Modeling the dielectric behaviors of perovskites : ferroelectrics and incipient ferroelectrics

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Modeling the dielectric behaviors of perovskites : ferroelectrics and incipient ferroelectrics

 

Author: Deng, Haiyao
Title: Modeling the dielectric behaviors of perovskites : ferroelectrics and incipient ferroelectrics
Degree: Ph.D.
Year: 2012
Subject: Oxide minerals.
Perovskite -- Electric properties.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xvi, 186 p. : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2551256
URI: http://theses.lib.polyu.edu.hk/handle/200/6776
Abstract: This dissertation is devoted to an atomistic understanding of the dielectric properties of oxide perovskites. I shall concentrate on the long-known barium titanate (BTO) and strontium titanate (STO), although other related materials will also be discussed. The BTO is known as a ferroelectric, which undergoes a series of polar structural phase transitions as temperature goes down. In the work presented here, the focus is placed on the tetragonal-cubic transition that occurs about 410K. A lot of theoretical work have been invested to clarify the transition mechanism, which pertains basically to the behaviors of the static dielectric constant and hence to the lowest frequency phonon mode. Those work were largely based on First-priciples computations or models whose origin need be revealed. Despite their usefulness, they shed little light upon the modes other than the lowest frequency one and their inter-relations. This situation may be partly due to the difficulty in establishing an atomistic model that is amenable to intuitive analysis. In this dissertation, I try to fill this gap and show how such a model can actually be achieved. In doing so, insights into other problems are obtained.
A central issue concerns the identity of the observed peaks in the imaginary part of the dielectric function. The assignment of these peaks are accomplished by comparison with the said model. Accordingly, the peaks around 180 cm⁻¹ and 510 cm⁻¹ are considered to correspond to O vibrations along and normal to the Ti-O-Ti bonds, respectively. The perceived anistropy is found to arise from the strong Ti-O covalent bonding. All remaining peaks are considered to correspond to Ti motions. These motions are highly anharmonic. As a result, these peaks are prone to temperature changes and spectrual weight transfer has to happen between them. Based on the model, one can explain (1) the huge ratio εa/εc and (2) the polar chain formation in the cubic phase. STO, on the contrary, is only an incipient ferroelectric. The model is equally applicable to this compound. The O vibrations are found at similar but slightly shifted frequencies. Such shifts are comprehensible by this model. In this compound, a key issue relates to the correlations between a zone boundary mode and a zone center mode. Both display softening behaviors. However, the softening with the latter is incomplete. The physics behind this incompleteness has been a matter for debate. In this dissertation, it is attributed to the inter-relations between these two modes. I explore this idea from both a vibronic and a phenomenological point of view. A fingerprint is found. Ferroelectricity can be induced in STO by replacing O¹ ⁶ with O¹⁸. The isotope effect is remarkable: the transition temperature can be as high as 24K. Different views have appeared to explain the effects. Based on the aforementioned model, I put forth a different idea, which states that, the main effects of the isotope substitution are to suppress the Ti zero-point fluctuations. This is signified by a decrease in the Ti tunneling frequency. In addition, I discuss the implications for cuprate superconductors.

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