Hydrogen trapping

Hydrogen in metals is trapped by all maimer of lattice impurities, including other interstitials and lattice defects. Trapping reduces the mobility of hydrogen but its solubility is usually improved. This prevents precipitation, and the crystal degradation that it causes is avoided. 

In tantalum hydride, TaHo.ose, at room temperature the solid solution, a-phase, hydrogen occupies Djd tetrahedral sites and shows two broad transitions in the INS, at 911 and 1315 cm [59]. As the temperature falls the P-phase precipitates such that at ca 100 K this is complete, its spectrum is sharper and the lower transition is at 968 cm.  

When nitrogen impurities are introduced into the tantalum (they reside on octahedral sites) the hydride, TaNo.oo6Ho.oo3, behaves differently. Firstly, they retain their tetrahedral frequencies, discounting suggestions that the hydrogen atoms have moved to octahedral sites. Moreover, this spectrum remains unchanged down to the lowest temperatures, 1.5 K, showing that the solid solution phase has not precipitated [59]. 

The lanthanum in LaNis is essentially a high level impurity in these technologically important battery materials, again serving to maintain hydrogen solubility. INS work on these systems showed that at low concentrations hydrogen occupies both octahedral and tetrahedral sites [61]. 

Although the enhanced solubility of hydrogen in nanocrystalline palladium was originally associated with the relatively large number of grain boundaries in these materials [62], this is now seen to be more a question of sample preparation than of the final sample morphology [63]. 

---

One of them is the ortho-para conversion of hydrogen trapped in the frame copper during the production process (see ref. [100-102] and Section 2.2). 

Fig. 5. The hydrogen profile in an Al/Si sample measured with the 15N nuclear reaction, showing hydrogen trapped at the Al/Si interface (Liu et al., 1990).

Another example for a bimetallic NHC complex is the combination of a ruthenium hydride fragment with an ytterbium NHC complex. The NHC serves partly as the hydrogen trap [Eq. (39)]. ... 

Various additives can scavenge these intermediates. Added hydrogen traps the diphenyl diradical, while acetylene combines with the dehydrobenzene. Ammonia forms addition compounds with all three intermediates to give aminobenzaldehyde, benzamide, aniline and carbazol. 

Figure 1. Schematic diagram of the MIT hydrogen trap, with magnetic field profile. (From ref. 7.)...

Figure 2. Pictorial diagram of the hydrogen trap. The magnets are immersed in liquid helium the inner region of the trap is connected to a dilution refrigerator.

We summarize hoe some considerations affecting the potential accuracy of a spectroscopic measurement of the IS ->2S transition. We shall not dwell on the challenging problems of laser stabilization and optical frequency metrology, but only on the atomic considerations. In particular, we shall consider the major sources of line broadening and possible systematic shifts. We discuss below some of the factors which govern the accuracy of IS —>2S spectroscopy in the hydrogen trap. 

A.A. Haasz, P. Franzen, J.W. Davis, S. Chiu, C.S. Pitcher, Two-region model for hydrogen trapping in and release from graphite, J. Appl. Phys. 77 (1995) 66... 

The strong prevalence for allyl radicals to cyclize in 5-exo fashion as well as the accelerating effect of geminal diester substitution was also observed in iodide atom transfer reactions of ally lie iodides. The ratio of S-exo to 6-endo product is even higher than for hydrogen trapping, probably also due to the lower temperature in this photolytically initiated reaction (equation 6). Allylic dimers were again isolated as side products. No... 

---