Sr4Ru3.05O12: A New Member of the Hexagonal Perovskite Family

Sr4Ru3.05O12crystals have been synthesized under hydrothermal conditions (500°C, 1800 bars). The crystal structure was determined by X-ray single-crystal diffraction. The unit cell is hexagonal and presents a superstructure (as= ab=9.641(2) ? andcs=cb=18.186(5) ?. The average structure has been determined (P63mcspace group,ab=5.566(4) ?,cb=18.186(5) ?, andZ=2). The structure refinement converged toR=0.057 andRw=0.032 for 186 independent reflections. The crystal structure is based on the 8H (cchc)2close-packed stacking of SrO3layers with face-sharing octahedra partially occupied by the ruthenium atoms. Electron diffraction observations and high-resolution electron microscopy image observations recognize the X-ray average structure. The superstructure spots appear clearly in the ED patterns and most of the crystals display diffuse streaks alongc*, which are consistent with the disorder clearly imaged in the HREM images. Heating Sr4Ru3.05O12crystals leads to the disappearance of the superstructure.