Sr₂RhO₄ and Sr₂IrO₄: Structural and magnetic studies of 4d and 5d transition metal analogs of La₂CuO₄

X-ray and neutron diffraction studies of Sr₂IrO₄ and Sr₂RhO₄ reveal structural distortions from the ideal K₂NiF₄ structure, in which the IrO₆ and RhO₆ octahedra are rotated about the crystallographic c-axes by 11° or 9.7°, respectively. These rotations reduce the symmetry from I4 mmm to I4₁ acd. We find that Sr₂IrO₄ is an electrical insulator which exhibits weak ferromagnetism at 250 K, attributable to canted antiferromagnetism associated with long-range three-dimensional ordering of the Ir⁴⁺ moments. Although no ferromagnetic ordering is found in Sr₂RhO₄, susceptibility data shows a peak possibly associated with two-dimensional short-range antiferromagnetic order. Doping of electrons in Sr₂RhO₄ results in compositions which show weak ferromagnetism, perhaps arising from double-exchange-type magnetic interactions due to the presence of rhodium in 4+ and 3+ oxidation states.