Publication: Transport, electronic, and structural properties of nanocrystalline CuAlO_(2) delafossites
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2011-11-24
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Durá, O. J.
Boada, R.
Bauer, E.
López de la Torre, M. A.
Chaboy, J.
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American Physical Society
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Abstract
This work reports on the effect of grain size on the electrical, thermal, and structural properties of CuAlO_(2) samples obtained by solid-state reaction combined with ball milling. Electrical characterization made in microcrystalline and nanocrystalline samples shows that the electrical conductivity decreases several orders of magnitude for the nanocrystalline samples, and, in addition, there is a large discrepancy between the activation energies associated to thermoelectric power ES . The study of the Cu K-edge x-ray absorption spectra of the CuAlO_(2) samples shows that the local structure around Cu is preserved after the sintering process, indicating that the observed behavior of the electrical conductivity is of intrinsic origin. Complex conductivity measurements as a function of frequency allow us to discard grain-boundaries effects on the electrical transport. Thus, the changes in σ(T ) and S(T ) are interpreted in terms of charge localization in the framework of small polarons. This is in agreement with the analysis of the near-edge region of the absorption spectra, which indicates that sintering favors the Cu-O hybridization. As a consequence, oxygen atoms progressively lose their capability of trapping holes, and the electrical conductivity is also enhanced.
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© 2011 American Physical Society. We acknowledge financial support by JCCM through Projects No. PAI-05-013 and PCI08-0091, and the Spanish CICYT through Grants No. MAT2008-06542-C04-01 and MAT2008-06517-C02. R.B. acknowledges support from the Ministerio de Ciencia e Innovación of Spain. We are grateful to J. Santamaría for helpful discussions, and to L. Vázquez for the AFM images. The assistance of the SpLine staff during the experimental work at ESRF is acknowledged.
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