Effects of NaOH in Solid NaH: Solution/Segregation Phase Transition and Diffusion Acceleration

Abstract

The presence of approximately 10 mol % NaOH mixed with NaH powder is shown to result in much more rapid hydrogen motion in the NaH above 150 °C, as indicated by the onset of hydrogen NMR line narrowing at this temperature. A similar result appears for air-exposed NaH due to the formation of hydroxide from atmospheric H2O. The NMR line narrowing is too rapid, as a function of temperature, to be described by thermal activation; rather, it is suggestive of a phase transition. Indeed, differential scanning calorimetry finds, after an initial thermal cycle, a reversible thermal anomaly indicating a phase transition near 150 °C. Powder X-ray diffraction with an excess of NaOH displays a remarkable lattice expansion of the NaH in the temperature range of 100–240 °C where the (200), (220), and (311) reflections indicate a volume lattice expansion of up to 11%; the expansion is reversible upon cycling. The data thus point to a reversible phase transition in which NaOH enters the NaH structure above 150 °C and exits the NaH below that temperature. First-principles calculations using OH-substituted NaH supercells indicate significant solubility in the NaH lattice and find a transition temperature and enthalpy change that are in approximate agreement with differential scanning calorimetry (DSC) measurements, confirming the presence of a phase transition.