Effect of chloride substitution on the order–disorder transition in NaBH4 and Na11BD4

Abstract

Phase transition associated with anion disordering over two orientations in Na11BD4 (NaBH4) and its solid solutions with NaCl, Na(11BD4)1−xClx, is investigated with powder diffraction (neutron and synchrotron radiation), differential scanning calorimetry and Raman spectroscopy. Upon heating, the transition temperature extrapolated to zero rate of heating is 192.2 K for Na11BD4, ΔS = 4.41 J/mol K, hysteresis 1.7 K and the volume increase 0.43%. Thermal parameters of the transition in Na(11BD4)1−xClx follow a colligative-property model of an ideal solution, with x = 0.158(1) as the critical concentration at which the ordering interactions and the transition itself are eliminated. On approaching this limit, the tetragonal distortion of the ordered structure decreases somewhat towards the cubic average, and this is associated with a partial disorder of the tetrahedral anions seen by diffraction methods. In fact, a 3% disorder is already present in the pure solvent of the solid solution (Na11BD4) at 8 K.