X-ray Diffraction and Density Functional Theory Studies of R2FeCoO6 (R=Pr, Nd, Sm, Eu, Gd)
Winnie K. Wong-Ng, Guangyao Liu, Igor Levin, Izaak Williamson, P Ackerman, Kevin R. Talley, Khawla AlHamdan, James A. Kaduk, Lan Li
The structure and thermoelectric properties of a series of lanthanide iron cobalt oxides, R2FeCoO6 (R = Pr, Nd, Sm, Eu and Gd), have been investigated. The space group of these compounds was confirmed to be orthorhombic Pnma (No.62), Z=4. From Gd to Pr, the lattice parameter a varies from 5.50710(13) Å to 5.46635(13) Å, b from 7.56175(13) Å to 7.7018(2) Å, c from 5.29200(8) Å to 5.44338(10) Å, and unit-cell volume V from 229.170(9) Å3 to 220.376(9) Å3, respectively. While the trend of V follows the trend of the lanthanide contraction, the lattice parameter a increases as the ionic radius r(R3+) decreases. X-ray diffraction and transmission electron microscopy confirm that Fe and Co are disordered over the octahedral sites. The structure distortion of these compounds is evidenced in the tilt angles theta, phi, and omega, which represent rotations of an octahedron about the pseudo-cubic perovskite p, p and p axes. All three tilt angles increase across the lanthanide series (for R=Pr to R= Gd: increases from 12.27 deg to 15.15 deg, from 7.52 deg to 15.78deg, and from 14.36deg to 21.74deg), indicating a greater octahedral distortion as r(R3+) decreases. The bond valence sum for the 6-fold (Fe/Co) site and the 8-fold Ba site of R2FeCoO6 reveal no significant bond strain. Density Functional Theory calculations for Pr2FeCoO6 support the disorder of Fe and Co and suggest that this compound is a semiconductor with a band gap of 0.43 eV. X-ray diffraction patterns of the R2FeCoO6 samples were submitted to the Powder Diffraction File.