HTD2: a single-crystal X-ray diffractometer for combined high-pressure/low-temperature experiments at laboratory scale

The design and operation of a newly commissioned single-crystal X-ray diffractometer (HTD2) are presented. The device enables experiments under simultaneous low-temperature and high-pressure conditions using a laboratory X-ray source.

High-pressure (HP) X-ray diffraction experiments at low temperature (LT) require dedicated instruments as well as non-standard sample environments and measuring strategies. This is especially true when helium cryogenic temperatures below 80 K are targeted. Furthermore, only experiments on single-crystalline samples provide the prerequisites to study subtle structural changes in the p–T phase diagram under extreme LT and HP conditions in greater detail. Due to special hardware requirements, such measurements are usually in the realm of synchrotron beamlines. This contribution describes the design of an LT/HP diffractometer (HTD2) to perform single-crystal X-ray diffraction experiments using a laboratory source in the temperature range 400 > T > 2 K while applying pressures of up to 20 GPa.

Chromous siloxides of variable nuclearity and magnetism

Treatment of Cr[N(SiMe₃)₂]₂(thf)₂ with HOSiR₃ (R = Et, iPr) in THF afforded the bridged Cr^II siloxide complexes Cr₃(OSiEt₃)₂(μ-OSiEt₃)₄(thf)₂ and Cr₂(OSiiPr₃)₂(μ-OSiiPr₃)₂(thf)₂ in high yield. Exposure of these compounds to vacuum in aliphatic solvents led to the loss of coordinated THF and to the formation of the homoleptic chromous siloxides Cr₄(μ-OSiEt₃)₈ and Cr₃(OSiiPr₃)₂(μ-OSiiPr₃)₄, respectively, in moderate to high yield. Use of TMEDA as a potentially bidentate donor molecule gave the monomeric cis-coordinated siloxide Cr(OSiiPr₃)₂(tmeda) (tmeda = N,N,N',N'-tetramethylethane-1,2-diamine). Oxidation of Cr₂(OSiiPr₃)₂(μ-OSiiPr₃)₂(thf)₂ with CHI₃ and C₂Cl₆ produced the trigonal bipyramidal chromic compound Cr^III(OSiiPr)₃(thf)₂ and asymmetrically coordinated Cr₂Cl₃(OSiiPr₃)₃(thf)₃, respectively. Magnetic measurements (Evans and SQUID) hinted at (a) antiferromagnetic interactions between the Cr^II centres, (b) revealed higher effective magnetic moments (μ_eff) for cis-coordinated monomeric heteroleptic complexes compared to trans-coordinated ones, and (c) pointed out the highest (μ_eff) for the tetranuclear complex Cr₄(μ-OSiEt₃)₈ (6.26μ_B, SQUID, 300 K; Cr⋯Cr_adjacent avg. 2.535 A).

Beyond Takai's Olefination Reagent: Persistent Dehalogenation Emerges in a Chromium(III)-μ3 -Methylidyne Complex

Reaction of CHI3 with six equivalents of CrCl2 in THF at low temperatures affords [Cr3 Cl3 (μ2 -Cl)3 (μ3 -CH)(thf)6 ] as the first isolable high-yield CrIII μ3 -methylidyne complex. Substitution of the terminal chlorido ligands via salt metathesis with alkali-metal cyclopentadienides generates isostructural half-sandwich chromium(III)-μ3 -methylidynes [CpR 3 Cr3 (μ2 -Cl)3 (μ3 -CH)] (CpR =C5 H5 , C5 Me5 , C5 H4 SiMe3 ). Side and decomposition products of the Cl/CpR exchange reactions were identified and structurally characterized for [Cr4 (μ2 -Cl)4 (μ2 -I)2 (μ4 -O)(thf)4 ] and [(η5 -C5 H4 SiMe3 )CrCl(μ2 -Cl)2 Li(thf)2 ]. The Cl/CpR exchange drastically changed the ambient-temperature effective magnetic moment μeff from 9.30/9.11 μB (solution/solid) to 3.63/4.32 μB (CpR =C5 Me5 ). Reactions of [Cr3 Cl3 (μ2 -Cl)3 (μ3 -CH)(thf)6 ] with aldehydes and ketones produce intricate mixtures of species through oxy/methylidyne exchange, which were partially identified as radical recombination products through GC/MS analysis and 1 H NMR spectroscopy.

Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31  K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb_{4}S_{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.