Designing New Structures of Magnetic Materials: Cases of Metal Borides and Metal Chalcogenides
- Author(s): Scheifers, Jan Phillip
- Advisor(s): Fokwa, Boniface P. T.
- et al.
We are in the middle of a changing paradigm in solid-state chemistry: shifting away from analyzing the structures of crystalline compounds to understand the structure-property relations towards the targeted synthesis of new materials with properties desirable for specific applications – known as the materials-by-design strategy.
This approach requires control over the obtained products and their structures, which is still an enormously challenging task, especially for intermetallics. Intermetallics are non-molecular crystalline solids based on two or more metals with a stoichiometry that is not continuously variable and for which the crystal structure differs from the structure of the involved elements. Metal-rich borides are a subgroup of intermetallics with huge structural variety and highly interesting properties. Many metal borides are hard (or even superhard), have extremely high melting points, and exhibit fascinating magnetic properties (if they contain magnetically active elements).
This work primarily focuses on metal-rich borides and their structural relations but it includes some chalcogenides as well. A ternary niobium boride, Nb1-xOs1+xB with a new crystal structure was discovered, presented itself to be a missing link. Its discovery led to the identification of a new class of borides containing over a dozen different crystal structures. The structure-building principles were explored, and it was found that all of these crystal structures can be described using the same primary and secondary building blocks. The structure of Ti5-xFe1-yOs6+x+yB6, another boride in a new crystal structure with interesting magnetic properties due to 1-dimensional Fe-chains, can be explained in the same conceptual framework. Moreover, for the first time, hitherto unknown crystal structures with exciting magnetic properties can be predicted by applying these structure-building principles. The new TiFe1-xOs2+xB2, which is predicted using such principles, is one of the few ternary metal-borides without rare-earth elements that is ferromagnetic above room temperature.
The structure-building principles with their predictive power can serve as guidelines for obtaining metal borides with specific properties. Therefore, they are an important step towards materials-by-design. Moreover, they could aid in understanding how and why observed structures form while others do not, a crucial step to control and ultimately manipulate the formation of these compounds in the future.