Symmetry analysis of hydrogen related structural transformations in Laves phase intermetallic compounds

Absorption of hydrogen in the intermetallic compounds of Rare Earths and Yttrium with 3-d metals causes structure distortions and transformations depending on hydrogen content and temperature. One of the most intriguing problems for the unsaturated hydrides is a tendency for ordering of hydrogen atoms. To find the explanation of the observed transformations and hydrogen ordering the symmetry analysis method in the frame of theory of space groups and their representations is applied for Laves phase hydrides RMn2 (R = Rare Earth and Yttrium). There are analysed all possible transformations of the parent structure with symmetry space group Fd-3m leading to the structures of lower symmetry, for k = (0,0,0) and selected transitions to the translation subgroup corresponding to k = (0,0,1). In such structures of lower symmetry the hydrogen interstitial positions split into subgroups with differentiated local symmetries and abundances. The hydrogen interstitial positions of A2B2 type and AB3 type are taken in to account. If one of such super-lattices is occupied by hydrogen and the others are empty, we receive effect of hydrogen ordering. All transformations allowed by symmetry rules are presented and discussed with the aim to analyse and understand the conditions determining phase transitions and hydrogen ordering.