Abstract
Sc 5Si 3C x carbides (0.01 < x < 1.0) have been made by arc melting Sc 5Si 3 with carbon. X-ray powder methods have failed to reveal any D8 8 superstructure phases and this is used as the basis of a discussion on hydride formation for Sc 5Si 3. Elevated pressures and temperatures do not result in an Sc 5Si 3H x phase. Limited solid solubility in the Y 5Si 3-Sc 5Si 3 system has been established. The reaction of samples from the two single-phase regions Y 5− a Sc aSi 3 and Sc 5− a Y a Si 3 for a in the range 0.05 < a < 1.0 has been studied. The substitution of scandium into Y 5Si 3 raises the equilibrium pressure and reduces the temperature at which a half mole of hydrogen per Y 5− a Sc a Si 3H 2.5 can be reversibly stored; ΔH and ΔS values have been established from pressure-composition isotherms. Substitution of scandium by yttrium in Sc 5− a Y a Si 3 results in hydride formation when a ⪢ 0.5; the pressure dependence of this hydride composition has been established and reversing temperatures and enthalpies estimated.
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