The effect of the cluster structure in the phase transition during hydrogen absorption. M. Suleiman 1 , N. M. Jisrawi 2 , O. Dankert 1 , C. Bähtz 3 , R. Kirchheim 1 and A. Pundt 1 1 Institute of Material Physics, University of Göttingen, D-37073 Göttingen, Germany 2 Department of Physics, Birzeit University, POBox 14, Birzeit, Palestine 3 Darmstadt University of Technology, Institute of Materials Science, D-64287 Darmstadt, Germany The physical and chemical properties of nanometer sized materials, such as nanocrystalline materials and clusters, are of particular interest because they are often size dependent and different to bulk properties [1]. The clusters are synthesised by an electrochemical method [2], in which a simple electrolysis cell consisting of two electrodes ( a Pd anode and cathode ) is used. Applying constant current to the electrodes causes dissolution of the Pd anode, which reduces in the vicinity of the cathode into the so called “ad-atoms”. In the electrolyte, the ad-atoms aggregate into clusters, which are stabilised by a surfactant. In this work we are using in-situ XRD measurements which give structural information and enables us to monitor the sample structure directly during hydrogen loading. The aims of this work are to 1) determine the lattice parameter of the cluster as a function of the partial hydrogen pressure, 2) identify the effect of the cluster size and structure on the lattice expansion and phase transition, during hydrogen absorption. The hydriding behaviour of Pd-clusters was studied in-stiu. For these measurements a special high vacuum gas loading cell was constructed that allows stepwise controlled hydrogen loading and unloading between 10 and 10 5 Pa. Each loading cycle starts at a base pressure of about 10 -3 Pa, the hydrogen pressure was increased stepwise. The measurements are restricted with the time it takes to reach equilibrium pressure and the time needed to take one diffractogram at a selected 2 range (24°-70°) with reasonable statistics. The lattice structure of the clusters was obtained using x-ray analysis. The x-ray patterns of each sample was obtained at about 10 -3 Pa.. Comparing the experimental XRD patterns with XRD patterns obtained by MD-simulations [3] shows that the 3.8-nm cluster has icosahedral lattice structure, while the 6.0-nm cluster has cubic lattice. During hydrogen loading a shift in the diffraction patterns is observed to lower 2, indicating a lattice expansion for the cluster. Figure 1 shows diffraction patterns of the 6.0-nm and 3.8- nm cluster samples at four different pressure steps monitoring the lattice expansion. It is clearly seen that the lattice expansion of the 6.0-nm cluster is larger than that for the 3.8-nm cluster. Also, in the 6.0-nm cluster a transition is observed. Since these clusters have cubic lattice structure and also the observed transition occurs at a pressure typical for bulk palladium, this transition is interpreted as a (- ´) phase transition,figure1(a). No (- ´) phase transition was found in the 3.8-nm cluster sample, as shown in figure 1(b), increasing the hydrogen pressure leads to very small dilation of the lattice, shifting the Bragg reflection to smaller 2. Moreover , there is no peak intensity drop, which excludes a possible structural change [4].