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Two methods to make Y 5 Si 3 N x nitrides in an arc furnace are described. When YN is reacted with Y+Si single phase nitrides are produced as long as x ≤0.3. When Si 3 N 4 is used multiphase products containing Y 5 Si 3 (N) x and Y 5 Si 4 (N) y with x + y ≤0.3 result. In reactions with hydrogen the single phase material is shown to quickly saturate with H≥4.5 (fu) −1 and become amorphous. In contrast the multiphase products only slowly reach saturation and can be recycled many times until eventually their reactivity increases and they saturate. This behaviour is related to a loss of nitrogen as ammonia from the nitrides. Evidence for this reaction is given. Reaction enthalpies from Van’t Hoff isopleths have been measured. Curved isopleths similar to those found for Y 5 Si 3 C x samples are observed in most cases.

Carbides of Y 5 Si 3 , of general formula Y 5 Si 3 C x , where x =0.05–0.7, have been prepared by arc melting and their reaction with hydrogen studied. Single-phase carbides can only be made melting pre-formed Y 5 Si 3 with carbon. An activation process, which earlier work did not find, has been developed to induce reaction of the carbide with hydrogen. It involves temperature cycling in gas pressures above 2 atm to higher than 550°C with the severity of these conditions depending on the carbon content. Higher T and P are needed to condition samples of increased carbon content. Conditioned samples, where x ≥0.3 in Y 5 Si 3 C x , will repeatedly cycle 2.5 H (fu) −1 between room temperature and 500°C with between 0.9 and 1.2 H (fu) −1 being desorbed or absorbed with fast kinetics within a very small temperature range. This behaviour is believed to arise from a polymorphic change between an α-form of the D8 8 structure and a supercell β-form when x x >0.3. The first phase change is identified from the rapid expulsion of hydrogen to be close to 438°C and the second at 446°C. Hydrogen saturation of the samples is close to 7 H (fu) −1 and is related to the carbon content. Y 5 Si 3 C 0.5 H 7.33 appears to be an upper limit. The large amount of hydrogen in the crystal structure does not result in loss of good crystallinity or loss of hydrogen reactivity that is always observed for samples without carbon, such as Y 5 Si 3 H ≈5 . The significantly enhanced crystal stability and the greatly improved cycle lifetime of the carbide is analysed in terms of a structural disorder caused by the presence of carbon in at least two crystallographic sites. An attempt to quantify the enhanced stability, compared to Y 5 Si 3 , is made by measuring reaction enthalpies using Van’t Hoff isopleths. To achieve this, rigorous conditions were imposed on the reaction of the binary alloy with hydrogen in order to obtain reliable and repeatable values for Δ H abs and Δ H des .