Hydrogen sorption properties of D8₈-type systems II: Sc₅Si₃ and Y₅Si₃-Sc₅Si₃ solid solutions

Sc₅Si₃C_x carbides (0.01 < x < 1.0) have been made by arc melting Sc₅Si₃ with carbon. X-ray powder methods have failed to reveal any D8₈ superstructure phases and this is used as the basis of a discussion on hydride formation for Sc₅Si₃. Elevated pressures and temperatures do not result in an Sc₅Si₃H_x phase. Limited solid solubility in the Y₅Si₃-Sc₅Si₃ system has been established. The reaction of samples from the two single-phase regions Y_5-aSc_aSi₃ and Sc_5-aY_aSi₃ for a in the range 0.05 < a < 1.0 has been studied. The substitution of scandium into Y₅Si₃ raises the equilibrium pressure and reduces the temperature at which a half mole of hydrogen per Y_5-aSc_aSi₃H_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-aY_aSi₃ results in hydride formation when a ≫ 0.5; the pressure dependence of this hydride composition has been established and reversing temperatures and enthalpies estimated.