Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB.

Case studies of 1T-TiSe and YBaCuO have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the of approx.

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

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 - phase diagram under extreme LT and HP conditions in greater detail.

Chromous siloxides of variable nuclearity and magnetism.

Treatment of Cr[N(SiMe)](thf) with HOSiR (R = Et, Pr) in THF afforded the bridged Cr siloxide complexes Cr(OSiEt)(μ-OSiEt)(thf) and Cr(OSiPr)(μ-OSiPr)(thf) 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 Cr(μ-OSiEt) and Cr(OSiiPr)(μ-OSiPr), respectively, in moderate to high yield. Use of TMEDA as a potentially bidentate donor molecule gave the monomeric -coordinated siloxide Cr(OSiiPr)(tmeda) (tmeda = ,,','-tetramethylethane-1,2-diamine).

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

Reaction of CHI with six equivalents of CrCl in THF at low temperatures affords [Cr Cl (μ -Cl) (μ -CH)(thf) ] as the first isolable high-yield Cr μ -methylidyne complex. Substitution of the terminal chlorido ligands via salt metathesis with alkali-metal cyclopentadienides generates isostructural half-sandwich chromium(III)-μ -methylidynes [Cp Cr (μ -Cl) (μ -CH)] (Cp =C H , C Me , C H SiMe ). Side and decomposition products of the Cl/Cp exchange reactions were identified and structurally characterized for [Cr (μ -Cl) (μ -I) (μ -O)(thf) ] and [(η -C H SiMe )CrCl(μ -Cl) Li(thf) ].

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}.

Dispersion Forces Drive the Formation of Uranium-Alkane Adducts.

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η-H, C metal-alkane interactions in the [((ArO)tacn)U(cy-C6)]·(cy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-O and U-N bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap.