Group of Dr. Constantin Hoch - Faculty for Chemistry and Pharmacy

Research Interests

Reconciling Salts and Metals:

Chemistry between ionic and metallic bonding

Our group deals with structural solid state chemistry and concentrates on compounds in which some kind of mixture of metallic and ionic bonding contributions occur. We follow two conceptual routes: (1) introducing Coulombic polarity into intermetallic phases by creating partial electron transfer from a less noble metal to a noble metal with low electron affinity, and (2) creating crystal structures with spatially separated metallic or ionic structure compartments. We call them 'chemical twins'. They can show different manifestations of polarity: magnetic or electric dipoles may occur.

Our chemistry typically combines explorative preparative work with crystallographic structural studies, accompanied with differential calorimetry, DFT calculations of the electronic structures, solid state NMR spectroscopy and characterisation of physical properties. Preparative techniques involve glove box and schlenk inert gas preparations, (earth) alkali metal works, electrochemistry under inert gas conditions, tantalum ampoule high temperature syntheses, complex chemistry in polar aprotic solvents and others. Analytical methods include X-ray crystallography at different temperatures, DTA/DSC/TG, temperature-dependent powder X-ray diffraction, solid state NMR spectroscopy (together with Dr. Thomas Bräuniger), IR and Raman spectroscopy, physical property determination (specific resistivity, impedance etc, together with Prof. Dr. Dirk Johrendt), DFT calculations (together with Prof. Dr. Ján Minár), Mössbauer spectroscopy (together with Prof. Dr. Rainer Pöttgen) and others.

Amalgams of less noble metals as model systems for compounds with polar metallic bonding and 'bad metal behaviour' Subvalent metalates as examples for compounds with spatially separated ionic and metallic structural compartments
Amalgams of less noble metals are systems with a high but never complete electron transfer from the less noble component (alkali, earth alkaline, lanthanoid metal) to mercury. The electronegativity difference leads to high Coulombic polarity, and the endothermic electron affinity of mercury prevents the formation of Zintl-like anions. All amalgams behave as metals but show strong 'bad metal' characteristics, especially when Hg-rich. The compounds usually are very air- and moisture-sensitive and have low peritectic decomposition temperatures. We synthesize them via a newly developed preparation method that we call 'isothermal electrocrystallisation'. It is based on the cathodic deposition of the less noble metal from a solution on elemental mercury which acts as the cathode. by this, we obtain typically phase-pure samples in gram scale and high crystallinity. Physical properties and crystal structures are characterised. From solid state NMR spectroscopic measurements of the Knight shift of 199Hg statements about the polarity within the amalgams can be derived. Crystal structures in which spatially separated compartments in which either ionic or metallic bonding contributions prevail are present e. g. in the class of alkali metal suboxides or earth alkaline metal subnitrides. There, the anionic structural part consists of isolated O2- or N3- anions confined in metallic clusters. They are ionic on theri inner side but metallic on the outside so that additional purely metallic bound atoms can be intercalated between them in an ordered crystal structure. We extend this structural principle by extending the anionic part. Instead of monoatomar anions we create similar structures from complex anions, e. g. tetrahedral oxometalate [MOn]y- or nitridometalate [MNm]z- anions. They can combine to larger anionic entities and are embedded in a substructure from (inter-)metallic units, such as alkali metal cubes, icosahedra of earth alkaline metals and others. Both structural parts together form a regular crystal structure which, in total, behaves as a good metal. We plan to introduce anionic entities which bring magnetic or electric dipoles into the structure, creating materials which are both metallic and ferromagnetic or, more interestingly, ferroelectric. Multiferroic materials are interesting candidates for new data storage devices.

Single crystals of Tetramethylammoniumamalgam, [N(CH3)4]Hg8, grown by isothermal electrocrystallisation at -20 °C

Crystal structure of Ba23Na11(TaN4)4 with tetrahedral [TaN4]7- anions (green), Na-centered [Ba10Na2] icosahedra (dark blue) and Na-centered [Na8] cubes (light blue)
Other topics of interest in our group: preparation of thermodynamically unstable solids such as EuI3 or intercalation amalgams, dental amalgams, metallic compounds of Hgδ+, ternary amalgams, Beryllium amalgams and polar Be intermetallics, lithium gallides, combined crystallography from X-ray and NMR, solid state ion conductors and electroactive materials, partially oxidised Zintl compounds, ...
Zinkphosphat-Dihydrat Cs9InO4 BaHg11 LiHg3