Density Functional Theory of MH–MOH Solid Solubility (M = Alkali) and Experiments in NaH–NaOH

We present first-principles solubility calculations of H^–/[OH]⁻ mixing in binary alkali metal hydrides MH and their corresponding hydroxides MOH, for M ={Li, Na, K, Rb, Cs}. Solid solubility in the MH–MOH system may play an important role in solid-phase reactions involving the MH system, including for example many aluminum-based complex hydrides of the alkali metals, such as NaAlH₄ and LiAlH₄. Our results indicate that the available cell volume for H^– and OH^– groups correlates strongly with mixing, and MOH is soluble in MH for M ={Na, K, and Rb} where available volumes for H^– and OH^– anions differ by less than about 15%, very similar to a Hume–Rothery type rule for intermetallics. The predicted mixing temperatures for the K and Rb systems are lower than for the Na system, in part because of the similarity in MH and MOH primitive cell volumes. Critical temperature diagrams for the formation of solid solution MH–MOH mixtures as a function of MOH concentration are calculated using a free energy minimization in the grand canonical ensemble. Differential scanning calorimetry and in situ X-ray diffraction measurements of the NaH_1–<i>x</i>(OH)_<i>x</i> system are presented for a range of compositions (0.3 ≤ <i>x</i> ≤ 1.0). As the temperature is raised, the polymorphic phase transitions present in NaOH occur concomitantly with H^–/OH^– mixing, eventually forming a single-phase cubic structure; the behavior is fully reversible on cooling. Finally, the formation of solid solution MH/MOH reduces the decomposition temperature of NaH_1–<i>x</i>(OH)_<i>x</i> to lower temperatures than pure NaH while increasing the stability in the KH and RbH systems