High pressure transformations of NaOH

When Quantum Fluctuations Meet Structural Instabilities: The Isotope- and Pressure-Induced Phase Transition in the Quantum Paraelectric NaOH

Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope effects: NaOD undergoes a first-order transition, which is absent in NaOH. By combining ab initio electronic structure calculations with Feynman path integrals, we show that NaOH is an unusual example of a quantum paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds (HBs) into a region where they are unstable and break. By strengthening the HBs via isotope substitution or applied pressure, the system can be driven to a broken-symmetry antiferroelectric phase. In passing, we provide a simple quantitative criterion for HB breaking in layered crystals and show that nuclear quantum effects are crucial in paraelectric to ferroelectric transitions in hydrogen-bonded hydroxides.

Selective adsorption and dissociation of NO, NO2, and N2O molecules on Si-doped haeckelite boron nitride nanotube: an investigation for sensitive molecular sensors and catalysts

The current study describes the investigation of the adsorption NO, N₂O, and NO₂ on haeckelite boron nitride nanotube doped with Si (Si-doped haeck-BNNT) by means of density functional theory calculation (DFT). The obtained results confirmed the energetic stability of the optimized geometries and revealed that the adsorption of the gas molecules with the nanotube sidewall is a spontaneous process. The calculated work function of Si-doped haeck-BNNT in the presence of gas molecules is greater than that of a bare Si-doped haeck-BNNT sheet. The energy gap of the Si-doped haeck-BNNT is sensitive to the adsorption of the gas molecules, which implies possible future applications in gas sensors. For most of the adsorption configurations studied, the adsorption energies for the Si_B-doped haeck-BNNT are higher than those for Si_N-doped haeck-BNNTones. The N₂O gas molecule is totally dissociated into N₂ and O species through the adsorption process, while the other gas molecules retain their molecular forms. Thus, the Si_N-doped haeck-BNNT is a likely catalyst for dissociation of the N₂O gas molecule. Our findings divulge promising potential of the doped haeck-BNNT as a highly sensitive molecular sensor for NO and NO₂ detection and a catalyst for N₂O dissociation.

Pressure-induced localisation of the hydrogen-bond network in KOH-VI

Using a combination of ab initio crystal structure prediction and neutron diffraction techniques, we have solved the full structure of KOH-VI at 7 GPa. Rather than being orthorhombic and proton-ordered as had previously be proposed, we find that this high-pressure phase of potassium hydroxide is tetragonal (space group I4/mmm) and proton disordered. It has an unusual hydrogen bond topology, where the hydroxyl groups form isolated hydrogen-bonded square planar (OH)4 units. This structure is stable above 6.5 GPa and, despite being macroscopically proton-disordered, local ice rules enforce microscopic order of the hydrogen bonds. We suggest the use of this novel type of structure to study concerted proton tunneling in the solid state, while the topology of the hydrogen bond network could conceivably be exploited in data storage applications based solely on the manipulations of hydrogen bonds. The unusual localisation of the hydrogen bond network under applied pressure is found to be favored by a more compact packing of the constituents in a distorted cesium chloride structure.