H. Fuess

Preparation and crystal structure of a new high-pressure phase (V0.5Re0.5)O2 with rutile-type structure

Abstract The new complex oxide (V0.5Re0.5)O2 has been synthesized under high pressure of 60 kbar at 1000°C, and its crystal structure determined by single crystal X-ray diffraction data analysis (S.G.: P42/mnm, a = 4.6357(4) Å, c = 2.8292(3) Å, V = 60.80(1) Å3). This compound adopts a rutile-type structure with random cat ion distribution on one crystallographic site. The substitution of 50 vanadium by rhenium stabilizes the high temperature form of VO2 at room temperature. The cation ratio of 1 : 1 was confirmed by EDX analysis performed by electron microscopy on the same crystals used for X-ray diffraction.

The quasi-binary phase diagram BaF2-BaBr2 and its relation to the x-ray storage phosphor BaFBr : Eu2+

In order to understand the formation and stoichiometry of the x-ray storage phosphor BaFBr : Eu 2+ , the phase diagram of the quasi-binary BaF2–BaBr2 system has been investigated. The phase diagram was obtained by means of differential thermal analysis and temperature controlled x-ray diffraction experiments. The resulting phase diagram indicates that BaFBr forms a compound with no detectable solid solubility for neither BaF2 nor BaBr2. Experiments to obtain non-stoichiometric BaFBr via the synthesis route using BaF2 and NH4Br as proposed in the literature could not be verified. It will be shown that the type of colour centre created during x-ray irradiation is related to the non-stoichiometry of the starting compositions before sintering. A surplus of either barium fluoride or barium bromide during sintering allows the controlled formation of F(Br − )- and F(F − )-centres, respectively.

Crystal structure and magnetic properties of Mo-substituted ‘Ba2(Fe,W)2O6’ double-perovskites: a synchrotron diffraction, magnetization and Mössbauer study

Abstract Synchrotron diffraction and Mossbauer spectroscopy on Ba 2 (Fe,W) 2 O 6 confirmed an Fe excess in this double-perovskite, better described as Ba 2 Fe(Fe x W 1− x )O 6− δ with x =0.2. The crystal structure is cubic ( Fm 3 m ) down to 10 K, and a recently reported tetragonal structure of Ba 2 FeWO 6 is critically reviewed. Magnetic properties of Ba 2 (Fe,W 1− x Mo x ) 2 O 6 compounds were studied by SQUID measurements and revealed weak ferromagnetism at low temperatures, and T C increases with more substitution of Mo for W.

Preparation, crystal structure and magnetic properties of the high-pressure phase MnReO4 with a wolframite-type structure

The ternary oxide MnReO(4), manganese rhenium oxide, has been synthesized under a high pressure of 5.5 GPa at 1473 K and its crystal structure has been determined by single-crystal X-ray diffraction. MnReO(4) crystallizes in a wolframite-type structure with average bond lengths of Re-O = 1.935 and Mn-O = 2.160 A that are in good agreement with the ionic radii of Re(6+) and Mn(2+). The magnetic properties of MnReO(4) have been studied by SQUID measurements, revealing magnetic ordering below 275 (10) K and a weak ferromagnetic component of the ordered magnetic structure.

Synthesis and crystal structure of a new rhenium scandium oxide, Sc 6 ReO 12

A number of Ln6MO12 phases, where Ln = rare earth cation and M = Mo, W, Re or U, crystallizes in a distorted rhombohedral fluorite structure with space group R-3 [1, 2]. The same crystal structure is observed for Sc6MO12 compounds, where M = Mo and W [3]. In our work a new complex rhenium scandium oxide, Sc6ReO12, has been synthesized as a single phase in a sealed silica tube and by high-pressure high-temperature synthesis, and its crystal structure has been determined by single crystal X-ray diffraction. This compound crystallizes in a rhombohedral structure (space groupR-3, a= 9.248(2) A, c = 8.720(2) A, Z = 3). Figure 1 demonstrates the dependence of the cell parameters ’’a’’ and ’’c’’ on the ionic radius of the trivalent cations in the isostructural compounds Ln6ReO12 (Ln = Sc, Ho, Er, Tm, Yb and Lu). This structure represents a three-dimensional framework formed by ReO6 and ScO7 polyhedra connected via edges and corners. ReO6 octahedra are undistorted, the Re-O interatomic distance dRe-O= 1.926 A is in a good agreement with the Re-O distances observed for other Re containing oxides with a formal rhenium oxidation state +6. ScO7 polyhedra represent capped prisms with six close oxygens (dSc-O = 2.027 2.230 A, in agreement with average Sc-O distances in other Sc-containing oxides), and one more distant oxygen (dSc-O = 2.67 A), which is included in the coordination sphere due to calculated bond-valence parameters. The synthesis of Sc6ReO12 under high pressure indicates the stability of its structure up to 50 kbar and 1300 C. The magnetic properies of Sc6ReO12 were also investigated.

