The Ca–Tb–Co–O system: phase diagram and crystal structures

Chem. Met. Alloys 17 (2024) 32-37

Oksana ZAREMBA, Nataliya-Mariya ANTONYSHYN, Roman GLADYSHEVSKII

https://doi.org/10.30970/cma17.0442

The phase diagram of the CaO–Tb₂O₃ (Tb₇O₁₂)–CoO system was built based on X-ray diffraction of 21 ceramic polycrystalline samples synthesized by a two-stage solid-state reaction method in air. The formation of two four-component oxides was observed. The existence of the CaTbCoO₄ phase (structure type La₂CuO₄, Pearson symbol oS28, space group Cmce, a = 0.53021(5), b = 1.1778(1), c = 0.52331(5) nm, R_B = 0.079) was revealed for the first time, whereas the formation of the Ca₂TbCo₂O₆ compound (structure type Ca₃Co₂O₆, Pearson symbol hR66, space group R-3c, a = 0.9040(1), c = 1.0625(2) nm, R_B = 0.146) was confirmed under the experimental conditions. Examination of other Ca_0.5R_0.5CoO₃ samples (R = rare-earth element) indicated the absence of perovskite phases in the corresponding systems, but confirmed the formation of CaRCoO₄ compounds with the tetragonal K₂NiF₄ (R = Pr³⁺, Nd³⁺, and Sm³⁺) or orthorhombic La₂CuO₄ (R = Eu³⁺ and Gd³⁺) structure types.

Arrangement of CoO₆ octahedra in CaTbCoO₄ (left) and Ca₂TbCo₂O₆ (right).

Keywords

Oxides / Solid-state synthesis / X-ray diffraction / Phase diagram / Crystal structure