Investigation of an unusual low-temperature phase transformation in RbBH4 by neutron diffraction

To investigate the previously reported low-temperature phase transition in rubidium borohydride (RbBH4) near 48.5K, we carried out neutron powder diffraction and vibrational spectroscopy measurements both above and below this temperature on an isotopically-enriched sample of Rb11BD4. Our diffraction data reflected an average cubic Fm3¯m structure with BD4− anion orientational disorder at all temperatures, with no hint of extra Bragg peaks due to long-range orientational order below the transition temperature as reported by others. These structural results and careful analysis of torsional vibrations in RbBD4 corroborate the results of prior neutron vibrational spectroscopy measurements suggesting that the low-temperature RbBH4 structure indeed possesses some orientational ordering of the BH4− anions, but of a shorter-ranged nature insensitive to powder diffraction methods. The neutron powder diffraction pattern of RbBD4 below the phase transition temperature (shown here in black) is indistinguishable from that collected above the phase transition temperature. The inset depicts the cubic structure that fits the data at both temperatures. [Display omitted] •We investigated the nature of the RbBD4 phase transition using NVS and NPD.•A change in shape of the RbBD4 torsion mode was observed across the transition.•The RbBD4 diffraction pattern across this phase transition was unchanged.•The phase transition in RbBD4 appears to produce only short-range ordering of BD4−.