The crystal structure of LnFeAsO 1Ày (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO 1Ày is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs 4 -tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs 4 -tetrahedrons.

The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

The crystal and magnetic structure of Ca 2 RuO 4 ͑CRO͒ has been studied by powder neutron diffraction. CRO was synthesized in two different modifications ͑stoichiometric and containing excess oxygen͒ whose crystal structures are, in spite of strong differences in the lattice constants, closely related. Both structures are derived from the ideal K 2 NiF 4 structure type by a rotation of the RuO 6 octahedra around the long axis, combined with a tilt around an axis lying in the RuO 2 plane. The orientation of the tilt axis seems to distinguish the two room-temperature symmetries; the excess oygen compound is characterized by a smaller tilt angle and shorter Ru-O in-plane bond distances. Stoichiometric CRO undergoes large structural changes on cooling, though no symmetry change was detected. In contrast, the excess oxygen containing compound undergoes a first-order structural phase transition accompanied by a change from metallic to insulating behavior in the electric resistivity. Both compounds exhibit antiferromagnetic order below 110-150 K; for the stoichiometric sample, the onset of magnetic order is associated with several structural anomalies.

The phase diagram of Ca 2−x Sr x RuO 4 has been studied by neutron diffraction on powder and single-crystalline samples. The experiments reveal antiferromagnetic order and structural distortions characterized by tilts and rotations of the RuO6-octahedra. There is strong evidence that the structural details of the isovalent samples tune the magnetic as well as the electronic behavior. In particular we observe for low Sr-concentration a metal insulator transition associated with a structural change and magnetic ordering.