Structural transitions related to order-disorder and thermal desorption of D atoms in TbFe$_{2}$D$_{4.2}$

Abstract

TbFe$_{2}$D$_{4.2}$ deuteride crystallizes in a monoclinic structure ($Pc$ space group) with deuterium inserted into 13 [Tb$_{2}$Fe$_{2}$] and 5 [TbFe$_{3}$] tetrahedral interstitial sites. Its structural evolution versus temperature has been investigated by combining in-situ X-ray and neutron diffraction (XRD and NPD) with differential scanning calorimetry (DSC) experiments.Upon heating, the deuteride undergoes a reversible order-disorder transition from an ordered monoclinic structure to a disordered cubic structure between 320 and 380 K. Then, a multipeak thermal desorption occurs between 400 and 550 K, which can be explained by the transitions between different cubic deuterides separated by two-phase ranges. After controlled partial D desorption of TbFe$_{2}$D$_{4.2}$,the XRD patterns of several TbFe$_{2}$D$_{x}$ deuterides were measured ex-situ using synchrotron radiation at room temperature, revealing the formation of different phases with cubic or monoclinic structures separated by two phase ranges.A tetragonal superstructure was observed for a phase with $x$ = 2. This work can explain previous results of the literature indicating the existence of cubic and or rhombohedral hydrides depending on the hydrogenation conditions and the H content. The monoclinic structures reported here correspond to a slight distortion of the previous rhombohedral structures described by other authors.