Ytterbium dihydride

Because europium and ytterbium dihydrides resemble the ionic alkaline earth dihydrides, it is expected that these hydrides would be poor electronic conductors (insulators or semiconductors). There have been no reported electronic measurements on EuHj, but Heckman (1969) obtained a resistivity value of about lO ohm-cm for YbHi.90 at room temperature. Heating the hydride resulted in a monotonic decrease in resistivity to a value of 2.5 x lO" ohm-cm at 150°C. The logarithm of the resistivity varied linearly with reciprocal temperature which is indicative of semiconductor behavior. 

Eu—H system.—The lighter rare earth trihydrides have a face centered cubic structure whilst those of samarium and the heavier rare earths are hexagonal close packed. Europium and ytterbium, however, form orthorhombic dihydrides [236]. Although it has been possible to prepare [237] a hydride of ytterbium with empirical formula YKH2.55 all attempts to prepare the europium trihydride failed. 

The preparative reaction with europium dihydride failed to produce analogous carbide hydrides. Obviously, the failure is consistent with the high stability of divalent europium. In YbCo 5H, YbCHo.s and YbOo.sCo.s, ytterbium is clearly trivalent, as shown by magnetic data (Haschke 1975), and for La and Y the hexagonal phases have also been found (Lallement and Veyssie 1968). Therefore, it can be expected that the hexagonal RCo.sH phases probably exist across the lanthanide series, which exhibit a trivalent state as their stable valence state, with the exception of europium. 

There has been one report on the preparation of californium hydrides [235]. The hydrides were prepared by reaction of californium metal with hydrogen at elevated temperatures. It was believed that the stoichiometries were close to that for the dihydride (CfHj+The products exhibited fee structures with an average lattice parameter of Oo = 0.5285 nm, which is slightly larger than expected for the compound based on extrapolations of parameters for preceding actinide dihydrides. This larger parameter and the inability to prepare a trihydride of californium were believed to reflect a tendency for californium to be divalent In the lanthanide-hydrogen system, the hydrides of divalent europium and ytterbium metals deviate from the behavior of the other lanthanide hydrides [149]. 

As discussed earlier, europium and ytterbium differ from the other rare earth hydrides because of their more stable +2 oxidation state. Therefore, the magnetic moment of europium dihydride (7/it) is closer to the theoretical value for the state (7.94 it) than the Eu state (zero). Because of the seven... 

In a later paper Chouinard and Gustafson (1971) reported angular correlations of positron annihilation in erbium, gadolinium, holmium and ytterbium hydrides. The correlations were again claimed to be consistent with the predictions of the protonic model. Recent studies on yttrium dihydride (Sabin et al., 1972 Rozen-feld and Debowska, 1975) were interpreted on the basis of the proton model, but it was pointed out that the model fails for higher hydrogen concentrations, i.e. YH2-YH3 (Rozenfeld and Debowska, 1975). 

---