Mostaed, Ali (2017) Atomic structure study of pyrochlore ytterbium titanate. PhD thesis, University of Warwick.

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Abstract

There has been great interest in the magnetic behaviour of pyrochlore oxides with the general formula A2B2O7, in which rare-earth (A), and transition metal (B) cations are ordered on separate interpenetrating lattices of corner-sharing tetrahedra. Such materials exhibit behaviours including quantum spin-ice, (quantum) spin-liquid, and ordered magnetic ground states. Yb2Ti2O7 lies on the boundary between a number of competing magnetic ground states. Features in the low-temperature specific heat capacity that vary in sharpness and temperature from sample to sample suggest that in some cases the magnetic moments order, while in others the moments remain dynamic down to temperatures as low as ~16 mK. In this work, three different Yb2Ti2O7 single crystal samples, all grown by the optical floating zone technique but exhibiting quite different heat capacity behaviour, are studied by aberration-corrected scanning transmission microscopy (STEM). Atomic-scale energy-dispersive X-ray (EDX) analysis shows that a crystal with no specific heat anomaly has substitution of Yb atoms on Ti sites (“stuffing"). In fact, EDX analysis shows for the first time that “stuffing” of Yb (A) cations onto Ti (B) sites in the lattice can be observed directly in the pyrochlore structure. Moreover, I show that the detailed intensity distribution around the visible atom columns in annular dark field STEM images is sensitive to the presence of nearby atoms of low atomic number (in this case oxygen) and find significant differences between the samples that correlates both with their magnetic behaviour and measurements of Ti oxidation state using electron energy loss spectroscopy. These measurements support the view that the magnetic ground state of Yb2Ti2O7 is extremely sensitive to disorder. On the other hand, structural modification of ytterbium titanate by deviation from stoichiometry as well as electron irradiation is studied in this work. I show that Ti excess is accommodated by Yb cation vacancies while Yb excess is compensated by Yb interstitials. Furthermore, two MATLAB programs, Detect Columns and BurgersVectors, to analyse STEM images and calculate the dislocation density tensor of STEM images as well as their Burgers vector(s) are respectively introduced in this work.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Ytterbium, Titanates, Rare earth metals -- Magnetic properties, Transition metals -- Magnetic properties, Electron microscopy
Official Date:	April 2017
Dates:	Date Event April 2017 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Beanland, R. ; Balakrishnan, Geetha
Format of File:	pdf
Extent:	xxiii, 111 leaves : illustrations, charts
Language:	eng

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