Prof. Dr. Richard Dronskowski

Chair of Solid-state and Quantum Chemistry

Research

In a nutshell, the Dronskowski group is specializing in the fields of synthetic and quantum-theoretical solid-state chemistry, bordering with materials science, solid-state and theoretical physics, crystallography, as well as quantum and computational chemistry. In detail, we are synthesizing novel, sometimes extremely sensitive, compounds and elucidate their compositions and crystal structures by means of X-ray and neutron diffractional techniques. The characterization of their physical properties, that is, electronic transport and magnetism, also plays a very important role.
We regularly perform solid-state quantum-chemical calculations from first principles to yield the electronic (band) structures and, in particular, to extract the important chemical bonding information needed to thoroughly understand the interplay between chemistry and physics. We have been a major player in making theoretical approaches an essential part of solid-state chemistry. Indeed, the ultimate quest for prediction and understanding is at the very center of our research; thus, syntheses are theory-driven and experiments challenge theories. Alternatively expressed, we have given up any unnecessary fencing between synthetic and theoretical approaches.
At the present moment, we expect the most exciting research opportunities in the fields of extended nitrides and oxynitrides, magnetic intermetallics, cyanamides and carbodiimides, and, also, in the quantum-chemical modeling of hypothetical phases and very large solid-state systems. In addition, there is an enormous scientific and, possibly, economical potential in (pathological) biomineralization, namely on the borderline between chemistry and medicine. For reasons of brevity, let's merely mention a small number of typical examples to illustrate our research philosophy and how it has evolved over the years:

Without the slightest doubt, extended nitrides are clearly becoming the electronic (non-oxidic) materials of the 21st century, not only because of the fascinating optoelectronic properties of gallium nitride, GaN, and similar phases. There has been an enormous progress in the synthesis of more complex (ternary or quaternary) nitrides, and our group has specialized, some years ago, in the prediction of new nitride materials, yet to be made, or the characterization of existing nitrides [1]. Within the last years, we have studied, by means of first-principles total-energy calculations, the ternary system MFe₃N (where M stands for another transition metal), derivatives of the famous Fe₄N phase which is a magnetically as well as mechanically important material. Because of these calculations and the theoretically derived enthalpy-pressure diagrams (see right figure), we have predicted the existence of the compound RhFe₃N and the high-pressure phase IrFe₃N [2]. Before having synthesized the Rh-based material, we knew that this compound would behave ferromagnetically; in addition, the theory is able to correctly predict the conditions of its synthesis.

Indeed, RhFe₃N has been obtained a few months later by our group through a special chemical route of the coupled-reduction type (see figure). The new phase is indeed a very strong ferromagnet, and additional quantum-chemical calculations suggest the material to be semihard in magnetic nature [3]. With respect to IrFe₃N, high-pressure routes will be needed to prepare the phase starting from FeN, Ir and molecular nitrogen, and we would be happy to either cooperate with high-pressure groups to eventually synthesize this exciting compound. For the future, we are planning to extend the theoretical as well as synthetic high-pressure nitride chemistry, in particular with respect to mixed transition-metal/rare-earth metal nitride systems because the 3d/4f electronic interplay in such metal-rich phases will guarantee even more exciting magnetic properties. On the theoretical side, the most recent result is the first-principles falsification of platinum nitride (PtN) which does not exist although it has been claimed by others. Instead, PtN₂ adopting the pyrite structure was predicted by us, and this platinum diazenide (not nitride) has been made, independently, by other experimentalist in the meantime [4].

Also, our group has specialized in the complex chemistry of novel metal oxynitrides, a very interesting class of materials; the German Science Foundation has recognized the importance of oxynitride chemistry by having started a special research initiative in which our group plays its role. For oxynitrides, the chemical tuning of the materials properties does not occur in the cationic but in the anionic substructure. Because these compounds are much more difficult to make, empirical knowledge is rare and a deep understanding based on quantum-chemical calculations may turn out extremely useful. As an example, we have cooperated in characterizing novel zirconium oxynitride films from the plasma phase [5] and we have theoretically characterized CoO_0.5N_0.5 (see above figure), the first oxynitride of a magnetic transition metal [6] as enthalpically unstable. This oxynitride only exists because of the existence of likewise unstable zinc blende-type CoO.

Another important step [7] has been the prediction of a stoichiometrically precise vanadium oxynitride, VON, and the high-pressure conditions of its synthesis (see left figure). Also, the high-pressure phases of NbON and TaON have been predicted and wait for their discoveries. Many more synthetic and theoretical investigations are planned for the chemistry of oxynitrides, and applications are particularly obvious for the realm of, say, heterogeneous catalysis and non-toxic pigments. Note that the color of these phases (which simply reflect the topology of the band structure in the visual regime) may be nicely controlled by the stoichiometry of the anionic sub-structure.

Starting in the late 1990s, our group has paved the way for a chemical understanding of itinerant (that is, delocalized) magnetism of the transition metals and their alloys. We have succeeded to re-formulate the phenomena of both itinerant ferro- and antiferromagnetism into a chemical bonding theory in which the tendency to behave either as a ferro- or antiferromagnet simply results from occupied anti- and nonbonding states (see right figure) at the Fermi level [8]. Based on these predictions, we have been the first to rationally design and prepare ferro- and antiferromagnets in the quaternary system alkaline-earth metal/rhodium/transition metal/boron, and the synthesis of the three new compounds Sc₂FeRh₅B₂, Sc₂MnRu₂Rh₃B₂ and also Sc₂FeRu₃Rh₂B₂ [9] has corroborated our theoretical efforts. Within the last two years, we have continuously made a dozen of similar intermetallic compounds (see structural sketch on the left), yet to be published, in which their valence electron concentrations are carefully adjusted between 60 and 68 electrons per formula unit, and this corresponds to a fine-tuning of the Mn/Fe electron density in fractions of an electron! This research topic has turned out extraordinarly fruitful, and we plan to heavily invest in the crystal chemistry of such novel and complex magnetic intermetallics. In general, the bonding theory of such metal-rich intermetallic phases is quite underdeveloped despite the fact that a chemical understanding is a prerequisite for truly rational syntheses. On the other side, complex intermetallic magnets are true bonanzas for the restless synthetic solid-state chemist because there are infinite possibilities in terms of structure and composition.

