Structural analysis of excess-anion C-type rare earth oxide: a case study with Gd_1−xCe_xO_1.5+x/2 (x = 0.20 and 0.40)

Structural analysis of anion-rich C-type Gd₂O₃ was carried by the Rietveld refinement of the powder X-ray diffraction data for compositions Gd_0.8Ce_0.2O_1.60 and Gd_0.6Ce_0.4O_1.70. Both compounds have a body-centred cubic lattice (space group Ia, No. 206, Z = 32) with unit-cell parameters of 10.8488 (1) and 10.8542 (1) Å, respectively. Both of these compounds are iso-structural with the C-type rare earth oxides, with excess anions as required for charge balance. The structural analysis reveals that there are two different kinds of metal ion site, namely 8b (M1) and 24d (M2), and two different kinds of anion sites, namely 48e (O1) and 16c (O2). The excess anions occupy the 16c (xxx) sites. The two metal ions each form an approximately eightfold-coordination polyhedron with O1 and O2. The details of these two compositions are explained and compared with both the CeO₂ structure and the Gd₂O₃ structure, i.e. the end member.