Hydrides tetrahedral site

Hexagonal AB5 compounds form orthorhombic hydrides. Basal plane expansion, caused by hydrogen occupation of interstitial sites (4), change the compound s structure and can be ordered or disordered. For example, Kuijpers and Loopstra (4) found, by neutron diffraction, that the deuterium atoms in PrCo5D4 were ordered on certain interstitial octahedral and tetrahedral sites. The sites occupied by hydrogen in various AB5 hydrides are not fully understood because of insufficient neutron diffraction data. The available information is considered in the section on configurational entropies. 

Although the model of three octahedral and six tetrahedral sites seems satisfactory for hydrides (AB5Hn) where n < 4, several models yield comparable results for the three hydrides where n > 4. To select the best model, additional neutron diffraction data for hydrides with n > 4 would be useful. 

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. Calculated properties of hydride of BCC metal (atomic radius unity) as a function of relative radius R of hydrogen in one tetrahedral site on face, alternating (0,1/2, 3/4) and (1/2, 0,1/4) Unprimed numbers refer to case where metal atoms are at minimum separation primed numbers to case of minimum molar volume...

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

On hydrogenation, hydrogen atoms vhll occupy specific interstitial sites. The interstitial sites in three major crystal structures of hydrides are shown in Figure 4.6. Only octahedral (O) and tetrahedral (T) sites are shown because they are the only ones occupied by hydrogen atoms. However, some distinction should be made between the different structures. In the fee lattice, the Tand O sites are, respectively, enclosed in regular tetrahedral and octahedral formed by metal atoms. At low or medium H concentration, the preferred interstitial sites are octahedral. In the hep lattice, the tetrahedral or octahedral sites become distorted as the ratio of lattice parameters eja deviates from the ideal value of 1.633. Tetrahedral sites are favored at low H concentration in hep metals. In fee and hep lattices, for each metal atom, there is one octahedral site and 2 tetrahedral sites available for hydrogen after considering the Westlake criterion. 

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. ... 

An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOsioC(CO)24] . Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [OsioC(CO)24] from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The Os satellite pattern associated with the hydride signal in the H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal. 

Many of these compounds (MHn) show large deviations from ideal stoichiometry (n = 1,2, 3) and can exist as multiphase systems. The lattice structure is that of a typical metal with atoms of hydrogen on the interstitial sites for this reason they are also called interstitial hydrides. This type of structure has the limiting compositiOTis MH, MH2, and MH3 the hydrogen atoms fit into octahedral or tetrahedral holes in the metal lattice, or a combination of the two types. In fee and hep structures, we find 1 octahedral site (r = 0.414) and 2 tetrahedral sites (r = 0.255), and in bcc (body-centered-cubic) structures we find 3 octahedral site (r = 0.155) and 6 tetrahedral sites (r = 0.291). 

The relative increase in intensity of the 1.6 eV peak at higher Ep is therefore evidence of a hydride phase increasing in H concentration with depth. The system is disordered, consistent with random occupation d tetrahedral sites, and a CeH (x < 2) stoichiometry. Heating to 550°C leads to partial H desorption, but at lower temperatures a different hydride phase may become established with possible octahedral sites occupation near the surface. 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

Finally, neutron-scattering experiments on several RH(D)2+x-systems revealed localized vibrational modes ascribed to H vibrations on O and on T sites, wbich are summarized in table 17. A third (higher-energy) peak is sometimes observed in superstoichiometric samples and attributed to the lowered symmetry of a tetrahedral site (T ) in the neighborhood of an x-atom. The role of SRO and LRO in the Ho-sublattice upon the evolution of the local modes (both their concentration- and temperature dependence) in a series of RH(D)2+x hydrides (R=Y, La, Ce, Tb, Dy) was demonstrated recently by... 

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. 

In scandium hydride, Weaver (1972a) observed two activation energies at compositions close to stoichiometry (see table 26.10). At the lower compositions and temperatures, he proposed a simple vacancy mechanism, t-t. However, at the higher temperatures and hydrogen contents, more octahedral sites are occupied and the mechanism becomes t-o-t. It is of interest to note that on the basis of energetic considerations, Khodosov (1971) proposed that in fluorite-type hydrides at sufficiently high temperatures, there may be more hydrogen atoms in octahedral sites than in tetrahedral sites so that the diffusion mechanism would be o-t-o. 

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