6 research outputs found
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 CrB
We report the low-temperature properties of B-enriched single-crystal
CrB as prepared from high-purity Cr and B powder by a solid-state
reaction and optical float zoning. The electrical resistivity, ,
Hall effect, , and specific heat, , are characteristic of an
exchange-enhanced Fermi liquid ground state, which develops a slightly
anisotropic spin gap below . This observation is corroborated by the absence of a Curie dependence in
the magnetization for reported in the literature. Comparison of
with , where we infer lattice contributions from
measurements of VB, reveals strong antiferromagnetic spin fluctuations with
a characteristic spin fluctuation temperature
in the paramagnetic state, followed by a pronounced second-order mean-field
transition at , and unusual excitations around . The pronounced anisotropy of above is
characteristic of an easy-plane anisotropy of the spin fluctuations consistent
with the magnetization. The ratio of the Curie-Weiss to the Nel
temperatures, , inferred from the
magnetization, implies strong geometric frustration. All physical properties
are remarkably invariant under applied magnetic fields up to ,
the highest field studied. In contrast to earlier suggestions of local-moment
magnetism our study identifies CrB 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 up to 14GPa close to a putativequantum phase transition. Combining hydrostatic, uniaxial, and quasi-hydrostatic pressure, the emergenceof incipient superconductivity in CrB is observed. The temperature dependence of the electrical resistivity inCeCuAl is consistent with emergent Kondo correlations and an enhanced coupling of magneto-elastic excitationswith the conduction electrons at low and intermediate temperatures, respectively