As has been alluded to already, nitrogen-containing solids are a hot topic in contemporary solid-state chemistry, and we are also specializing in more complex phases with involve carbon-nitrogen anions such as cyanamides and carbodiimides as structure-building ingredients. In fact, the majority of all presently known solid cyanamides/carbodiimides has been made by our group. As a recent accomplishment, there is the phase HgNCN which represents the first solid state cyanamide/carbodiimide polymorphism ever encountered [10] as well as more much complex rare-earth carbodiimides such as LiEu₂(NCN)I₃ which are comprised of rare-earth tetrahedral clusters [11]; here, the NCN^2- units serve as linkers between the magnetic europium units; clearly, there is more structural flexibility for NCN^2- than for the simple N^3- (nitride) ion such that even condensed and oligomeric cluster structures can be generated [12]. Also, because of the reduced anionic charge of NCN^2- compared to N^3-, there is more covalency present in these "2nd generation" nitrogen-based solid-state systems.

Interestingly, no cyanamide/carbodiimide phase of the magnetic transition metals has ever been reported which involves a non-d¹⁰ electron count. This is particularly puzzling because, as we suspect, a couple of groups have been trying to reach that goal. In 2004, we had started a massive (almost combinatorial) computational project, funded by the von Humboldt foundation and targeted at the structure prediction of these phases, and the results indicate that all MNCN phases - before they have been made - are thermodynamically unstable with respect to the elements [13]. Given this information on hypothetical phases, the synthesis of MnNCN, the first member of a novel class of magnetic materials, is straightforward by removing another side product from the chemical equilibrium. As predicted, MnNCN (see left ORTEP sketch) is a high-spin antiferromagnetic material [14]. Other phases (such as CuNCN) can also be made [15]; we believe that there is an infinite number of ternary, quaternary or even higher magnetic carbodiimides waiting to be realized, and we have just begun to explorate this exciting kind of synthetic chemistry.

A new, truly interdisciplinary direction of research is given by pathological biomineralization for which accurate analytical techniques, modern diffractional methods and, sure enough, intense medical research must be synergetically combined. Did you know that the calcification of the aortic valve of the human heart is one of the major killers in all western societies? Almost nothing is known about the cause of this "calcification" (see right figure), and even the technical term is entirely misleading because the mineral deposits are composed of Ca-deficient hydroxyapatite which is, chemically, a simple phosphate [16]. We have found that - very surprisingly and counter-intuitively, too - only genetic reasons determine whether or not individuals will suffer from that disease, and we also believe that our recent crystallographic and statistical studies [17] (see left figure) will make it easier for the medical people to successfully navigate in the search for a cure for this disease. Without much doubt, there is enormous potential lurking in this hybrid of solid-state chemistry and medical research despite the fact that the experimental techniques involved, with the exception of X-ray crystallography, are relatively simple and inexpensive.

With the advent of very fast (parallel) computer architectures, density-functional theory-based molecular-dynamics simulations for molecules and solids are becoming increasingly popular, both on the pico- and nanometer length scales. For huge systems, we think that alternative parameterizations (which require a crystal-chemical understanding) may - sometimes - serve as cheaper (that is, much faster) alternatives [18]. In addition, order-N scaling electronic-structure methods using super-localized atomic orbitals are another alternative although their usefulness differs drastically from system to system. Also, entirely new approaches are needed to more systematically deal with periodic systems in which electron correlation plays a major, if not dominating, role such that density-functional theory appears as an invalid tool [19]. Upon searching for intermetallic compounds in the realm of anticorrosive alloys (binary system Sn/Zn), however, we have found a promising combination of density-functional theory on the one side and classic thermochemical approaches [20]. Here, too, the research is motivated by the paramount importance of corrosion protection; in Germany alone, corrosion destroys on the order of Euro 50 billion (!) each year; that's a lot of money. This kind of metal-oriented research will also become more important in the near future, in particular for high-performance steel-related activities.

Last, let's mention a few all-time research favorites of this group which we consider a useful supplement of a modern solid-state chemical laboratory. By pure curiosity, we have always been interested in the crystal chemistry of small high-energy molecules. As only one example, one might mention the crystal-structure determination and the study of the phase transformation of solid S₄N₄ in the proximity of its detonation temperature [21]. On the other side, metastable (albeit non-explosive) extended solids, for example binary indium halides such as In₄Br₇, have contributed to our ability to solve very complex structures from powder data and understand second-order Jahn-Teller distortions close to absolute zero temperature; such studies demand high-resolution neutron powder data [22]. Clearly, these "finger exercises" have no direct relationship with economically important materials but they provide and guarantee the skill of the coworkers to competently deal with any material. If one can handle and characterize air- and light-sensitive explosives at any temperature, the structural characterization of ionic liquids is not too difficult [23], and the characterization of oxidic ferroelectrics seems almost trivial [24]. There must always be a little room for unconventional, out-of-the-ordinary ideas.

Selected Publications