Hydrogen tetrahedral interstitial sites

Point defects and complexes exliibit metastability when more than one configuration can be realized in a given charge state. For example, neutral interstitial hydrogen is metastable in many semiconductors one configuration has H at a relaxed bond-centred site, bound to the crystal, and the other has H atomic-like at the tetrahedral interstitial site. 

Throughout the remaining sections of this chapter, various configurations for complexes that include hydrogen will be discussed. Figure 1 shows a schematic of a [110] plane that includes a substitutional impurity. The following sites for an H atom attached to the impurity are labeled the bond-centered site (BC), the tetrahedral interstitial site (T), the antibonding site (AB), and the C-site (C). 

One of the earliest studies was an MNDO-cluster treatment by Corbett et al. (1983) of the isolated interstitial hydrogen and the corresponding molecule. In this early study, the isolated H was found to be stable at the M-site in silicon. This is directly between two adjacent C-sites, where the C-site is directly between next-near-neighbor silicons. (We note that in these calculations, the C- and M-site energies are very similar.) It was not known at that time, however, that the BC site is the stable location for neutral isolated interstitial hydrogen (see Chapter 16). In the Corbett study, an H2 molecule was found to be stable (or at least, metastable) in the tetrahedral interstitial site when oriented along a (111) direction. The... 

The close correspondence between the properties of Mu in Si as determined by /u,SR and pLCR and those for the AA9 center produced by implanting hydrogen in silicon shows that Mu in silicon and the AA9 center are isostructural and in fact almost identical. They are neutral isolated bond-centered interstitials. Numerous theoretical studies support this conclusion. The observation of such similar centers for muonium and hydrogen supports the generalization that hydrogen analogs of many of the muonium centers exist. Of course, this assumes that the effects of the larger zero-point vibration of the muon relative to the proton do not make a major contribution to structural differences. The p-SR experiments, reinforced by theory, demonstrate that another structure also exists for muonium in silicon, called normal muonium or Mu. This structure is metastable and almost certainly is isolated neutral muonium at a tetrahedral interstitial site. 

As this takes place, hydrogen atoms can occupy quite definite positions in the lattice of metal (octahedral, tetrahedral, or both octa- and tetrahedral interstitial sites). 

In the case, when hydrogen atoms are distributed in octahedral and tetrahedral interstitial sites of single-component fullerite, the maximum possible concentration of hydrogen atoms in fullerite at T—>0 is defined by the relative number of interstitial sites and equals... 

Figure 14.9 Unit cells of fullerite with fee (a) and bee (b) structures with lattice hydrogen only. (Shaded circle) Sites of crystalline lattices, in which fullerenes molecules are distributed ( ) octahedral interstitial sites (o) tetrahedral interstitial sites, in which atoms of lattice hydrogen are located. (Reprinted from Ref [80] with permission from Elsevier.)...

Fig. 3.19. Schematic density of states g(E, x, T) for hydrogen in amorphous Ni-Zr on tetrahedral interstitial sites comparing rapidly quenched (a) and mechanically alloyed (b) materials [3.30]...

The dynamical properties of hydrogen in Sc are also quite remarkable. NMR measurements of the proton spin-lattice relaxation rate in a-ScH [132] have revealed a localized H motion with the characteristic jump rate tf of about 10 s at 50 K. This localized motion is evident from an additional frequency-dependent peak at low temperatures (35-80 K). The structure of the sublattice of tetrahedral interstitial sites in a h.c.p. metal suggests that the localized H motion corresponds to jumps between two nearest-neighbor sites separated by about 1.0 A in the c direction. QENS measurements on a-ScH [133, 134] have revealed the existence of a still faster localized motion with the jump rate passing through a minimum of approximately 7x10 ° s i near 100 K and increasing to 10 s at 10 K. 

Diffusivity and diffusion coefficient are synonymous [25]. In dense membranes, diffusivity is associated with the rate of movement of dissociated hydrogen from site to site within a crystal lattice. In general, diffusivity of hydrogen is greater in metals with the body centered cubic structure (bcc) relative to metals with the face centered cubic stmcture (fee) [26]. According to Wipf, dissociated hydrogen occupies tetrahedral interstitial sites in bcc metals and hops between such sites. Tetrahedral interstitial sites are only 1.01-1.17 A apart in common bcc metals [26]. This relatively short distance permits quantum mechanical tunneling, which accord-... 

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