Single crystals of the Kitaev spin-liquid candidate $\alpha -{\mathrm{RuCl}}_3$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at $T_N=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a ${120}^{\circ }$ spiral order or a collinear structure with a spin direction of ${35}^{\circ }$ away from the $a$ axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) ${\mu }_B$ per ${\mathrm{Ru}}^{3+}$ ion.

We have combined single-crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na${}_2$IrO${}_3$. The system orders magnetically below $18.1\left(2\right)$ K with Ir${}^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of $0.22\left(1\right){\mu }_{\text{B}}/\text{Ir}$ site. Such a configuration sharply contrasts with the Néel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO${}_6$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much needed experimental insights into the magnetic and crystal structure that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the $5d$-electron-based honeycomb lattice.

Bulk rutile ${\mathrm{RuO}}_2$ has long been considered a Pauli paramagnet. Here we report that ${\mathrm{RuO}}_2$ exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately $0.05{\mu }_B$ as evidenced by polarized neutron diffraction. Density functional theory plus $U$ ($\mathrm{DFT}+U$) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard $U$ of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Néel temperature and small itinerant moments make ${\mathrm{RuO}}_2$ unique among ruthenate compounds and among oxide materials in general.