Unit cell expansion in Laves phase‐type compounds caused by interstitial hydrogen

Abstract

The Laves phase‐type compounds of yttrium and rare earths with manganese (RMn2) easily absorb hydrogen which locates at the interstitial sites. It is well known that the lattice expands with increasing hydrogen concentration and the volume increases up to about 30%. The increase of the unit volume is nonlinear as a function of hydrogen content. To explain this behaviour a model in which the volume changes were correlated with the changes of the bulk modulus with hydrogen concentration was proposed by Hirata and co‐workers. This model describes properly the initial volume increase, but does not reflect the changes at higher concentrations. For the hydrides with R = Y, Gd, Er, Dy, Tb, Ho at x ≈ 3 the volume of AB3 interstitials becomes large enough for hydrogen to enter. According to this observation we extended this model. We show that the calculated volume expansion fits very well to the experimental data for RMn2 (R = Y, Gd, Dy, Tb, Ho). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and magnetic properties of C15 HoMn2 hydrides

“…The process is reversible and decreasing the temperature forces a return to the ı structure. The related behavior (such a high temperature of the first structural transformation) was observed in GdMn 2 H x [9] and it seems to be the result of filling AB3 interstitial sites by hydrogen atoms and it is in agreement with the Hirata-Figiel model [15]. Smaller volume of that type of interstitial site in comparison with A2B2 site could lead to an unstable cubic structure and in consequence the C15 unit cell transforms to the rhombohedral system with lower symmetry but larger volume of the unit cell.…”

“…1) was fitted with Eq. (2) according to the model proposed in [15] yielding parameters: B 0 = 6 ± 0.5, b = 10 ± 1,…”

Effect of Ni concentration on the structural and hydrogen storage characteristics of Zr–Mn based laves phase system

“…As can be seen, the XRDs of hydrogenated samples of ZrMnNi and ZrMn 0.75 Ni 1.25 exhibit a considerable shifting toward the lower angle side. The shifting arises due to the fact that the insertion of hydrogen in the interstitial sites does lead to expansion of lattice (the upper limit of expansion is known to be one-fourth of the actual lattice parameter) [23]. The peak shifting gives us a clue that hydrides of both the materials could be stable under room temperature.…”

Structural and magnetic transformations in NdMn2Hx hydrides

“…Table 1a shows the relative cell volume change V/V 0 for all the phases detected at 300 K. Its values increase steadily with increasing hydrogen concentration attaining ∼20% at x = 4.0 H/f.u. The lack of data points below x < 2.0 H/f.u., which is due to the instability and irreproducibility of the samples in this concentration region, precludes a reliable interpretation within the Hirata-Figiel model [13]. Nevertheless, it is instructive to compare the RT V/V 0 values obtained for NdMn 2 H x to those reported for the other analyzed hydrides (Gd, Tb, Dy, Ho, Er)Mn 2 H x [8,13] (Fig.…”

“…The A2B2 sites are occupied by hydrogen atoms at low hydrogen concentrations (up to x ∼ 3.0-3.5 H/f.u. ), whereas for higher x the H atoms start to occupy the AB3 sites as well [12,13].…”