Hydrides hydrogen sublattice

Band structure calculations of metal hydrides have provided understanding of the bonding characteristics of these materials and clarified the mechanisms involved in various physical properties such as superconductivity. Since the electronic structure of stoichiometric hydrides has been discussed extensively in the literature, in this article we will focus on the methodology and results of disordered hydrides. This disorder can occur on the metal site of the hydride by considering random substitutions of the host metal or on the hydrogen sublattice where vacancies appear. 

The first group consists of hydrides of La, Ce, Pr and Nd. The dihydrides are cubic, with H atoms occupying the T sites of a fee metal sublattice (fluorite structure). The trihydrides of the first group are also cubic and form solid solutions with the dihydrides. The third H per formula unit occupies an site in the fee metal sublattice. Hydrogen atoms occupy 0, sites before all the T sites are filled, so that at the stoichiometric composition, MHj, there is an equal concentration of vacant T sites and occupied Oj sites. ... 

In this article we discuss TB-CPA calculations for TiFeH and for di-hydrides such as TiH. For these calculations we follow closely Faulkner s formalism,[5] and make the assumption that the metal sublattice is perfectly periodic, while the non-metal sublattice is randomly occupied by hydrogen or vacancies. 

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. 

As more hydrogen is added to the hydrogen-deficient dihydride, the hydrogen atoms commence to randomly occupy the octahedral positions in the metal sublattice (see section 2.2) and the resistivity increases as illustrated for several hydrides in fig. 26.9. It is of interest to note that the resistivities of several different hydrides all fall on approximately the same curve and that the variation in temperature appears to have negligible effect. The latter observation indicates that although the resistivity rises with hydrogen content, the samples remain... 

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