Formation, thermal redox reaction and crystal structure of δ-CaCr₂O₄

The thermal redox reactions of a Ca–Cr–O system under different atmospheric conditions were investigated by cyclic thermodiffractometry and thermogravimetry. In the first heating process under a reductive atmosphere, CaCrO₄ decomposed into CaO and δ-CaCr₂O₄, a metastable form, with the liberation of free oxygen. In the second process under an oxidizing atmosphere, the CaO and δ-CaCr₂O₄ produced in the first reduction process reacted to regenerate CaCrO₄, indicating that this redox reaction could proceed reversibly upon controlling the atmosphere. The crystal structure of δ-CaCr₂O₄ was determined, for the first time, on the basis of high-resolution synchrotron powder X-ray diffraction data. δ-CaCr₂O₄ crystallized in a monoclinic structure with the space group P2₁/m. The structure can be described as a stacking of CrO₂ layers composed of edge-sharing CrO₆ octahedra, with calcium ions residing on-site between the layers. The calcium ions were coordinated by six oxygen atoms to form a trigonal prism geometry between the CrO₂ layers. The structure of δ-CaCr₂O₄ was different from those of low-temperature β-CaCr₂O₄ (distorted CaFe₂O₄-type) and high-temperature α-CaCr₂O₄ (P2-type layered structure).