Mg-N-H system

Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, T. Noritake, S. Towata, S. Orimo, Hydrogen storage properties of Li-Mg-N-H systems , J. Alloys Compd. 404-406 (2005) 396-398. 

J. Yang, A. Sudik, C. Wolverton, Activation of hydrogen storage materials in the Li-Mg-N-H system Effect on storage properties , J. Alloys Compd. 430 (2007) 334-338. 

T. Ichikawa, K. Tokoyoda, H. Leng, H. Fujii, Hydrogen absorption properties of Li-Mg-N-H system , J. Alloys Compd. 400 (2005) 245-248. 

The PC isothermal properties on the above Li-Mg-N-H system have been reported by a few groups for desorption [9, 20-22], absorption [23] and both absorption and desorption reactions [24—26], as shown in Figure 6.2 (desorption) and Figure 6.3 (absorption). However, thermodynamically accurate results have not yet been reported, because the kinetic properties are significantly worse, even around... 

Recently, Wu [37] reported the crystal structure of the ternary imide Li2Ca(NH)2 which was determined using neutron powder diffraction data on a deuterated sample. In his paper, the reaction, Li2Ca(NH)2 + H2 LiNH2 + LiH + CaNH, was indicated. However, with respect to the reaction mechanism of the Li-Mg-N-H system [38], the reaction process in this report was thought to be stopped halfway. 

Using this NH3-mediated model, the reaction between LiH and Mg(NH2)2 can also be explained, with the Li+ cation being supplied from LiH after the reaction between LiH and NH3. Here, we discuss the transformation of the phases during the dehydrogenating reaction processes in the Li-Mg-N-H systems. Rijssenbeek et al. [81], for the case of Li/Mg = 6/3 , determined three new imide crystal structures by high-resolution X-ray and neutron diffraction, identifying a new family of imides with formula LLt 2xMgx(NH)2 (up to 6 mass% of H2, at 220°C). 

For the case of Li/Mg = 8/3 , Nakamura et al. determined the phase of Li2.6MgN2Di 4 as a dehydrogenated state (in vacuum conditions, at 200 °C) by using synchrotron radiation (SR)-X-ray and neutron diffraction [109]. For Li/Mg = 6/3, 8/3, 12/3 systems, Aoki et al. performed PC-isotherm measurements only for the dehydrogenation at 250 °C and observed the complex imide Li2Mg(NH)2 as a dehydrogented state by SR-XRD [110]. Considering all these experimental results, the reaction steps of the Li-Mg-N-H system can be stated as below,... 

However, the fact that experimentally NH3 evolution continues to be observed on decomposition of the amide with hydride present suggests that unlike the LiNH2 -I- LiH desorption step, the kinetics of the reaction between MgH2 and NH3 are slow. This was one further motivation for considering the Li-Mg-N-H system (and the reaction between LiH and Mg(NH2)2 as described in Section 16.4.1). 

Two independent studies rapidly followed considering the alternative pathway to mixed imides in the Li-Mg-N-H system, namely combining magnesium amide with lithium hydride [84, 85]. The two studies differed in the ratios of starting materials considered. The former took the 1 2 ratio of amide hydride and by analogy to Eq. (16.22) sought to liberate hydrogen as a by-product of mixed imide formation, viz. Eq. (16.23) (hereafter referred to as the 1 2 reaction based on the Mg(NH2)2 LiH ratio) ... 

As example, the Li-Mg-N-H system was studied by differential scanning calorimetry by Gross s group [53]. The major issue for metal-N-H storage systems is the formation of NH3, that takes place in parallel with H2 release, and that acts as a... 

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