Third Time is a Charm - Protonating Tricarboxybenzenes

Triprotonation of the three constitution isomers of tricarboxybenzene was accomplished. Furthermore, the preparation of selected mono- and dications showed the sequence of protonation steps. The cations were mostly isolated as [SbF6] and [AsF6] species, which were characterized by Raman and NMR spectroscopy as well as X-ray structure determination. To further elucidate the experimental results, quantum chemical calculations are employed, especially in regard to charge distribution with NPA charges and aromaticity by NICS(0) values. Thermal decomposition of the compounds was investigated in order to explore the possibility of acyl cation formation. The influence of the aromatic system, substitution pattern and anomeric effect concerning the properties of the respective compounds was thus explained

Ultra-high vacuum compatible induction-heated rod casting furnace

We report the design of a radio-frequency induction-heated rod casting furnace that permits the preparation of polycrystalline ingots of intermetallic compounds under ultra-high vacuum compatible conditions. The central part of the system is a bespoke water-cooled Hukin crucible supporting a casting mold. Depending on the choice of mold, typical rods have a diameter between 6 mm and 10 mm and a length up to 90 mm, suitable for single-crystal growth by means of float-zoning. The setup is all-metal sealed and may be baked out. We find that the resulting ultra-high vacuum represents an important precondition for processing compounds with high vapor pressures under a high-purity argon atmosphere up to 3 bar. Using the rod casting furnace, we succeeded to prepare large high-quality single crystals of two half-Heusler compounds, namely the itinerant antiferromagnet CuMnSb and the half-metallic ferromagnet NiMnSb.Comment: 5 pages, 4 figure

Low-temperature properties of single-crystal CrB2_{2}

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.Comment: 15 pages, 9 figure

Compositional Studies of Metals with Complex Order by means of the Optical Floating‐Zone Technique

The availability of large high-quality single crystals is an important prerequisite for many studies in solid-state research. The optical floating-zone technique is an elegant method to grow such crystals, offering potential to prepare samples that may be hardly accessible with other techniques. As elaborated in this report, examples include single crystals with intentional compositional gradients, deliberate off-stoichiometry, or complex metallurgy. For the cubic chiral magnets Mn1–xFexSi and Fe1–xCoxSi, single crystals are prepared in which the composition is varied during growth from x ¼ 0 to 0.15 and fromx ¼ 0.1 to 0.3, respectively. Such samples allow us to efficiently study the evolution of the magnetic properties as a function of composition, as demonstrated by means of neutron scattering. For the archetypical chiral magnet MnSi and the itinerant antiferromagnet CrB2, single crystals with varying initial manganese (0.99–1.04) and boron (1.95–2.1) content are grown. Measurements of the low-temperature properties address the correlation between magnetic transition temperature and sample quality. Furthermore, single crystals of the diborides ErB2, MnB2, and VB2 are prepared. In addition to high vapor pressures, these materials suffer from peritectic formation, potential decomposition, and high melting temperature, respectively

High‐Pressure Studies of Correlated Electron Systems

Tuning the electronic properties of transition-metal and rare-earth compounds by virtue of changes of the crystallographiclattice constants offers controlled access to new forms of order. We review the development oftungsten carbide and moissanite Bridgman cells conceived for studies of the electrical resistivity up to 10GPa,as well as bespoke diamond anvil cells developed for neutron depolarization studies up to 20GPa. For the diamondanvil cells, the applied pressure changes as a function of temperature in quantitative agreement withthe thermal expansion of the pressure cell. A set-up based on focussing neutron guides for measurements of thedepolarization of a neutron beam by samples in a diamond anvil cell is described. Resistivity measurementsand neutron depolarization provide evidence of ferromagnetic order in SrRuO$_3$ up to 14GPa close to a putativequantum phase transition. Combining hydrostatic, uniaxial, and quasi-hydrostatic pressure, the emergenceof incipient superconductivity in CrB$_2$ is observed. The temperature dependence of the electrical resistivity inCeCuAl$_3$ is consistent with emergent Kondo correlations and an enhanced coupling of magneto-elastic excitationswith the conduction electrons at low and intermediate temperatures, respectively