Abstract. At 320 K, the crystal structure of CeRuSn is commensurate with the related CeCoAl type of structure by the doubling of the c lattice parameter. However, with lowering the temperature it becomes incommensurate with x and z position parameters at all three elemental sites being modulated as one moves along the c-axis. The resulting crystal structure can be conveniently described within the superspace formalism in (3+1) dimensions. The modulation vector, after initially strong temperature dependence, approaches a value close to q nuc = (0 0 0.35). Below T N = 2.8 (1) K, CeRuSn orders antiferromagnetically with a propagation vector q mag = (0 0 0.175), i.e. with the magnetic unit cell doubled along the c-axis direction with respect to the incommensurate crystal structure. Ce moments appear to be nearly collinear, confined to the a − c plane, forming ferromagnetically coupled pairs. Their magnitudes are modulated between 0.11 and 0.95 µ B as one moves along the c-axis.

We report on the crystal and magnetic structures, magnetic, transport and thermal properties of U 2 Rh 2 Sn single crystals studied in part in high magnetic fields up to 58 T. The material adopts a U 3 Si 2 -related tetragonal crystal structure and orders antiferromagnetically below T N = 25 K. The antiferromagnetic structure is characterized by a propagation vector k = (0 0 1 2 ). The magnetism in U 2 Rh 2 Sn is found to be associated mainly with 5f states. However, both unpolarized and polarized neutron experiments reveal at low temperatures in zero field non-negligible magnetic moments also on Rh sites. U moments of 0.50(2) µ B are directed along the tetragonal axis while and Rh moments of 0.06(4) µ B form a non-collinear arrangement confined to the basal plane. The response to applied magnetic field is highly anisotropic. Above ∼ 15 K the easy magnetization direction is along the tetragonal axis. At lower temperatures, however, a stronger response is found perpendicular to the c axis. While for the a axis no magnetic phase transition is observed up to 58 T, for the field applied at 1.8 K along the tetragonal axis we observe above 22.5 T a field-polarized state. A magnetic phase diagram for the field applied along the c axis is presented.

To study the influence of the sample preparation procedure on the superconducting properties of FeSe x Te 1−x , we have grown two FeSe 0.4 Te 0.6 crystals and investigated their superconducting properties. One of the crystals possessed a secondary phase of Fe 3 Se 2.1 Te 1.8 , while the other was a high-quality FeSe 0.4 Te 0.6 single crystal. We have checked the sample compositions and phases via energy dispersive x-ray spectroscopy to obtain a more sophisticated picture of the inclusions. Our susceptibility measurements under hydrostatic pressure show that neither pressure up to 9 kbar nor stress is sufficient to obtain a superconducting state in the homogeneous crystal. The critical temperature of two-phase FeSe 0.4 Te 0.6 increases at 8.9 kbar from 12.3 K to T c =17.9 K. Therefore, we conclude that inhomogeneities are a necessary feature for providing superconductivity in this iron chalcogenide system. We have prepared Fe 3 Se 2.1 Te 1.8 polycrystals and studied their magnetic properties for comparison.

Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe 0.6 Se 0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: They consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20-30 vol.%). These have different crystal structure and chemical composition and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe 0.6 Se 0.4 is non-superconducting.

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure contains two different crystallographic Ce sites, we observe that a Ce site with larger nearest-neighbor distances is clearly more susceptible to the applied magnetic field, whereas the other is hardly polarizable. This finding clearly documents that different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce 3+ and nonmagnetic Ce (4−δ)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a redistribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions coexist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce 3+ state (at sites with the largest Ce-Ru interatomic distances), whereas Ce (4−δ)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous γ -α transition in elemental cerium is discussed.