Structure-Composition-Property Relationships of 6H-BaTi1-yCoyO3-delta (0.1 <= y <= 0.4)

[摘要]：A solid solution of 6H-type BaTi1-yCoyO3-delta samples where 0.1 <= y <= 0.4 and delta <= 0.05 has been prepared in air and characterized by a combination of X-ray, neutron, and electron diffraction, X-ray absorption spectroscopy, hydrogen-reduction thermogravimetric analysis, high resolution electron microscopy, magnetic susceptibility measurements, and impedance spectroscopy. For y = 0.1, Co acts as an acceptor dopant and is incorporated as Co-III ions with charge balanced achieved by the formation of O(2) oxygen vacancies in the h-BaO3 layers that separate pairs of face-sharing octahedra. For samples with y > 0.1, Co is present as both Co-III and Co-II ions and in all cases delta similar to 0.03. This indicates a change from primarily aliovalent (Co-III) to isovalent (Co-IV) doping with increasing Co-content with a general formula BaTi1-yIV((Co0.06Coy-0.06IV)-Co-IV)O-2.97 for 0.2 <= y <= 0.4. All samples are semiconducting at room temperature with relative permittivity in the range similar to 20-50. The bulk conductivity increases with y and displays complex, non-Arrhenius-type behavior. All samples show paramagnetic behavior that can be fitted to the Curie-Weiss law from similar to 150 to 300 K with a negative Weiss temperature indicating antiferromagnetic interactions between the magnetic Co ions. X-ray absorption spectroscopy data on a sample of y = 0.1 that retained the 6H-structure after heat treatment under reducing conditions revealed the existence of mixed state Co-II and Co-III ions. A solid solution volume therefore exists for 6H-Ba(Ti,Co)O3-delta where the oxidation state of Co can vary from II to IV depending on the preparative conditions.

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