The layered honeycomb magnet $\alpha -{\mathrm{RuCl}}_3$ has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled $j_{\mathrm{eff}}=\frac{1}{2}{\mathrm{Ru}}^{3+}$ magnetic moments. Here, we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the $j_{\mathrm{eff}}=\frac{1}{2}$ picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the $b$ axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below $T_{\mathrm{N}}\approx 13$ K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons¹, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state². Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric () to non-centrosymmetric (R3c) transition in metallic LiOsO₃ that is structurally equivalent to the ferroelectric transition of LiNbO₃ (ref. 3). The transition involves a continuous shift in the mean position of Li⁺ ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

Powder diffractometers are naturally suited for short-pulse spallation sources, as they optimally exploit the brilliance from the sharp neutron pulses, with a relative time-resolution constant over a broad wavelength range. Their design has considerably progressed over the last three decades, in particular due to advances in detector technology. In the 80s, the first instruments such as HRPD at Argonne and HRPD at ISIS employed backscattering geometry, where the instruments are naturally focussed, and have long primary flight-paths in order to acquire high-resolution data. To improve count rates, large solid angles could be covered but detectors were positioned on a locus that allowed geometrical focusing, where time-of-flight (TOF) histograms produced by individual detector elements have equal resolution and can be directly grouped into a single diffraction pattern.

Zeolites that prefer alkynes: Alkenes such as ethylene and propene must be separated from alkynes before they can be converted in polymers. Drawbacks in current methods, such as hydrogenation of alkynes producing unwanted alkanes, has spurred interest in sorption separation methods. Zeolites have generally been inefficient, given the similar sizes and volatilities of the molecules. Chai et al. incorporated atomically dispersed divalent transition metal cations into faujasite zeolite and found that the nickel-containing analog efficiently removed alkynes from olefins through chemoselective binding at open nickel(II) sites. At ambient conditions in the presence of water and carbon dioxide, the zeolites retained separation selectivities of 100 and 92, respectively, for acetylene over ethylene and propyne over propylene for 10 adsorption-desorption cycles. Science , this issue p. 1002

In rhombohedral CaMn7O12, an improper ferroelectric polarization of magnitude 2870\,\,$\mu${}C\,m-2 is induced by an incommensurate helical magnetic structure that evolves below TN1=90\,\,K. The electric polarization was found to be constrained to the high symmetry threefold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn7O12 gives rise to the largest magnetically induced, electric polarization measured to date.