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Niobium dioxide can exhibit negative differential resistance (NDR) in metal-insulator-metal (MIM) devices, which has recently attracted significant interest for its potential applications as a highly non-linear selector element in emerging nonvolatile memory (NVM) and as a locally-active element in neuromorphic circuits. In order to further understand the processing of this material system, we studied the effect of thermal annealing on a 15 nm thick NbO2 thin film sandwiched inside a nanoscale MIM device and compared it with 180 nm thick blanket NbOx (x = 2 and 2.5) films deposited on a silicon dioxide surface as references. A systematic transmission electron microscope (TEM) study revealed a similar structural transition from amorphous to a distorted rutile structure in both cases, with a transition temperature of 700 °C for the NbO2 inside the MIM device and a slightly higher transition temperature of 750 °C for the reference NbO2 film. Quantitative composition analysis from electron energy loss spectroscopy (EELS) showed the stoichiometry of the nominal 15 nm NbO2 layer in the as-fabricated MIM device deviated from the target 1:2 ratio because of an interaction with the electrode materials, which was more prominent at elevated annealing temperature.

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 NaAlH 4 and LiAlH 4 . 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−x (OH) x system are presented for a range of compositions (0.3 ≤ x ≤ 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−x (OH) x to lower temperatures than pure NaH while increasing the stability in the KH and RbH systems.

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 H 2 O. 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.