Single crystals of CaIrO3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca4IrO6 structure. The IrO6 octahedra are significantly distorted, in contrast to other octahedral Ir4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling’s rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO3-type post perovskite MgSiO3 crystals may develop under the shear flow in the Earth’s mantle.

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