CrzGezTes is a new layered material belonging to the lamellar ternary MzXzTen chalcogenides family (where M is B 3+ oxidation stale metal and Xz a silicon or a germanium pair). It was synthesized from a stoichiometric mixture of the element5 and heated in a sealed evacunep quam tube for 20 d at 700'C. The crystal symmetry is rhombohedral. of space group R3, with the following hexagonal cell pameters: (1 = h = 0.68275(4) nm. c = 2.056 19(9) nm. V = 0.830 l(1) nm3 and 2 = 3. The crystal StNcture of CrzGezTes w3s solved using both x-ray single-crystal diffraction and neumn powder diffraction data. Growth defects were detected and investigated using high-resolution elecmn microscopy. The magnetic structure and properties of CrzGezTen have been determined by neutron powder d i h c t i o n and susceptibility and magnetization measurements. Below 61 K, CrzCqTeg is ferromagnetic with spins aligned along the c axis ofthe cell (p(&+) = 2.80(4)pe at 5 K). The thermal evolution of the magnetic moment5 below Tc is given. At mom temperature, CrzGezTen present5 a p = 0.02 z2 cm resistivity. Above Tc, the thermal evolution of the resistivity can be fitted 3s P = exp(A/kT), where A = 0.2 eV. Band-stmctum and crystal orbitals of CrzGezTen have been calculated using the extended Hiickel method and indicate no gap but highly l o d i Cr 3d stales. giving evidence of a hopping mechanism for CaGezTeg elrctiiul conductivity.

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.