EXAFS and XRD investigation of crystal structure in Cr doped YMn2 deuterides

摘要

Among the number of novel hydrides synthesized recently under high hydrogen pressure from various Laves phases, very specific ones are RMn2H6 (where R=Y, Er, Ho, Dy and Gd) with space group Fm-3m. These hydrides (or deuterides) can be formed independently on the initial structure (C14 or C15) of their parent RMn2 compounds. Surprisingly, in contrast to hydrides/deuterides derived so far from the Laves phases, RMn2H6 compounds are not interstitial but are complex compounds. In order to understand better the role played by Mn in formation of the specific RMn2D6 deuterides we tested the possibility of manganese substitution by chromium, the element preceding Mn in the 3d series. Two YMn_(2-x)Cr_x alloys (where X=0.1 and 0.2) were submitted to treatment at 100 °C and high deuterium pressure. For X=0.1 the majority (88%) phase was Fm-3m space group whereas for X=0.2 the dominant (around 78%) was deuteride with expanded C15 Laves phase lattice (Fd-3m) like for YMn2 and the second phase (only 22%) was the Fm-3m. This means that substitution of Mn with chromium by amount higher than X=0.1 makes the formation of Fm-3m single phase increasingly difficult. It was also confirmed that Cr preferably locates in 4a positions of the Fm-3m structure of YMn2D6, which might be responsible for its destabilization.