Two-dimensional (2D) water, confined by atomically flat layered materials, may transit into various crystalline phases even at room temperature. However, to gain full control over the crystalline ...state, we should not only confine water in the out-of-plane direction but also restrict its in-plane motion, forming 2D water clusters or ribbons. One way to do this is by using an electric field, in particular the intrinsic electric field of an adjacent polar material. We have found that the crystalline phases of 2D water clusters placed between two hexagonal boron nitride (h-BN) nanoribbons are crucially determined by the nanoribbons’ edges, the resulting polarity of the nanoribbons, and their interlayer distance. We make use of the density functional theory with further assistance of molecular dynamics simulations to establish the comprehensive phase diagrams, demonstrating transitions between liquid and solid phases and between the states of different crystalline orders. We also show that the crystalline orders are maintained when water flows between h-BN channels under external pressure. Our results open a promising pathway toward the control of the water structure and its flow by the use of the microscopic electric field of polar materials.
Alkali metal $\beta$/$\beta^{\prime\prime}$ aluminas are among the fastest
ionic conductors, yet little is understood about the role of defects in the ion
transport mechanism. Here, we use density ...functional theory (DFT) to
investigate the crystal structures of $\beta$ and $\beta^{\prime\prime}$
phases, and vacancy and interstitial defects in these materials. We find that
charge transport is likely to be dominated by alkali metal interstitials in
$\beta$-aluminas and by vacancies in $\beta^{\prime\prime}$ aluminas. Lower
bounds for the activation energy for diffusion are found by determining the
minimum energy paths for defect migration. The resulting migration barriers are
lower than the experimental activation energies for conduction in Na $\beta$
and $\beta^{\prime\prime}$ aluminas, suggesting a latent potential for
optimization. The lowest activation energy of about 20 meV is predicted for
correlated vacancy migration in K $\beta^{\prime\prime}$ alumina.
2D water, confined by atomically flat layered materials, may transit into various crystalline phases even at room temperature. However, to gain full control over the crystalline state, we should not ...only confine water in the out of plane direction but also restrict its in plane motion, forming 2D water clusters or ribbons. One way to do this is by using an electric field, in particular the intrinsic electric field of an adjacent polar material. We have found that the crystalline phases of 2D water clusters placed between two hexagonal boron nitride hBN nanoribbons are crucially determined by the nanoribbons edges, the resulting polarity of the nanoribbons, and their interlayer distance. We make use of density functional theory with further assistance of molecular dynamics simulations to establish the comprehensive phase diagrams demonstrating transitions between liquid and solid phases and between the states of different crystalline orders. We also show that the crystalline orders are maintained when water flows between hBN channels under external pressure. Our results open a promising pathway towards the control of water structure and its flow by the use of the microscopic electric field of polar materials.
Alkali metal \(\beta\)/\(\beta^{\prime\prime}\) aluminas are among the fastest ionic conductors, yet little is understood about the role of defects in the ion transport mechanism. Here, we use ...density functional theory (DFT) to investigate the crystal structures of \(\beta\) and \(\beta^{\prime\prime}\) phases, and vacancy and interstitial defects in these materials. We find that charge transport is likely to be dominated by alkali metal interstitials in \(\beta\)-aluminas and by vacancies in \(\beta^{\prime\prime}\) aluminas. Lower bounds for the activation energy for diffusion are found by determining the minimum energy paths for defect migration. The resulting migration barriers are lower than the experimental activation energies for conduction in Na \(\beta\) and \(\beta^{\prime\prime}\) aluminas, suggesting a latent potential for optimization. The lowest activation energy of about 20 meV is predicted for correlated vacancy migration in K \(\beta^{\prime\prime}\) alumina.
