STRUCTURE AND PROPERTIES OF 2D AND 3D NETWORK BORIDES

Boron exists in several allotropes representing structural complexity, unusual bonding situations, and a variety of derivative structures. Properties of boron compounds are very diverse. High boron content compounds are used as refractory, corrosion-resistance and super-hard materials. They possess strong covalent boron-atom networks which deliver those appealing mechanical and chemical characteristics. In recent years the new application fields of borides have emerged, e.g. thermoelectric and energy conversion materials. Boron atom icosahedral framework compounds are in majority p-type electrical conductors; this property is inherited from p-type semiconducting boron matrix. The insertion of metal atoms into the atomic boron network may induce new functionality of materials, such as e.g. n-type semiconducting behavior, or possible metallization, and development of superconductivity. Therefore, it is of scientific and practical interest to investigate new derivative compounds of 3D icosahedral boride groups or even expand the studies to structural families containing covalent boron clusters which have higher metal-to-boron ratio. In this connection it is useful to consider net-like layered materials, which exhibit the infinite 2D nets composed of boron and intercalated metal atoms. The important fact about borides with 2D layered structures related to MgB2 is that their properties can be varied from semiconducting to superconducting by sandwiching different metal species between the B sheets. Hence, the ternary boron net-like layered structures, exhibit interest also as the research field for the search of superconductors. The given project is aimed on exploration of new complex boron atoms 2D- and 3D-covalent network structures which will be obtained by variation of the metal component within the ranges of rare earth and platinum metals. The study will focus on new icosahedral- and planar- B network solids providing the understanding of their structural and physical properties by integration of structural chemistry, physics, and calculational methods.

Boron and borides are fascinating materials with diverse unusual structures and attractive functional properties. Because of strong covalent boron?boron and metal?boron bonds, the borides are characterized by high melting points, excellent corrosion and wear resistance, chemical inertness, etc. In recent years new application fields of borides exhibiting three-dimensional (3D) boron atomic frameworks have emerged, i.e. as HT thermoelectric and energy conversion materials. In the current project we mainly focus on the discovery of new borides, exploration of their atomic network structure and symmetry in crystal structure and investigation of their physical properties.One particularly studied subject within the project was the firstly discovered anomalous zirconium doping effect in ?-rh boron, which was clearly demonstrated to modify the crystal structure of boron leading to a drastic change in the sign of Seebeck coefficient. The ability of boron to form strong chemical bonds with itself and with metallic elements (e.g. Y, rare-earth (RE) and transition metals) led us to the synthesis and identification of new examples of cage like structures in which the structural arrangement plays a large role for the physical properties. Revisiting the phonon glass electron crystal REB66 phases, we obtained and for the first time studied in details the structural arrangements of boron-rich binary ytterbium borides (YB66- and YB50-type structures). We also discovered that small amounts of transition metal can act sometimes as bridging sites and result in formation of novel boron cluster structures (e.g. Pt doped YB50-type phase). Such illustration of the possibility to control the properties can expand the prospects of borides as application materials.The phases with moderate boron content are characterized by two-dimensional boron sub-lattices. The family of 2D atomic nets structures was expanded in our work by new representatives using both experimental and theoretical methods (e.g. ScRu2B3, demonstrating a unique arrangement of puckered infinite honeycomb boron layers intercalated with the metal-rich CeCo3B2-type related slabs; (Sc,Y)2RuB6 exhibiting infinite planar layers of boron pentagons, hexagons and heptagons; etc.).Binary platinum borides tend to form defect boron aggregates, e.g. defect planar boron 63 nets in the anti-MoS2 structure of Pt~2B and defect zigzag chains with ordered boron/vacancy arrangement in the unique structure of Pt2B. The application of metal fluxes (Cu, Al) for the synthesis of Pt(Pd) borides has been investigated and resulted in a number of novel ternary complex structures for which low temperature superconducting transitions have been observed and studied.To summarize, by synergy of experimental (solid state chemistry, solid state physics, structural chemistry) and theoretical (DFT calculations) methods, new materials based on various boron atomic networks were developed or powerful methods for modification were demonstrated.