Order-Disorder Phase Transition in NaBD4
By means of neutron diffraction the low-temperature crystal structure of NaBD4 has been determined. At 10 K the lattice parameters are a = 4.332(1) Å and c = 5.869(1) Å. Deuterium is found in a tetrahedral arrangement [sites (8g)] around B. The symmetry corresponds to space group P42/nmc. For room temperature the structure model for NaBD4 of Davis and Kennard with disordered deuterium distributed over two sites has been revised to space group Fm-3 m. Thus the 190 K phase transition known from specific heat measurements is of order-disorder type, caused by reorientations of BD4 tetrahedra.
Yvonne Andersson, Eric J. Mittemeijer and Udo Welzel
P. Fischer and A. Züttel, "Order-Disorder Phase Transition in NaBD4", Materials Science Forum, Vols. 443-444, pp. 287-290, 2004