Form Ternary Hydrides

Formation of Reversible Metal Hydrides 1.12.8. to Form Ternary Hydrides 1.12.8.1. from Intermetallics... 

The rare earth-transition metal intermetallics are to be distinguished from alloys that normally form between two or more metallic elements of similar size and crystal structure and can be regarded as solid solutions. The rare earths are generally larger than most of the metallic elements so they tend to form compounds rather than alloys in combination. Many of these species form ternary hydrides ... 

This chapter commences with a review of a limited number of ternary hydride systems that have two common features. First, at least one of the two metal constituents is an alkali or alkaline earth element which independently forms a binary hydride with a metal hydrogen bond that is characterized as saline or ionic. The second metal, for the most part, is near the end of the d-electron series and with the exception of palladium, is not known to form binary hydrides that are stable at room temperature. This review stems from our own more specific interest in preparing and characterizing ternary hydrides where one of the metals is europium or ytterbium and the other is a rarer platinum metal. The similarity between the crystal chemistry of these di-valent rare earths and Ca2+ and Sr2+ is well known so that in our systems, europium and ytterbium in their di-valent oxidation states are viewed as pseudoalkaline earth elements. 

The ternary hydride was formed also at a reduced pressure and temperature of 200 psi and 200°C, respectively, after several cycles of hydriding and decomposition. Mg2NiH4, a rust-colored solid with a nonmetallic luster, reacted sluggishly with water but more vigorously with nitric acid solution, giving off hydrogen. Mg2NiH4 appeared to be unreactive to air upon short exposure. 

Since AG is always negative, P is always higher than P. Therefore, the formation of a solid solution with a non-forming hydride has the effect of destabilizing the AH, hydride. The above relation holds when the alloy decomposes upon hydrogenation. However, most of the time, the AB alloy will not decompose upon hydrogenation but instead react as a ternary hydride. 

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... 

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