Miedema rule

The discovery of a cluster of six face-sharing [M6N] octahedra, shown in Figure 3.1-7, in a compound that contained 14 additional Na atoms per cluster, seemed to violate the validity of Miedema s rule, because the central atom refined perfectly as K in the X-ray structure determination. 

More general is the rule of reversed stability (Miedema model) the more stable an intermetallic compound, the less stable is the corresponding hydride, and the other tvay around [36]. This model is based on the fact that hydrogen can only participate on a bond with a neighboring metal atom if the bonds between the metal atoms are at least partially broken (Figures 5.25 and 5.26). 

Relation (4.19) is usually called Miedema s rule of reversed stability and states that the heat of formation of a ternary hydride is the difference between the sum of the heat of formation of the elemental hydride and the alloy enthalpy of formation. Because atom A is hydride forming, the first term of the right-hand side is negative and has a large absolute value while the second term is small (or even positive) and... 

A scheme for correlating hydride stabilities, the so called rule of reversed stability see e.g., 18,19), states that for a series of analogous alloys, the more stable the alloy, the less stable (i.e., higher dissociation pressure) the corresponding hydride. Using Miedema s formula (20), the calculated heat of formation for LaNis is — II.2 kj/mol and for LaAls is — 42.1 kj/mol. Since LaAls is more stable (more negative AH) than LaNis, the rule of reversed stability predicts the LaNis- rAh hydrides to be less stable than LaNisHe contrary to observation. Similarly, Shinar et... 

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