Layered intermetallic borides

Boron has a high melting point, low density and high hardness. It forms with metals a wide range of compounds which includes various stoichiometries exhibiting diverse structural arrangements. Among them, the layered borides, particularly those, related to the AlB2 structure are of fundamental interest and practical importance because of their excellent mechanical, thermal, chemical and electronic properties which enable the applications of these materials as wear protective coatings, primary battery electrodes, high-temperature structural materials, etc. Moreover, the AlB2 related borides have been under intense scrutiny since the discovery of superconductivity in MgB2. In these layered structures, the metal layers interleave with boron layers; their exceptional properties are attributed to the strong covalent network formed by boron nets interconnected with metal-boron bonds and can be tuned by the appropriate selection of the transition metal species. The interplay between metal atoms and boron may results in even more varied materials properties. Therefore, novel layered transition metal boron-based materials, having both strong metal-boron and boron- boron bonds are at the focus of the actual research proposal. Another group of materials with a great future which will be studied in the current project concerns the layered borides structurally related to MAX phases. In contrast to AlB2 related structures, the MAX structures are dominated by strong intralayer and weak interlayer interactions and consist of metal boride sublattice interleaved by layer(s) of pure aluminium. Due to selective oxidation of Al and formation of continuous and protective Al2O3 scales on the substrates, these phases demonstrate excellent oxidation resistance and could be used in demanding applications in harsh environments, e.g. for high-temperature applications in air. This group of materials will be extended in the current project through the discovery and thorough characterization of new transition metal borides with aluminium, gallium and indium. Borides with the layered structures can have extremely high anisotropy of physical properties. Having accurate structural models for boride phases provides valuable insight toward understanding the physical phenomena and more directly predicts the means of manipulating the crystal chemistry of these compounds. Besides the structural features, bonding and electronic properties, the application required characteristics are deeply related to composing elements. Among the perspective elements, the 3-, 4- and 5-d metals are the most appealing constituents for binary and ternary borides in view of potential applications as ceramic materials as well as to search for materials exhibiting high hardness and new superconductors with enhanced TC. Therefore the exploration of new transition metal boride systems towards identifying layered structures envisaged in the current research proposal is becoming a particularly important up-to-date task.

Boron-rich compounds exhibit many interesting properties, resulting from their crystal structure and bonding. These compounds are promising for a variety of technical applications. Parallel to the development of the chemistry and physics of higher borides, a lot of research work is currently devoted to borides that have chains of B atoms or boride networks nested with metal atoms. A topic particularly investigated in the project was the modification of the structural and physical properties of moderate/high boron compounds via partial B/M substitution in the boron atom framework. New ternary compounds REPtxB6-2x (CaB6-type disordered variants) and new representatives of the Er4NiB13 type (UB4-type derivative structure) were discovered. The structure of YPtxB6-2x is derived from the fragments of YB6- and YPt3-types, with an YB6-block, containing B8 rings and Y, and an YPt3-type fragment, containing Y/Pt layers. The Er4NiB13-type structure features infinite, slightly puckered B-networks of fused 7- and 4-membered rings, interleaved with layers of RE, M and B atoms. Since no ternary compounds with extended B-B bonding have been observed hitherto with Pt, the architecture of Y(Yb)PtxB6-2x provides a new understanding of the formation of B-B aggregates in this compositional space. In contrast, the concentration range within 40-60 at%B in RE-M-B systems (M=Ni,Os,Rh) is well populated with various structures, e.g. Y2ReB6-type, YCrB4-type, the REM'4B4 structures, etc. The layered Y2ReB6-type compounds in these systems have been studied by various experimental and theoretical methods and prove to be promising hard materials. By revisiting the concentration space of the Y-Os-B superconducting phase, the structure of YxOs4B4 was elucidated (incommensurate assembly of Y columns extending along [001] in an Os4B4 framework). The family of compounds with an M:B ratio of ~2.4-3.0 (and higher) was complemented by borides with new structural arrangements, using experimental and theoretical methods (layered heavy fermion YbPt5B2, superconducting cage compound Sc5Pt24B12, etc.). Recently, a number of chemically and structurally distinct layered compounds have been categorized as MAB structures based on their structural arrangements and physical properties. In order to expand the element and concentration space of layered structures towards these new phases, new ternary M-A-B and M-M'-B systems, as well as quaternary M-M'-A-B systems (M,M'=transition metals; A=Al,Ga,In) were explored. A number of ternary Al- and Ga-containing borides as well as ternary aluminides/gallides were obtained. Their crystal structures were investigated, together with electronic and physical properties . Experimental and theoretical investigations of the layered phases with Al and Ga versus In show that the first two elements are promising candidates to obtain structures with MAB features. These findings call for further experimental studies in context with computational screening in the relevant element and concentration range for the targeted synthesis of the indicated new materials.