Crystal Structure of the Zirconium Hydride Polymorphs determined by Neutron and Synchrotron X-ray Powder Diffraction A. Steuwer 1, J. Blomqvist 2, T. Maimaitiyili 2, O. Zanellato 3, C. Curfs 4, H.E. du Plessis 5, C. Bjerken 2 1 ESS AB, Stora Algatan 4, Lund, Sweden and NMMU, Port Elizabeth, 6031 South Africa, 2 Malmö University, Malmö, Sweden 3 Mateis, INSA, Lyon, France 4 ESRF, rue J Horowitz, Grenoble, France 5 SASOL, Sasolburg, South Africa Corresponding author: ESS AB, Stora Algatan, Lund, Sweden, prepared for ECNS 2011, Abstract: Zirconium alloys, widely used in the nuclear industry, Fig 1, have a strong affinity for hydrogen that leads to hydrogen pick up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are presumed to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature, exact structure and stability of the γ -ZrH phase, which is closely related to the  -ZrH phase through ordering of the hydrogen on tetrahedral sites on alternating 110 planes. Aim: Develop a recipe for in-situ preparation of γ -ZrH/D (conflicting reports exist) with purpose of high resolution neutron powder diffraction studies on deuterated Zr powder samples in order to re-determine and verify the reported structures that essentially date back to the 1960s [1], and compare those with high resolution synchrotron X-ray powder diffraction, and supporting results from other techniques. Fig. 1: Typical fuel assembly, uranium dioxide, pellets and fuel assembly (with zircalloy tubing/cladding) Background: The three crystal structures of ZrH 2-x are a) for x=0 the tetragonal ε -phase (not shown), b) x=0.34 the cubic, disordered δ -phase, and x=1 the ordered γ -phase, with hydrogen atoms in tetrahedral positions on alternating 110 planes as stated in [1,2]. The illustration on the right shows the δ and γ phase crystal structures visualised with full filling of the tetrahedral sites. The controversy of the ordered phases surrounds its RT stability and structure [3]. References: [1] S. S. Sidhu, N. S. Satyamurty, F. P. Campos, and D. D. Zauberis, Neutron and X-ray Studies on Non-Stoichiometric Metal Hydrides, in Advances in Chemistry, (American Chemical Society, Washington, DC, 1963), Vol. 39, p. 87. [2] E. Zuzek, J. P. Abriata, A. San-Martin, and F. D. Manchester, Bulletin of Alloy Phase Diagrams (American Society for Metals, Metals Park, Ohio, 1990), Vol. 11, No. 4. [3] Steuwer, A; Santisteban, JR; Preuss, M; Peel, MJ; Buslaps, T; Harada, M. Acta Mat. Vol 57, Iss 1, p , 2009  -ZrH  -ZrH Experimental procedure: a) Deuteration: The powder sample which was used in experiment was prepared in strict order following literature: First, the sample baked three days at 300 C (baking at P=5x10 10 mbar). Then, the sample exposed to a volume with a given amount of deuterium, when the volume had reached a low pressure (deuterium absorbed in the sample), the volume was refilled. This cycle was repeated until the desired amount of absorbed deuterium was achieved. During the entire sample preparation process the powder was handled in a glove box filled with Argon to avoid any sort of contamination or oxidation. The final Zr:D ratio (atomic) was almost 1:1, the ideal stochiometric ratio of the γ-phase. After preparation, the H/D rich ε-phase (x=0), as well as the more stable δ-phase and remaining α-phase (pure Zr) are present in the as prepared powder. b) in-situ experiments: SPODI: the wave length of the neutron kept as constant ( λ = Å). The powder was tested both at room temperature (as prepared) and elevated temperature to check the stability of the phases, in particular any transition around 180 C and/or 286 C, which has been suggested in the literature. However, very little γ phase in the annealed sample was detected, Fig.3 top. ANSTO: a separate in-situ preparation route (different thermal cycle) was tried subsequently at the ECHIDNA (HRPD) diffractometer at ANSTO. A new thermal cycle was found to successfully produce more traceable amounts of γ -ZrD, see Fig 3, middle. Additionally, the samples, as prepared and annealed (FRM2) have been measured on ID31 ( λ = Å) at the ESRF for comparison, Fig.3 bottom. The second set of samples (post- ECHIDNA) are considered for more ID31 measurements. Acknowledgements: FRM2 and ANSTO are gratefully acknowledged for the provision of beam time. Figure 2: c) Conclusions: Having finally identified a preparation route for γ - ZrD, we are now preparing to confirm the exact structure with simultaneous refinement of synchrotron X-ray and neutron diffraction data, as the obtained data is not in agreement with published structures, Fig 3. Fig.3: Preliminary Rietveld fits of the FRM2 (top) and ANSTO data (middle), and ID31 data (bottom) of the ZrD powders.