Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material α-RuCl 3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p ∼ 0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in α-RuCl 3 strongly competes with the crystallization of spin singlets into a valence bond solid. DOI: 10.1103/PhysRevB.97.241108 The Kitaev model on a honeycomb lattice has grown into a hot topic in the last decade due to its exact solubility and its quantum spin-liquid ground state, which would be relevant for, e.g., quantum computing [1,2]. It implies a bonddependent compass-type coupling K and strong intrinsic spin frustration [3]. A crucial ingredient for realizing the Kitaev model in real materials is a strong spin-orbit coupling together with a honeycomb structure. Recently, Kitaev interactions were identified in α-RuCl 3 , from its unusual magnetic excitation spectrum [4,5], its strong magnetic anisotropy [6], and electronic-structure calculations [7,8], which render this material an ideal platform for exploring Kitaev magnetism experimentally.α-RuCl 3 is a j eff = 1/2 Mott insulator with a twodimensional (2D) layered structure of edge-sharing RuCl 6 octahedra forming a honeycomb lattice. At ambient pressure, * g.bastien@ifw-dresden.de Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.the honeycomb layers are arranged in a monoclinic (C2/m) structure at room temperature with one of the three nearestneighbor (NN) Ru-Ru bonds slightly shorter than the other two [9]. A structural phase transition was reported at T S 60 K under cooling and T S 166 K upon warming, but the low-temperature crystal structure is still under debate and could be either rhombohedral (R3) [10,11] or monoclinic (C2/m) [12,13]. The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15]. While electronic-structure calculations indicate that K is ferromagnetic in α-RuCl 3 and indeed defines the largest exchange energy scale [7,8,14,15], the debate on the minimal effective spin model and precise magnitude of the different couplings is not fully settled yet. By applying a magnetic field in the basal plane, the magnetic zigzag ground sta...

At 120 K, the title compound [systematic name: (2RS,3SR,4RS,6SR)-3-benzoyl-4-hydroxy-2,4,6-triphenylcyclohexane-1,1-dicarbonitrile], C(33)H(26)N(2)O(2), has unit-cell dimensions apparently different from those reported at 294 K [Rong, Li, Yang, Wang & Shi (2006). Acta Cryst. E62, o1766-o1767]. The molecules are linked by two C-H...O hydrogen bonds and three C-H...N hydrogen bonds into complex sheets, and the hydrogen-bonded structures at the two temperatures are the same, although incorrectly described in the earlier report. The significance of the present study lies in its correct description of the hydrogen bonding and in its analysis of the unit-cell dimensions; the differences between the cell angles at 120 and 294 K arise from the fact that one such angle in the triclinic cell is extremely close to 90 degrees so that a very small change in this angle can induce significant changes in the reduced cell.

An efficient synthesis of 1-arylisochromeno[3,4-d][1,2,3]triazol-5(1H)-ones, involving the diazotization of 3-amino-4-arylamino-1H-isochromen-1-ones in weakly acidic solution, has been developed and the spectroscopic characterization and crystal structures of four examples are reported. The molecules of 1-phenylisochromeno[3,4-d][1,2,3]triazol-5(1H)-one, C 15 H 9 N 3 O 2 , (I), are linked into sheets by a combination of C-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, while the structures of 1-, each contain just one hydrogen bond which links the molecules into simple chains, which are further linked into sheets by -stacking interactions in (II) but not in (III). In the structure of 1-(4chlorophenyl)isochromeno [3,4-d][1,2,3]triazol-5(1H)-one, (IV), isomeric with (III), a combination of C-HÁ Á ÁO and C-HÁ Á Á(arene) hydrogen bonds links the molecules into sheets. When compound (II) was exposed to a strong acid in methanol, quantitative conversion occurred to give the ring-opened transesterification product methyl 2-[4-hydroxy-1-(2-methylphenyl)-1H-1,2,3-triazol-5-yl]benzoate, C 17 H 15 N 3 O 3 , (V), where the molecules are linked by paired O-HÁ Á ÁO hydrogen bonds to form centrosymmetric dimers.