Experimental magnetic form factors in Co 3 V 2 O 8 : A combined study of ab initio calculations, magnetic Compton scattering, and polarized neutron diffraction

We present a combination of ab initio calculations, magnetic Compton scattering and polarized neutron experiments, which elucidate the density distribution of unpaired electrons in the kagome staircase system Co3V2O8. Ab initio wave functions were used to calculate the spin densities in real and momentum space, which show good agreement with the respective experiments. It has been found that the spin polarized orbitals are equally distributed between the t2g and the eg levels for the spine (s) Co ions, while the eg orbitals of the cross-tie (c) Co ions only represent 30% of the atomic spin density. Furthermore, the results reveal that the magnetic moments of the cross-tie Co ions, which are significantly smaller than those of the spine Co ions in the zero-field ferromagnetic structure, do not saturate by applying an external magnetic field of 2 T along the easy axis a, but that the increasing bulk magnetization originates from induced magnetic moments on the O and V sites. The refined individual magnetic moments are mu(Co_c)=1.54(4) mu_B, mu(Co_s)=2.87(3) mu_B, mu(V)=0.41(4) mu_B, mu(O1)=0.05(5) mu_B, mu(O2)=0.35(5) mu_B, and; mu(O3)=0.36(5) mu_B combining to the same macroscopic magnetization value, which was previously only attributed to the Co ions.

The magnetic composition–temperature phase diagram of the kagome mixed system (CoxNi1−x)3V2O8

The magnetic composition–temperature (x–T) phase diagram of the mixed kagome system (CoxNi1−x)3V2O8 (CNVO) deduced from neutron powder diffraction and single-crystal heat capacity experiments is presented. CNVO changes its magnetic structure twice below 5.5 K as a function of the compositional parameter x. A sample with x = 0.98 has been found to be of a critical composition where both ferromagnetic and antiferromagnetic reflections of the Co3V2O8 (CVO) type magnetic structure have been observed in the diffraction pattern. Below this composition the ferromagnetic phase is completely suppressed and antiferromagnetic short-range order is present, which is modulated by a propagation vector k1 = (0,δ,0) with δ being temperature and composition dependent. Below 4.2 K δ has a constant value of 0.4 for 0.76<x<0.92. In the region 0.71<x<0.76 a change of the magnetic structure into the long-range ordered Ni3V2O8 (NVO) type with k2 = (δ,0,0), where δ is only dependent on x, takes place. Finally, for x<0.035 the positions of the magnetic reflections become temperature dependent again.

Magnetic structure of the Kagomé mixed compound (Co0.5Ni0.5)3V2O8

MS38 P05 Magnetic structure of the Kagomé mixed compound (Co0.5Ni0.5)3V2O8 N. Qureshi, H. Fuess, H. Ehrenberg, B. Ouladdiaf, T. C. Hansen, Th. Wolf, C. Meingast, Q. Zhang, W. Knafo, H. v. Löhneysen Institute for Materials Science, University of Technology, Darmstadt, Germany. Institut Max von Laue-Paul Langevin, Grenoble, France. Leibniz Institute for Solid State and Materials Research, Dresden, Germany. Research Center Karlsruhe, Institute of Solid State Physics, Karlsruhe, Germany. Physics Institute, Karlsruhe University, Karlsruhe, Germany. E-mail: navidq@st.tu-darmstadt.de

Coexistence of the canted ferromagnetism and canted antiferromagnetism in La0.6Pr0.4Mn2Si2

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s257 Acta Cryst. (2007). A63, s257 MS38 P01 Effect of monovalent doping in the A site on structural and physical propeties of La1-xCaxMnO3 manganese oxides A. Cheikhrouhou, W. Cheikh-Rouhou Koubaa, A. Mehri, M. Koubaa, Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B. P. 802, 3018 Sfax, Tunisia, Laboratoire Louis NEEL, CNRS, B. P. 166, 38042 Grenoble Cedex 9, France. E-mail: abdcheikhrouhou@yahoo.fr

Structural phase transitions in Sr2ScReO6

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s184 Acta Cryst. (2007). A63, s184 was heated under dynamic vacuum; at 130°C deuterium is released and LiMgAlD6 is formed: LiMg(AlD4)3 → LiMgAlD6 + 2Al + 3D2. At 180°C LiMgAlD6 decomposes: LiMgAlD6 → LiD + MgD2 + Al + 3/2D2. The first reaction is endothermic; the second exothermic, indicating that LiMg(AlD4)3 is not thermodynamic stable. LiMg(AlD4)3 crystallizes in the monoclinic space group P21/c with cell parameters: a = 8.37 Å, b = 8.74 Å, c = 14.30 Å and β = 124.83 °. The structure consists of isolated AlD4 tetrahedra that are connected by octahedral Mg and Li atoms. LiMgAlD6 crystallizes in the trigonal space group P321 with cell parameters: a = 7.98 Å and c = 4.38 Å. The structure consists of isolated AlD6 octahedra tied together by hexa-coordinated Li and Mg atoms. A small amount of TiCl3 was added to LiMg(AlD4)3 and mixed using the ball milling technique. The effect of addition of TiCl3 was also studied for LiMgAlD6.