We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225 K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4 mm above 225 K. At 16 K, the BD3-ND3 unit stacks along the c axis with a tilt angle of about 16° between the N-B bond and the c axis. As the temperature is increased, the BD3-ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition.
J. Yang et al., "Structural Evolution of Ammonia Borane for Hydrogen Storage," Applied Physics Letters, American Institute of Physics (AIP), Mar 2008.
The definitive version is available at http://dx.doi.org/10.1063/1.2889535
United States. Department of Energy
Keywords and Phrases
Ammonia; Crystal Structure; Crystallisation; Hydrogen Storage; Order-Disorder Transformations; Space Groups; Crystallization
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