggest separate modulations in the crystal structure of 11 Å and 35 Å. Larger quantitative modelling is required to confirm such mechanisms.
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Author(s)
Tung, Patrick
Supervisor(s)
Daniels, John
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Publication Year
2014
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Thesis
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Masters Thesis
UNSW Faculty
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