TEM Studies of Modulated Mixed A-site Perovskites

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Doctor of Philosophy (PhD)
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Materials Science & Engineering
Mechanics of Materials
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This dissertation focused on a unique family of mixed A-site (A+A3+)(B2+W6+)O6 (A+ = Na, K; A3+ = La, Ce, Pr, Nd, Gd, Tb; B2+ = Mn, Mg, Co, Ca, Sc) perovskites that form unusual and complicated structures comprised of periodic nanoscale modulations in composition, structure, and strain. In the first phases of the work these systems were investigated by electron diffraction, high resolution imaging, and Z-contrast imaging using a conventional TEM. In the later phases of the work the application of these techniques using aberration-corrected electron microscopes located at Oak Ridge National Laboratory yielded insights to local variations in chemistry and local displacements of ions that were inaccessible using the non-corrected instruments. (NaNd)(MgW)O6 was found to form a two-dimensional nanocheckerboard modulation with a 14ap x 14ap repeat involving both structural and chemical modulations. Two previously unknown perovskites, (KLa)(CaW)O6 and (NaLa)(CaW)O6, were prepared in which large Ca2+ cations were successfully incorporated onto the B-site. For (KLa)(CaW)O6, an incommensurate two-dimensional nanocheckerboard modulation with a ~9.4x9.4ap repeat was observed. (NaLa)(CaW)O6 instead showed a one-dimensional ~16(110)ap modulation. This system has one of the lowest tolerance factors (0.892) ever reported. Three of the above systems, (NaNd)(MgW)O6, (NaLa)(MgW)O6, and (NaLa)(CaW)O6 and one other sample, (Na0.8Nd1.07)(MgW)O6¬¬ were investigated by high resolution Z-contrast imaging using aberration-corrected STEM. In contrast to previously published reports, (NaLa)(MgW)O6 was not found to show any evidence for compositional modulations. However both (NaNd)(MgW)O6 systems and (NaLa)(CaW)O6 showed clear compositional segregation on the A-site. The sub-angstrom resolution of the STEM images allowed characterization of small (~0.15Å) correlated ionic displacements of the A-site columns from their ideal centered positions. A model for these displacements was developed based on an a-a-c+ tilt scheme recently found in the closely related, non-modulated, (NaLa)(MnW)O6 system. When combined with layered A-site ordering of an alkali and a rare earth, this tilt system has been shown to induce "improper" ferroelectricity and ferrielectric polarization from antiparallel movements of the A+/A3+ cations. This model invoked the formation of periodic ferrielectric domains with four possible in-plane (110)ap polar dipoles separated by a periodic array of 90° (100)ap and 180° (110)ap domain boundaries.

Peter K. Davies
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