Searching for ultra-incompressible materials under high-pressure conditions

One approach for the directed search for ultra-incompressible compounds is based on the idea that a combination of a high valence electron density in conjunction with covalent bonds should provide compounds with the desired properties. This has led to an extensive search for compounds containing heavy transition metal ions of period 6 (Ta,W,Re,Os,Ir) and a light element, such as carbon, nitrogen or boron. Some of these compounds, such as OsB2, have indeed shown outstanding physical properties, but several compounds, which are thought to be very promising, such as a rhenium boride with a B:Re ratio > 2, have not been synthesized yet. It is well established that high pressure/high temperature conditions may lead to unusual bonding environments and new structure types. However, our knowledge of the high-pressure behavior of binary period 6 transition metal borides is rather limited. While the corresponding carbides and nitrides are well investigated, no in-situ synthesis studies of borides at extreme p,T-conditions have been published yet and hence our knowledge of the p,T-phase stabilities of binary and ternary period 6 transition metal borides is extremely limited. Hence, the current proposal focusses on understanding phase-relations, structures and properties of binary and ternary period 6 transition metal borides obtained at very high pressures and temperatures in laser heated diamond anvil cells and large-volume high pressure devices, complemented by mechanochemically activated synthesis with high speed ball milling and density functional theory based atomistic model calculations. Samples will be characterized by numerous techniques, including microcalorimetry, vibrational spectroscopy and neutron diffraction on isotopically enriched compounds. The project fully exploits the complementary expertise and available experimental equipment in Innsbruck and Frankfurt and promises to yield novel compounds, a deeper understanding of the crystal chemistry of borides and possibly a route to synthesize materials with outstanding properties.

One aim of this study was the synthesis of new 5d transition metal borides under high- pressure and high-temperature conditions and furthermore their structural, physical, and mechanical characterization. The high-temperature syntheses were carried out either with a radio frequency furnace at the Leopold-Franzens-Universität Innsbruck or from our cooperation partners at the Johann Wolfgang Goethe-Universität Frankfurt with an arc melter. The high-pressure syntheses were carried out in a 1000 t Multianvil press with a Walker-type module at the Institut für Allgemeine, Anorganische und Theoretische Chemie at the Leopold-Franzens-Universität Innsbruck. During this project four new phases namely ß-Ir4B5, W 1.3(2)Re2.7(2)B2, Mn2IrB2, and Mn3-xIr5B2+x (0 = x = 0.5) could be synthesized for the first time. Furthermore, we were able to determine the structures of the new phases as well as different physical properties and could solve previously unresolved problems concerning the compressibilities of various rhenium borides. In earlier studies, Juarez et al.[1] determined the compressibilities of these rhenium borides experimentally and additionally via theoretical calculations. The experimental determined value of Re7B3 was unusually large (B0 = 438(16) GPa) and showed a huge discrepancy to the theoretical value (B0 = 385(1) GPa). A comparable discrepancy was observed for the values of Re3B. During our research we re-evaluated the compressibilities for the rhenium borides. Our findings for the phases (Re7B3: B0 = 391(5) GPa) are in much better agreement with the calculated data and showed that Juarez et al. overestimated the bulk modulus of Re7B3. Next to the re-evaluation of the compressibilities within the system Re B, we were able to synthesize four new phases and establish the bulk moduli of those and nine further phases within the various systems. By using neutron diffraction, we were able to determine the exact structure of ß-Ir4B5. Neutron diffraction was used as it is almost impossible to determine the exact position and occupation of very light (electron poor) elements such as boron next to heavy (electron rich) elements such as iridium solely based on X-ray diffraction data. The determination of the bulk moduli for the various iridium borides and elemental iridium showed that the incorporation of boron in the structures does not enhance the incompressibility but leads to a significant reduction of the bulk modulus in comparison to elemental iridium. This is in contrast to the observations on rhenium or osmium borides, where the incorporation of boron leads to a decrease of the compressibility with respect to the compressibility of the elements. [2] We were furthermore able to determine mechanical and physical properties (e.g. heat capacity, hardness, conductivity) of the various phases. In many cases the experimental data could be compared to theoretically obtained data via DFT calculations. These successful works have markedly deepened the knowledge in transition metal borides, especially due to the discovery and characterization of several new phases, which demonstrate impressively the potential of high-pressure/high-temperature syntheses.