Hexagonal hydride

Tka] Tkacz, M., Burtovyy, R., Isotope Effect in a Cu-H(D) System with Hexagonal Hydride Phase , J. Alloys Compd, 404/406, 368-371 (2005) (Crys. Structure, Thermodyn., Experimental, 14)... 

Hoff plots of the logarithm of the second plateau pressure vs reciprocal temperature are listed in table 26.6 for a few hexagonal hydrides where data are available. The values for samarium and gadolinium trihydrides were estimated from rather sparse data. Also listed in the table are enthalpy values for the transition MH2- iMH3. [including the reported (section 2.1) low temperature hexagonal neodymium hydride phase] as obtained by Mintz et al. (1976) from DTA curves. The agreements between the two sets of AH values are excellent for Ho and Er, and also are satisfactory for Sm and Gd in view of the uncertainty in the values obtained from van t Hoff plots of dissociation pressure data. 

Carbon also forms numerous cyclic hydrides of which benzene, is a well-known example. This has a planar, regular hexagonal... 

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

Reversibility of hydrogen charging was very good at room temperature. This is because the hexagonal lattice of the metal host, like this of LaNi, does not undergo major transformation as hydrogen is inserted interstitially. This was the first AB -type interstitial hydride in which hydrogen is stored between metal atoms. 

C.E. Lundin, F.E. Lynch, Modification of hydriding properties of AB5 alloy type hexagonal alloys through manganese substitution, Proceedings of the Miami International Conference Alternate Energy Sources, United States, 5-7 December 1977. University of Miami, (1978)... 

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

AB5 alloys are intermetallic compounds with hexagonal crystalline lattice. Constituting compounds are inter alia rare earth metals. These compounds are capable to form hydrides as the dissolve hydrogen. 

Both systems retain their rhombohedral or hexagonal symmetry upon hydriding. The RC05-H systems, on the other hand, were found to be degraded... 

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. 

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