Oxygen displacements and search for magnetic order in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$

The crystal structure of the layered ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ system has been analyzed by neutron powder diffraction methods. The structure is formed by stacking two blocks of distorted ${\mathrm{SrRuO}}_3$ perovskite along the c axis, interleaved with SrO layers. The neighboring corner-sharing octahedra in each double perovskite block are rotated with respect to each other about the vertical axis so that the Ru-O-Ru angle in the ${\mathrm{RuO}}_2$ planes is about 165° rather than 180°. These rotations are correlated within each double perovskite block, but they are not correlated along the c axis, resulting in an intrinsic disorder that emphasizes the layered nature of this material. The resulting structure has the symmetry of space group Pban and lattice parameters $a=b=5.5016\mathrm{}\left(1\right)\mathrm{}\AA$ and $c=20.7194\mathrm{}\left(5\right)\mathrm{}\AA$ at 295 K. Data taken to 9 K showed no evidence of any structural phase transitions occurring below room temperature. We also searched for the development of long-range magnetic order to temperatures as low as 1.6 K, but no evidence of either ferromagnetic or antiferromagnetic long-range order was observed, with an upper limit of $0.05{\mu }_B$ for any possible ordered moment. This result contrasts with a reported ferromagnetic ordering at 104 K with an ordered Ru moment of $1.3{\mu }_B,$ which we believe was due to a phase other than ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7.$ We also searched for an induced moment, for applied fields up to 7 T, but did not observe any induced ferromagnetic moment within the same experimental limit.