Crystal and magnetic structure of Ca 2 RuO 4 : Magnetoelastic coupling and the metal-insulator transition

The crystal and magnetic structure of ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ (CRO) has been studied by powder neutron diffraction. CRO was synthesized in two different modifications (stoichiometric and containing excess oxygen) whose crystal structures are, in spite of strong differences in the lattice constants, closely related. Both structures are derived from the ideal ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ structure type by a rotation of the ${\mathrm{RuO}}_{6}$ octahedra around the long axis, combined with a tilt around an axis lying in the ${\mathrm{RuO}}_{2}$ 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\char21{}150 K; for the stoichiometric sample, the onset of magnetic order is associated with several structural anomalies.