A Concept for Synthesis Planning in Solid-State Chemistry

Abstract

There is a widely-held belief that the preparation of new solid-state compounds based on rational design is not possible. Herein, we present a concept that points the way towards a rational design of syntheses in solid-state chemistry. The foundation of our approach is the representation of the whole material world, that is, the known and not-yet-known compounds, on an energy landscape, which gives information about the free energies of these compounds. From this it follows that all chemical compounds capable of existence are present on this landscape. Thus the chemical synthesis always corresponds to the discovery of compounds, not their creation. Consequently, the first step in planning a synthesis can and must be to identify a synthesizable compound. Up to now, materials capable of existence are discovered in the course of an experimental exploration of the energy landscape; however, an a priori identification of a synthesis goal requires an exploration using theoretical methods. In contrast to those computational approaches currently employed for structure determination for fixed composition and already known unit cells, our aims clash with such restrictions and full global optimizations have to be performed on the landscape. Although for reasons of computational feasibility the accuracy of the energy calculations is not yet as high as one would wish, our approach proves to be surprisingly robust. One always finds the already known compounds of a given chemical system, and, in addition, further plausible structure candidates are discovered. The second step of a rational planning of syntheses is the design of feasible synthesis routes. Modeling such routes requires highly accurate computations for realistic thermodynamic conditions, however this is usually beyond our current capabilities. Thus, we have not seriously pursued such a deductive approach; instead we have attempted, to reproduce the "computational annealing" employed during our structure predictions in the experiment. Educts, generated by vapor deposition methods, that are disperse on an atomic level are found to react with surprisingly low activation energies to give highly crystallized products. However, even this technique does not yet provide the possibility to selectively synthesize a specific solid compound. For this final step, modeling and experimental control of nucleation processes will be the key ingredient. Only when viewed superficially, our goal of a "rational design" of solid-state syntheses and the "high-throughput" syntheses are in contradiction. But an exhaustive exploration of the unimaginably large combinatorial diversity of chemistry remains beyond our capabilities, even with an exceedingly high throughput. The future of solid-state synthesis will be found in a union of these two conceptual approaches.

The Effect of Centrifugal Force on the Assembly and Crystallization of Binary Colloidal Systems: Towards Structural Gradients

“…While for ionic crystals the numerical proportion of the two spheres (atoms) is fixed due to charge balance, for intermetallic phases a whole range of different compositions can be probed correlating to an enormous variety of structures. Furthermore, it is well known that the very same compound can also exist in energetically less favorable modifications, so-called polymorphs [1]. However, the "disadvantages" of the model of atoms as spheres are that they are extremely small, that not every sphere size (or ratio of two sphere sizes) is available, and that the interaction cannot be adjusted for two given sphere sizes.…”

Phase stabilities at a glance: Stability diagrams of nickel dipnictides

“…As a result, the focus has been turned from the pure description of experimentally known compounds to the discovery of new materials by means of theoretical methods. A milestone to predict hitherto unknown compounds and structures was set by the work of Jansen, 3 emphasizing the role of theoretical methods in solid state chemistry synthesis planning. The introduction of the concept of an energy landscape that reflects the energy hypersurface of the material world and the exploration of this landscape delivers a key for rational design of syntheses.…”

Molecular Encoding at the Nanoscale: From Complex Cubes to Bimetallic Oxides

“…[4] Multimetallic oxides have been known in solid-state chemistry for a long time, but the high processing temperatures typically applied make them less suitable for the preparation of nanoscaled materials. [5] In addition, optimum dispersity of the two metals inside the nanostructures would play a pivotal role for the rational synthesis and adjustment of properties of those systems. Optimum dispersity is ensured when the respective elements are distributed on the molecular level.…”