Thermodynamic studies of hydrides of R 6Fe 23 (R ≡ Y, Er, Ho, Lu) and R 6Mn 23 (R ≡ Gd, Dy, Er, Ho)

Abstract

The results of a thermodynamic study of the dehydrogenation of R 6Mn 23 (R ≡ Gd, Dy, Ho, Er) are reported and correlated with earlier work on R 6Fe 23 hydrides (R ≡ Y, Er, Ho, Lu). The enthalpy and entropy changes ( ΔH, ΔS) associated with the desorption of hydrogen from particular hydride phases have been calculated from hydrogen pressure-composition isotherms and the van't Hoff expression. The thermodynamic work indicates four hydride phases for the iron-containing systems and three for the manganese-containing systems. A Lundin-type plot of the free energy of hydrogen desorption vs. the interstitial hole radius for the R 6Fe 23H x system illustrates the dependence of hydride stability on the lattice dimensions. The relative stabilities of the different hydrogen sites that are occupied for these R 6T 23-H systems were established by correlating thermodynamic and published structural information. In general, the affinity of an interstitial hole for hydrogen was found to follow R 6 > R 3 T > RT 3, where T ≡ Fe or Mn (spatially defined by R x T y ). Attention is drawn to the importance of H-H nearest-neighbor interactions in regard to the structures of these materials. Isotope effects are observed for the Ho 6Fe 23-H(D) systems.