Rare earth dihydride

Ratishvili I.G., Vajda P. "Hydrogen ordering in the superstoichiometric rare-earth dihydrides. The case of a system with an energy constanst ratio p = V2/Vi <1. LaH2+x". Phys.Rev. B (1996), 53, 581-587. 

Rare-earth hydrides are also prepared by methods other than direct reaction with Hj e.g., rare-earth dihydrides can be formed from the metal with H O vapor at 100-150°C ... 

Table 17. Crystal field ground state anticipated on the basis of a first-nearest-neighbor point-charge model for the rare-earth dihydrides. Both hydridic and protonic hydrogen are considered, and the known experimental results are shown. (146).

Based on a short discussion of the electronic structure of the rare earth dihydrides, we shall compare theoretical predictions with some experimental results concerning the initial stages of hydrogen uptake. 

Fe/Tb) films. As a preliminary remark one must draw attention to the fact that the indexing of XRD and EDP reported by different authors is somewhat erroneous. First, Sato and Habu (1987) indexed two X-ray diffraction lines, recorded at 19.15° and 26.20°, as the (10.1) and (110) planes relative to the rare earth and iron, respectively. However these two lines, with interplanar spacings as 0.235 nm and 0.1745 nm, do not correspond to the (10.1) plane of the hep rare earth (other hand, one intense line has been forgotten located at 16.90°, it corresponds to the (200) plane of the rare-earth dihydride (< 200 = 0-264 nm). 

Recently, M-S transitions with strong hysteresis effects have been observed by the group at Orsay in the heavy rare-earth dihydrides GdH2+j , H0H2+J , ErH2+j and in YH2+J , with a surprisingly low superstoichiometry, of the order of x 0.1-0.3 (Vajda and Daou 1991, 1994, Daou and Vajda 1994, see e.g. fig. 31). Again, it is clear that the M-S transitions are driven by the collapse of a superlattice but, this time, the structural... 

The heat of formation of magnesium hydride (—33 kJ/mol H) is similar to that of the transition between the rare earth dihydride and trihydride. This was essential to keep the hydrogen uptake reversible. 

A schematic generalized phase diagram for rare earth-hydrogen systems is shown in fig. 26.2. The two-phase cubic + hexagonal region does not occur in systems where the rare earth dihydrides form continuous solid solutions with their trihydrides in those cases the cubic hydride phase field extends to H/M = 3. The actual positions of the phase boundaries depend on the particular metal-bydrogen system involved, as well as the temperature. Room temperature existence ranges of the rare earth hydrides are summarized in table 26.1. 

Thermodynamic properties of rare earth dihydrides as calculated from dissociation... 

Partial molal entropies of cerium hydride as a function of r as calculated from the data of Lundin (1966), Streck and Dialer (1%0), and Hardcastle and Warf (1966) are shown in fig. 26.8. The minimum in the curve at about H/Ce = 2 is probably due to a maximum in the configurational entropy at this composition since hydrogen atoms enter the octahedral interstices in the rare earth dihydrides before all the tetrahedral sites are occupied (see section 2.2). Therefore, there is disorder in both the tetrahedral and octahedral sublattices near the stoichiometric dihydride composition. 

As is true for the transition metal hydrides in general, the rare earth dihydrides (except for YbH2 and EUH2) are metallic conductors. The hydrogen deficient dihydrides (MH, to MH1.9) ate actually better electronic conductors than the... 

Resistivity ratios of hydrogen-deficient rare earth dihydrides to the corresponding rare earth metal. 

Libowitz, G.G. and J.B. Lightstone, 1%7, Point Defects in Some Rare Earth Dihydrides, in Kohler, W.C. ed.. Proceedings of the Sixth Rare Earth Research Conference, Gatlinburg, Tenn. pp. 132-141. 

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