Abstract
With an aim to understand the origin of the low thermal expansion coefficients (TECs), cation-substituted YBaCo4O7-type oxides have been investigated by in-situ neutron diffraction, bond valence sum (BVS), thermogravimetric analysis, and dilatometry. The compositions YBaCo3ZnO7+δ, Y0.9In0.1BaCo3ZnO7+δ, and Y0.9In0.1BaCo3Zn0.6Fe0.4O7+δ were synthesized by solid-state reaction at 1200 °C. Rietveld refinement of the joint synchrotron X-ray and neutron diffraction data shows that the Zn and Fe dopants have different preferences to substitute the Co ions in the 6c and 2a sites. The bulk thermal-expansion coefficients of YBaCo3ZnO7+δ, Y0.9In0.1BaCo3ZnO7+δ, and Y0.9In0.1BaCo3Zn0.6Fe0.4O7+δ are, respectively, 9.42, 9.76, and 9.06 × 10−6 °C−1. Neutron diffraction data show that the low anisotropic TEC along the a-axis is the main contributor to the low bulk TECs. With the substitution of In, Zn, and Fe in Y0.9In0.1BaCo3Zn0.6Fe0.4O7+δ, the anisotropic and bulk TECs could be reduced to 8.94 and 9.06 × 10−6 °C−1, respectively, mainly due to the suppression of the change in Co–O bond length in CoO4 polyhedra. The observed weight loss during heating is due to the loss of interstitial oxide ions, as revealed by neutron diffraction and BVS map. Y0.9In0.1BaCo3Zn0.6Fe0.4O7+δ has the lowest area-specific cathodic polarization resistance of 0.14 Ω cm2 (Rtotal/2) at 700 °C in air.
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