Intermetallic Compounds by Reductive Annealing (Thesis)

Abstract 

The majority of the work described in this thesis focuses on the development of a new synthetic route to binary and ternary intermetallic compounds, which avoids the need for reaction at very high temperatures, multi-step reactions, reaction under vacuum and/or the synthesis of precursors. The route was adapted from a synthetic route reported for the synthesis of ternary and quaternary nitrides adopting the ε-carbide and filled β-manganese structures, where mixed metal oxides were reduced under an atmosphere of 10% hydrogen in nitrogen. In this work, binary intermetallic compounds including metal antimonides, germanides and bismuthides were prepared by reducing mixed metal oxides under 10% hydrogen in argon. A variety of high purity antimonides were synthesised, including NiSb, CoSb, CoSb3, Cu2Sb and SnSb. Tailoring of some of the properties of these antimonides was attempted by introducing other atoms into the crystal structures, thereby forming ternary compounds. The most notable ternary antimonides prepared include the solid solution of NiSb in CoSb, and the solid solution of RhSb3 in CoSb3. The Sb-Sb bond lengths in the solid solution of NiSb in CoSb display an unexpected trend, passing through a minimum. Reasons for this based on electronic grounds are suggested. Notably the Te-Te bond length within the solid solution of NiTe in CoTe does not display this variation. The solid solution of RhSb3 in CoSb3 is potentially very useful as the introduction of rhodium into the structure may alter the thermoelectric behaviour of the material. However, thermoelectric measurements on the compounds are yet to be carried out. Other work described in this thesis focuses on attempts to synthesise transition metal nitrides, firstly by extending a synthetic route reported for the synthesis of ε-carbide and filled β-manganese nitrides by introducing germanium into Fe3Mo3N, Co3Mo3N and the hypothetical compound „Ni3Mo3N‟, and secondly by the thermal treatment of potential precursors to binary transition metal nitrides. The synthesis and magnetic properties of Fe2GeMo3N, Co2GeMo3N and Ni2GeMo3N are discussed, and the synthesis of solid solutions of Fe2GeMo3N and Ni2GeMo3N in Co2GeMo3N are described. In addition, experiments to determine whether a small interstitial (in this case nitrogen) is needed to stabilise certain compounds adopting the ε-carbide and filled β-manganese structures are described. The preparation, crystal structures and thermal treatment of metal urea v complexes as potential precursors to binary nitrides are also discussed. Two of the metal urea complexes synthesised, ([Cr(urea)4(H2O)2][NO3]3 and [Fe(urea)6][NO3]), are particularly interesting: [Cr(urea)4(H2O)2][NO3]3 crystallises in a primitive, but close to C-centred unit cell, and [Fe(urea)6][NO3]3 crystallises in a C-centred cell which is related by a five-fold expansion to a smaller pseudo C-centred cell previously reported for the same structure.


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