Crystallographic Studies of Ba12Nb8-xTaxCo4O36 (x=1,3,4,5,7)
Winnie K. Wong-Ng, Guangyao Liu, Igor Levin, Izaak Williamson, James A. Kaduk, Lan Li, Andres C. Hernandez
Crystal structures and X-ray reference powder diffraction patterns have been determined for the Ba12Nb8-xTaxCo4O36 (x = 1, 3, 4, 5, 7) series. Ba12Nb8-xTaxCo4O36 crystallize in the cubic perovskite structure with a space group Pm3m (No. 221) and the lattice parameter increasing from a = 4.08732(3) Å to a = 4.08894 (2) Å as x varies from 1 to 7. The Goldschmidt distortion parameter for the entire series is about 1.05 (1.0488 to 1.0506), being similar to that of the cubic end member BaNb2CoO12. Transmission electron microscopy of the composition Ba12Nb4Ta4Co4O36 (or Ba3(NbTaCo)O9) revealed nanoscale 1:2 ordering of Nb/Ta and Co, manifested in the presence of the diffuse superlattice reflections. As this ordering is limited to short range, the average structures were still refined assuming the ideal-perovskite cubic symmetry. X-ray powder diffraction patterns of the studied compounds have been submitted to the Powder Diffraction File (PDF).