Order-Disorder Phase Transition in NaBD4

Abstract

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.