NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL-STRUCTURES OF SR2RUO4AND 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 diffr action technique and the Rietveld method of profile fitting. Sr2RuO4 C rystallizes with the symmetry of space group I4/mmm and room temperatu re 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 paramet ers a = 5.4994(l) and c = 25.7841(8) angstrom. The unit cells of the t wo 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 struc tures is that the MO6 octahedra (M = Ru, Ir) have a regular, undistort ed configuration in the ruthenium compound, while they are tilted by a bout 11-degrees around the c axis of the unit cell in the iridium comp ound. 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 mean s that the IrO6 octahedra assume two configurations and on a local lev el 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 t han 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) 19 94 Academic Press, inc.