<p>The synthesis, structural characterisation and the properties of the Zintl hydrides AeE<sub>2</sub>H<sub>2</sub> and AeAlSiH (Ae = Ba, Ca, Sr; E = Al, Ga, In, Si, Zn) are reported. The first hydride in this class of compounds is SrAl<sub>2</sub>H<sub>2</sub> which was discovered under an experiment by Gingl, who hydrogenated SrAl<sub>2</sub> at various temperatures. (Gingl et al, Journal of Alloys and Compounds 306 (2000) 127-132). The intention was to form alanates, e.g. AlH<sub>4</sub><sup>-</sup>, by terminating the three dimensional four connected aluminium network in SrAl<sub>2</sub>. The new hydride, SrAl<sub>2</sub>H<sub>2</sub>, has a partially conserved aluminium network. The three dimensional anionic network in SrAl<sub>2</sub> is reduced to two dimensions in the hydride, with aluminium bonded to both aluminium and hydrogen. This type of bonding configuration has not been observed before.</p><p>The hydrogenation of SrAl<sub>2</sub> is straight forward, 190 <sup>o</sup>C and 50 bar, compared to the difficult synthesis of alanates and alane, AlH<sub>3</sub>. The latter synthesises uses aluminium in its zero oxidation state in contrast to the synthesis of SrAl<sub>2</sub>H<sub>2</sub> from SrAl<sub>2</sub>. (In the SrAl<sub>2</sub>-precursor aluminium is reduced by the electropositive metal to -I.) Thus, the discovery shows a different route to alanates by using precursors with aluminium in a reduced state. If SrAl<sub>2</sub>H<sub>2 </sub>is further hydrogenated at 250 <sup>o</sup>C the two dimensional network breaks and Sr<sub>2</sub>AlH<sub>7 </sub>forms.</p><p>We wanted to investigate if SrAl<sub>2</sub>H<sub>2</sub> was a singularity or if other similar compounds exist. We wanted to study how hydrogenation of precursors similar to the aluminide result in 1) new routes to compounds with high hydrogen content, as alanates, 2) to investigate how the E-H bond is affected as function of the network composition among different ternary hydrides, in particular BaAl<sub>x</sub>Si<sub>2-x</sub>H<sub>x</sub>, and choice of active metal.</p><p>BaGa<sub>2</sub>H<sub>2</sub> and SrGa<sub>2</sub>H<sub>2</sub>, two hydrides isostructural with SrAl<sub>2</sub>H<sub>2</sub>, were synthesized from its precursors BaGa<sub>2</sub> and SrGa<sub>2</sub>. In addition three ternary hydrides BaAlSiH, CaAlSiH and SrAlSiH were manufactured from their related AeAlSi precursors.</p><p>All powders were characterized by neutron and x-ray diffraction methods.</p><p>An increased stability towards water/moisture compared to ordinary saline hydrides was noticed, especially for the ternary hydrides. Heat stability was measured with DSC (differential scanning calorimetry). The hydrides BaGa<sub>2</sub>H<sub>2</sub> and SrGa<sub>2</sub>H<sub>2</sub> decompose around 300 <sup>o</sup>C at 1 atm. This is similar to isostructural SrAl<sub>2</sub>H<sub>2</sub>. The ternary hydrides BaAlSiH and SrAlSiH decompose at 600 <sup>o</sup>C, at 1 atm, which is the highest noticed temperature for compounds with Al-H bonds. Inelastic neutron scattering experiments showed that these hydrides Al-H and Sr-H bonds are really weak, even weaker then the Al-H interactions in alanates and alanes. These hydrides are probably stabilized be their lattices. The electric properties among the ternary hydrides were measured with IR-spectroscopy (diffuse reflectance). The ternary hydrides, AeAlSiH, are indirect semi conductors. BaGa<sub>2</sub>H<sub>2</sub> and SrGa<sub>2</sub>H<sub>2 </sub>are conductors. The ternary hydrides, AeAl<sub>x</sub>Si<sub>2-x</sub>H<sub>x</sub>, may have adjustable band gaps, which we were not able to determine.</p><p>This work is leading into a new research area within the field of metal hydrides.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:su-7388 |
| Date | January 2008 |
| Creators | Björling, Thomas |
| Publisher | Stockholm University, Department of Physical, Inorganic and Structural Chemistry, Stockholm : Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
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