NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL-STRUCTURES OF SR2RUO4 AND SR2IRO4 AT ROOM-TEMPERATURE AND AT 10-K

The crystal structures of the compounds Sr2RuO4 and Sr2IrO4 have been analyzed at room temperature and at 10 K with the neutron powder diffraction technique and the Rietveld method of profile fitting. Sr2RuO4 Crystallizes with the symmetry of space group I4/mmm and room temperature lattice parameters a = 3.8730(3) and c = 12.7323(9) angstrom, while Sr2IrO4 has the symmetry of space group I4(1)/acd and lattice parameters a = 5.4994(l) and c = 25.7841(8) angstrom. The unit cells of the two compounds are related to one another by the transformation matrix (1, - 1, 0/ 1, 1, 0/0, 0, 2). The main difference between the two structures is that the MO6 octahedra (M = Ru, Ir) have a regular, undistorted configuration in the ruthenium compound, while they are tilted by about 11-degrees around the c axis of the unit cell in the iridium compound. The oxygen atoms of the IrO2 layers were found to be disordered over two sets of positions x, x, 1/4 with x = 1/4 +/- delta. This means that the IrO6 octahedra assume two configurations and on a local level their relative orientation does not obey the symmetry requirements of space group I4(1)/acd in all cases. The MO6 octahedra are elongated along the c axis, and this distortion is more pronounced in Sr2RuO4 than in Sr2IrO4. The coordination of the strontium atoms is ninefold in both compounds. Because of the tilting of the IrO6 octahedra, however, the coordination polyhedra are different in the two cases. No phase transitions have been observed down to 10 K in either compound. (C) 1994 Academic Press, inc.