Crystal structure and phase transitions in various functional perovskites
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There has been specific interest over the past decade in the discovery and development of new piezoelectric and ferroelectric materials for the use in functional devices, specifically with the aim of replacing the widespread use of PbZrₓTi₁₋ₓO₃. The work detailed in this thesis focuses on the structural characterisation and thermal behaviour of several perovskites possessing interesting physical characteristics, such as ferroelectricity or magnetism. Structural evolution and phase behaviour is characterised using Rietveld refinement techniques on high resolution powder neutron diffraction data. Additional analytical techniques such as symmetry mode analysis, permittivity measurements and second harmonic generation measurements are also often exploited. The work on the LiₓNa₁₋ₓNbO₃ system demonstrated a susceptibility to softening of the T₄ octahedral tilt mode up to a composition of at least x = 0.12, indicating that the LNN-X solid solution could yield a number of unique perovskite structures. A rationale for how this T₄ mode varies across the composition range is offered. The higher doped composition at a value of x = 0.20, displays even more intriguing structural behaviour with the adoption of not one but two variants of the very rare a⁺a⁺c⁻ Glazer tilt system. A detailed bond length/bond angle analysis as a function of temperature is used to rationalise the nature of the octahedral distortion that drives the c > a crossover in the rare earth orthoferrite LaFeO₃. Symmetry mode analysis is exploited to assist in the structural comparison to the related compound Bi₀.₅La₀.₅FeO₃, highlighting the anomalous behaviour it exhibits as a result of magnetoelectric coupling effects. The nature of the paraelectric – ferroelectric transition in the layered perovskitelike Dion Jacobson phase, CsBi₀.₆La₀.₄Nb₂O₇ is identified as a direct “avalanche” type transition, making it an example of a hybrid improper ferroelectric. Ferroelectricity in this case does not occur as a result of traditional second-order Jahn-Teller distortions, but is achieved via a mechanism known as trilinear coupling. Experimental analysis is important in understanding the intricacies of this trilinear coupling mechanism. Symmetry mode analysis of CsBi₀.₆La₀.₄Nb₂O₇ shows that two zone boundary primary order parameters (M₂⁺ and M₅⁻) associated with octahedral tilting condense simultaneously, and couple to a zone centre ferroelectric distortion mode (Γ₄⁻). The similar temperature dependency for the two octahedral tilt modes excludes the presence of an intermediary phase, suggesting that the trilinear coupling in this layered phase is strong. Detailed structural characterisations such as those highlighted in this thesis are of fundamental importance as they can identify new design-led approaches to functional materials.
Thesis, PhD Doctor of Philosophy
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