Basic concepts of hydrogenation

The sensitivity of the interactions in the f-electron systems to modification of the crystal structure makes the experimental techniques involving alternation of the atomic arrangement especially important. Various studies under hydrostatic or uniaxial compression are wellknown examples of such methods. From this point of view hydrogenation can be treated as a complementary technique that provides "negative" pressure. In the narrowest sense, the term hydride is used just in cases when metal lattice is changed upon hydrogen absorption. However we will use this term in wider sense. We will define hydrides as compounds for which the hydrogen absorption leads to the modifications of the crystal structure, such as pure lattice expansion or the formation of a new structure. Hydrogen absorption can be used to modify magnetic properties of the intermetallics. After interstitial hydrogen doping the crystal lattice can be several percent larger than before hydrogenation