The methods of structural characterization of metal hydrides are reviewed (see also [1]). The existing difficulties and problems are outlined and possible solutions presented. It is shown that powder diffraction is essential component of metal hydrides research where the structural characterization is currently undertaken by X-ray and neutron diffraction. In the case of light metal hydrides like borohydrides of light alkaline metals/earths X-ray diffraction alone can provide the structural parameters with sufficient accuracy. A crystallographer analyzing metal hydrides has to face numerous crystallographic challenges which include complex structures, superstructures, pseudo-symmetries, twinning, chemical and positional disorder, structural solution from low quality data (powder patterns), joint use of several data sets, resonant scattering and fast in-situ data collection.

Direct space approach is currently the powder diffraction method mostly used in metal hydride research for its simplicity of use, ability to work with powder patterns of low quality (broad peaks), easy way to treat the occupation disorder on hydrogen sites and active use of simple geometrical constraints. Crystal structures containing as many as 55 independent atoms (including hydrogen) have been fully characterized using powder diffraction. This is of great importance, because rapid collection of powder data thanks to modern synchrotron and neutron time-of-flight sources opens the possibility for fast in-situ studies, mapping of phase transitions induced by the temperature, pressure, hydrogen content, and chemical reactions. The progress in structural characterization of metal hydrides goes hand in hand with the progress in the powder diffraction methodology.

[1] Černý R.; Z. Kristallogr. (2008), in preparation

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