H-bonding in lazulite: a single-crystal neutron diffraction study at 298 and 3 K

The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, Dawson mining district, Yukon, Canada) was investigated by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction at 298 and 3 K. Its empirical formula, based on EMPA data, is: (Mg0.871Fe0.127)\u3a30.998Al2.030(P1.985Ti0.008Si0.007O4)2(OH)2. The neutron diffraction experiments at room and low T proved that the H-free structural model of lazulite previously reported, on the basis of X-ray structure refinement, is correct: the building unit of the lazulite structure consists of a group of three face-sharing (Al-octahedron) + (Mg,Fe-octahedron) + (Aloctahedron), connected to the adjacent one via a corner-shared OH-group and two corner-shared oxygen sites of the P-tetrahedron, to form a dense 3D-edifice. Only one crystallographically independent H site occurs in the structure of lazulite, forming a hydroxyl group with the O5 oxygen, with O5\u2013H = 0.9997 \uc5 at room temperature (corrected for riding motion effect). The H-bonding scheme in the structure of lazulite is now well defined: a bifurcated bonding scheme occurs with the O4 and O2 oxygen sites as acceptors. The two H-bonds are energetically different, as shown by their bonding geometry: the H-bond with the O2 site as acceptor is energetically more favorable, being O5\u2013H\ub7\ub7\ub7O2 = 152.67(9)\ub0, O5\ub7\ub7\ub7O2 = 3.014(1) \uc5 and H\ub7\ub7\ub7O2 = 2.114(1) \uc5, whereas that with O4 as acceptor is energetically more costly, being O5\u2013H\ub7\ub7\ub7O4 = 135.73(8)\ub0, O5\ub7\ub7\ub7O4 = 3.156(1) \uc5 and H\ub7\ub7\ub7O4 = 2.383(1) \uc5, at room temperature. No T-induced phase transition occurs within the T-range investigated. At low temperature, the O5\u2013H\ub7\ub7\ub7O2 bond is virtually identical to the room-T one, whereas the effects of T on O5\u2013H\ub7\ub7\ub7O4 are more pronounced, with significant differences of the Odonor\ub7\ub7\ub7Oacceptor and H\ub7\ub7\ub7Oacceptor distances. The experimental findings of this study do not support the occurrence of HPO4 or H2PO4 units into the structure of lazulite, recently reported on the basis of infrared and Raman spectra

Topological aspects of multi-k antiferromagnetism in cubic rare-earth compounds

We advertise rare-earth intermetallics with high-symmetry crystal structures and competing interactions as a possible materials platform hosting spin structures with non-trivial topological properties. Focusing on the series of cubic RCu compounds, where R = Ho, Er, Tm, the bulk properties of these systems display exceptionally rich magnetic phase diagrams hosting an abundance of different phase pockets characteristic of antiferromagnetic order in the presence of delicately balanced interactions. The electrical transport properties exhibit large anomalous contributions suggestive of topologically non-trivial winding in the electronic and magnetic structures. Neutron diffraction identifies spontaneous long-range magnetic order in terms of commensurate and incommensurate variations of ππ0 antiferromagnetism with the possibility for various multi-k configurations. Motivated by general trends in these materials, we discuss the possible existence of topologically non-trivial winding in real and reciprocal space in the class of RCu compounds including antiferromagnetic skyrmion lattices. Putatively bringing together different limits of non-trivial topological winding in the same material, the combination of properties in RCu systems promises access to advanced functionalities

Neutron scattering study of commensurate magnetic ordering in single crystal CeSb2_2

Temperature and field-dependent magnetization $M(H,T)$ measurements and neutron scattering study of a single crystal CeSb$_2$ are presented. Several anomalies in the magnetization curves have been confirmed at low magnetic field, i.e., 15.6 K, 12 K, and 9.8 K. These three transitions are all metamagnetic transitions (MMT), which shift to lower temperatures as the magnetic field increases. The anomaly at 15.6 K has been suggested as paramagnetic (PM) to ferromagnetic (FM) phase transition. The anomaly located at around 12 K is antiferromagnetic-like transition, and this turning point will clearly split into two when the magnetic field $H\geq0.2$ T. Neutron scattering study reveals that the low temperature ground state of CeSb$_2$ orders antiferromagnetically with commensurate propagation wave vectors $\textbf{k}=(-1,\pm1/6,0)$ and $\textbf{k}=(\pm1/6,-1,0)$, with N\'eel temperature $T_N\sim9.8$ K. This transition is of first-order, as shown in the hysteresis loop observed by the field cooled cooling (FCC) and field cooled warming (FCW) processes.Comment: 7 pages,9 figure

