Rotational and translational distortions of the crystal structure of the Sr2HrRuO6 (Hr = Ho, Dy, Gd, Eu) complex perovskites

Sr2HrRuO6 (Hr = Ho, Dy, Gd, Eu) complex perovskites were synthesized through the high-temperature solid-state reaction method, and their crystal structures were analyzed in detail as a function of the Hr-cation ionic radius. Results of powder XRD pattern measurement and Rietveld analysis of the experimental profiles show that the Sr2HrRuO6 compounds crystallize in a monoclinic distorted perovskite-like structure, P21/n (#14) space group, where the unit cell parameters are related to the primitive unit cell ap by image, image and c ≈ 2ap. The structures show an alternate distribution of the Ru5+ (2d: 0.5, 0, 0) and Hr3+ (2c: 0, 0.5, 0) making up RuO6 and HrO6 octahedra alternatively arranged in two interleaving fcc sublattices, where the O(1), O(2), and O(3) ions are localized at the corner of the octahedral, while the Sr2+ is located at the A-site, occupying the cavities built by the corner-sharing octahedra with Wyckoff position 4e. Due to the existence of mismatched ionic sizes between the ionic radii of the Sr2HrRuO6 compounds, the HrO6 and RuO6 octahedra are constrained to tilting around the [111]c, [001]c, and [110]c cubic directions so as to optimize the Sr–O inter-atomic bond lengths, tending to rotate the structure in order to fix the Ru5+ and Hr3+ ions on the M′ and M″ sites of the complex perovskites. The cell parameters a, b, and c, the inter-atomic bond angles, the inter-atomic bond lengths, and the tilting angles increase as the Hr-cation ionic radius increases. The mismatch that exists in the Sr2HrRuO6 ionic radius produces a large distortion from the ideal cubic symmetry. The pure perovskite-like phase of Sr2HrRuO6 is thermodynamically and kinetically stable at high temperatures above 1420 K, where it is entirely governed by the average size of the Hr3+ and Ru5+ cations.