Site occupancy interstitial

There have been numerous studies with the objective of gaining an understanding of the factors that influence the stability, stoichiometry, and H-site occupation in hydride phases. Stability has been correlated with cell volume [7] or the size of the interstitial hole in the metal lattice [8] and the free energy of the a p phase conversion. This has been widely exploited to modulate hydride phase stability, as discussed in Sec. 7.2.2.1. 

In a review of the subject, Ubbelohde [3] points out that there is only a relatively small amount of data available concerning the properties of solids and also of the (product) liquids in the immediate vicinity of the melting point. In an early theory of melting, Lindemann [4] considered that when the amplitude of the vibrational displacements of the atoms of a particular solid increased with temperature to the point of attainment of a particular fraction (possibly 10%) of the lattice spacing, their mutual influences resulted in a loss of stability. The Lennard-Jones—Devonshire [5] theory considers the energy requirement for interchange of lattice constituents between occupation of site and interstitial positions. Subsequent developments of both these models, and, indeed, the numerous contributions in the field, are discussed in Ubbelohde s book [3]. 

Virtually all of the reported structural data on titanium alloy hydrides and deuterides indicate that the solute atoms occupy tetrahedral interstitial sites in the metal lattice. Neutron diffraction data obtained for deuterium in Ti/34 atom % Zr and in Ti/34 atom % Nb (17) indicate tetrahedral site occupancy in the bcc /3-phase. Similarly, data reported for deuterium in Ti/19 atom % V and in Ti/67 atom % Nb (18) indicate tetrahedral site occupancy in the fee 7-phase. Crystallographic examination of the 7-phase Ti-Nb-H system (19) reveals that increasing niobium content linearly increases the lattice parameter of the fee 7-phase for Nb contents ranging from 0 to 70.2 atom %. Vanadium, on the other hand, exerts the opposite effect (6) at H/M = 1.85, the 7-phase lattice parameter decreases with increasing vanadium contents. 

Figure 3. Relative expansion of the lattice constants of V induced by interstitial hydrogen. Open bars = O-site occupation closed bars = T-site occupation.

Figure 5 shows that the hydrogen occupation at both interstitial sites is stable for the fee metals. For Ni and Pd, the O-site occupation is suggested to be more stable than the T-site occupation. As has been expected from the results, the O-site occupation for Ni and Pd has been experimentally observed.910 As for the other fee metals (Pt, Cu, Ag and Au), the T-site occupation is more stable than the O-site occupation. This suggests that the T-site occupation in these fee metals should be experimentally observed, although there is no reliable data to confirm it. 

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

Before closing this section on thermodynamics, it should be mentioned that two schemes are used for modeling PCI curves [54]. In one of them, mathematical expressions based on the interstitial site occupation of hydrogen are used to calculate pressure as a function of concentration [55, 56]. The other is based on the phase... 

A variety of other y -type phases with high Li+ conductivity are derived from the Li3X04 phases with X = P, As, or V. The substitution mechanisms are of the type X- - (Si, Ge, Ti) + Li, and lead to the creation of interstitial Li+ ions which are responsible for the high ionic conductivity. The highest conductivity at room temperature, 4 x 10 Scm , is found in the series Lis+ c(Ge cVi c)04. Neutron diffraction has been used to locate the interstitial lithium ions, to determine their site occupancy, and correlate the high ionic conductivity with the connectivity of the interstitial sites. ... 

Desorption isotherms for the hydrides of LaNi eAlo,] and LaNij sAlo.s are presented and values for the enthalpy and entropy changes of the hydriding reactions are calculated from the vant Hoff plots of log P vs, i/T. A crystallographic model of LaNij Al is shown and consideration of the nearest neighbor atom distribution leads to a rationalization of the observed linear relationship between the enthalpy change, aH, and the aluminum composition. Brief discussions of methods to predict dissociation pressures or interstitial site occupation are included. The cubic and hexagonal ABs phases are compared and, finally, the application of these alloys in chemical heat pump systems is noted. 

Shinar et al. (16) have proposed that specific interstitial site occupations can be determined by associating different binding energies with... 

The charges on the defects are measured relative to the normal site occupation and are indicated by the superscripts ( ) and ( ) for positive and negative charges, respectively, e.g., Zn represents a divalent zinc ion on an interstitial site and carries a charge of +2 relative to the normal, unoccupied, site. 

Fig. 18 An example of how structural constraints may impose near-degeneracy on the site-occupation energies of neighboring interstitial sites in an amorphous solid. The host atom tetrahedra defining the interstitial sites a, b, c, d all have the edge 1-2 as a common structural feature. As a consequence, near degeneracy of sites a and d, for example, only involves a near-similarity in the arrangement of pairs 3-4 and 3-5.

The distribution of hydrogen atoms among the available interstitial sites of the metal host structure is of both fundamental and practical interest with regards to the stabilities and compositions of the various hydride phases. Although x-ray diffraction is commonly used to determine hydride structures, this technique cannot identify the hydrogen site occupancy except under extremely favorable conditions (which are quite rare). Powder neutron diffraction is the conventional method to locate deuterium atoms in metal deuterlde structures. Because the Incoherent... 

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