OXYGEN DISPLACEMENTS AND SEARCH FOR MAGNETIC ORDER IN SR3RU2O7

The crystal structure of the layered Sr3Ru2O7 system has been analyzed by neutron powder diffraction methods. The structure is formed by sta cking two blocks of distorted SrRuO3 perovskite along the c axis, inte rleaved with SrO layers. The neighboring corner-sharing octahedra in e ach double perovskite block are rotated with respect to each other abo ut the vertical axis so that the Ru-O-Ru angle in the RuO2 planes is a bout 165 degrees rather than 180 degrees. These rotations are correlat ed within each double perovskite block, but they are not correlated al ong the c axis, resulting in an intrinsic disorder that emphasizes the layered nature of this material. The resulting structure has the symm etry of space group Pban and lattice parameters a = b = 5.5016(1) Angs trom and c = 20.7194(5) Angstrom at 295 K. Data taken to 9 K showed no evidence of any structural phase transitions occurring below room tem perature. We also searched for the development of long-range magnetic order to temperatures as low as 1.6 K, but no evidence of either ferro magnetic or antiferromagnetic long-range order was observed, with an u pper limit of 0.05 mu(B) for any possible ordered moment. This result contrasts with a reported ferromagnetic ordering at 104 K with an orde red Ru moment of 1.3 mu(B), which we believe was due to a phase other than Sr3Ru2O7. We also searched for an induced moment, for applied fie lds up to 7 T, but did not observe any induced ferromagnetic moment wi thin the same experimental limit.