Angular dependence of critical current density and magnetoresistance of sputtered high-T(sub c)-films

The angular dependence of the critical current density and the magnetoresistance of high-T(sub c)-films in high and low magnetic fields and for different temperatures were measured to investigate the flux pinning and the superconducting properties. A comparison of the results for the different superconductors shows their increasing dependence on the angle Theta between the magnetic field and the c-axis of the film due to the anisotropy of the chosen superconductor. Furthermore the influence of the current direction to the Theta-rotation plane is discussed

Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.Comment: 7 pages, 7 figure

Canted antiferromagnetism in phase-pure CuMnSb

We report the low-temperature properties of phase-pure single crystals of the half-Heusler compound CuMnSb grown by means of optical float-zoning. The magnetization, specific heat, electrical resistivity, and Hall effect of our single crystals exhibit an antiferromagnetic transition at $T_{\mathrm{N}} = 55~\mathrm{K}$ and a second anomaly at a temperature $T^{*} \approx 34~\mathrm{K}$. Powder and single-crystal neutron diffraction establish an ordered magnetic moment of $(3.9\pm0.1)~\mu_{\mathrm{B}}/\mathrm{f.u.}$, consistent with the effective moment inferred from the Curie-Weiss dependence of the susceptibility. Below $T_{\mathrm{N}}$, the Mn sublattice displays commensurate type-II antiferromagnetic order with propagation vectors and magnetic moments along $\langle111\rangle$ (magnetic space group $R[I]3c$). Surprisingly, below $T^{*}$, the moments tilt away from $\langle111\rangle$ by a finite angle $\delta \approx 11^{\circ}$, forming a canted antiferromagnetic structure without uniform magnetization consistent with magnetic space group $C[B]c$. Our results establish that type-II antiferromagnetism is not the zero-temperature magnetic ground state of CuMnSb as may be expected of the face-centered cubic Mn sublattice.Comment: 14 pages, 15 figure

On the complex H-bonding network in paravauxite, Fe2+Al2(PO4)2(OH)2 · 8 H2O

Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, playing an essential geochemical role in the evolution processes of pegmatites. A good knowledge of their crystal chemistry is therefore necessary to better understand the genesis of pegmatites. Paravauxite is a mineral found in hydrothermal tin veins and granite pegmatites. Its ideal chemical formula is Fe2+Al2(PO4)2(OH)2\ub78H2O. Its crystal structure was solved and refined by Baur in 1969 on the basis of single-crystal X-ray diffraction data. This structure model appears to be consistent. However, due to the technical limitations of X-ray diffraction, the refinement only provided the isotropic displacement parameters, and the positions of nine independent proton sites were assigned but not refined. This led to a poor description of (the expected) complex H-bonding scheme in the paravauxite structure. In light of this, the crystal structure of a natural paravauxite was reinvestigated using electron microprobe analysis in wavelength dispersive mode (EPMA-WDS) and single-crystal neutron diffraction in an attempt to resolve these open questions

A SINGLE-CRYSTAL NEUTRONS DIFFRACTION STUDY OF BRAZILIANITE, NAAL3(PO4)2(OH)4

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First Order Metamagnetic Transition in Ho2_{2 }Ti2_{2 }O7_{7} Observed by Vibrating Coil Magnetometry at Milli-Kelvin Temperatures

We report vibrating coil magnetometry of the spin-ice system Ho2Ti2O7 down to 0:04 K for magnetic fields up to 5 T applied parallel to the [111] axis. History-dependent behavior emerges below T0 0:6 K near zero magnetic field, in common with other spin-ice compounds. In large magnetic fields, we observe a magnetization plateau followed by a hysteretic metamagnetic transition. The temperature dependence of the coercive fields as well as the susceptibility calculated from the magnetization identify the metamagnetic transition as a line of first order transitions terminating in a critical end point at Tm' 0:37 K, Bm’ 1:5 T. The metamagnetic transition in Ho2Ti2O7 is strongly reminiscent of that observed in Dy2Ti2O7, suggestive of a general feature of the spin ices