We present the structural and dynamic nature of water ultraconfined in the quasi-two-dimensional nanopores of the highly disordered calcium-silicate-hydrate (C-S-H), the major binding phase in cement. Our approach is based on classical molecular simulations. We demonstrate that the C-S-H nanopore space is hydrophilic, particularly because of the nonbridging oxygen atoms on the disordered silicate chains which serve as hydrogen-bond acceptor sites, directionally orienting the hydrogen atoms of the interfacial water molecules toward the calcium-silicate layers. The water in this interlayer space adopts a unique multirange structure: a distorted tetrahedral coordination at short range up to 2.7 Å, a disordered structure similar to that of dense fluids and supercooled phases at intermediate range up to 4.2 Å, and persisting spatial correlations through dipole-dipole interactions up to 10 Å. A three-stage dynamics governs the mean square displacement (MSD) of water molecules, with a clear cage stage characteristic of the dynamics in supercooled liquids and glasses, consistent with its intermediate-range structure identified here. At the intermediate time scales corresponding to the β-relaxation of glassy materials, coincident with the cage stage in MSD, the non-Gaussian parameter indicates a significant heterogeneity in the translational dynamics. This dynamic heterogeneity is induced primarily because of the heterogeneity in the distribution of hydrogen bond strengths. The strongly attractive interactions of water molecules with the calcium silicate walls serve to constrain their motion. Our findings have important implications on describing the cohesion and mechanical behavior of cement from its setting to its aging.

We present a density functional theory (DFT) framework taking into account the finite temperature effects to quantitatively understand and predict charged defect equilibria in a metal oxide. Demonstration of this approach was performed on the technologically important tetragonal zirconium oxide, T-ZrO 2 . We showed that phonon free energy and electronic entropy at finite temperatures add a non-negligible contribution to the free energy of formation of the defects. Defect equilibria were conveniently casted in Kröger-Vink diagrams to facilitate realistic comparison with experiments. Consistent with experiments, our DFT-based results indicate the predominance of free electrons at low oxygen partial pressure ( P without extrinsic doping. The approach presented here can be used to determine the thermodynamic conditions that extremize certain desirable or undesirable defect to attain the optimal catalytic and electronic performance of oxides.

Hydrogen pickup and embrittlement pose a challenging safety limit for structural alloys used in a wide range of infrastructure applications, including zirconium alloys in nuclear reactors. Previous experimental observations guide the empirical design of hydrogen-resistant zirconium alloys, but the underlying mechanisms remain undecipherable. Here, we assess two critical prongs of hydrogen pickup through the ZrO 2 passive film that serves as a surface barrier of zirconium alloys; the solubility of hydrogen in it-a detrimental process-and the ease of H 2 gas evolution from its surface-a desirable process. By combining statistical thermodynamics and density-functional-theory calculations, we show that hydrogen solubility in ZrO 2 exhibits a valley shape as a function of the chemical potential of electrons, μ e . Here, μ e , which is tunable by doping, serves as a physical descriptor of hydrogen resistance based on the electronic structure of ZrO 2 . For designing zirconium alloys resistant against hydrogen pickup, we target either a dopant that thermodynamically minimizes the solubility of hydrogen in ZrO 2 at the bottom of this valley (such as Cr) or a dopant that maximizes μ e and kinetically accelerates proton reduction and H 2 evolution at the surface of ZrO 2 (such as Nb, Ta, Mo, W, or P). Maximizing μ e also promotes the predomination of a less-mobile form of hydrogen defect, which can reduce the flux of hydrogen uptake. The analysis presented here for the case of ZrO 2 passive film on Zr alloys serves as a broadly applicable and physically informed framework to uncover doping strategies to mitigate hydrogen embrittlement also in other alloys, such as austenitic steels or nickel alloys, which absorb hydrogen through their surface oxide films.

We demonstrate a thermodynamic formulation to quantify defect formation energetics in an insulator under a high electric field. As a model system, we analyzed neutral oxygen vacancies (color centers) in alkaline-earth-metal binary oxides using density functional theory, Berry phase calculations, and maximally localized Wannier functions. The work of polarization lowers the field-dependent electric Gibbs energy of formation of this defect. This is attributed mainly to the ease of polarizing the two electrons trapped in the vacant site, and secondarily to the defect induced reduction in bond stiffness and softening of phonon modes. The formulation and analysis have implications for understanding the behavior of insulating oxides in electronic, magnetic, catalytic, and electrocaloric devices under a high electric field.

In the energy-structure paradigm, we analyzed the defects that can arise in tetragonal zirconium oxide (T-ZrO2) involving the hydrogen atom or the hydrogen molecule using density functional theory. Our results indicate that the dominant hydrogen defect under reducing conditions is H(·)(0), a complex formed between the hydride ion and a doubly charged oxygen vacancy. This result is consistent with the experimental observation that under reducing conditions, the solubility of hydrogen is proportional to the degree of hypostoichiometry of T-ZrO2. Under oxidizing conditions we found three different hydrogen defects, each predominating in a specific range of the chemical potential of electrons. Starting from the valence band top toward the conduction band bottom, these defects are the interstitial proton, H(·)(i), a complex formed between two hydrogen species and a zirconium vacancy with a net effective charge of (2-), (2H)"(Zr), and finally a complex similar to the latter but with a net effective charge of (4-), (H(2))'''(Zr). In (2H)"(Zr)the two hydrogens exist in the form of hydroxyl groups, while in (H(2))" " (Zr) they exist in the form of a hydrogen molecule. In addition, we found that up to three hydrogen species can favorably accumulate in a zirconium vacancy under oxidizing conditions. The clustering of hydrogen in cation vacancies can be a precursor for the deleterious effects of hydrogen on the mechanical properties and stability of metal oxides, in analogy with hydrogen embrittlement in metals. Finally we observed a red-shift and a blue-shift for the vibrational frequencies of all the hydroxyl groups and all the hydrogen molecules, respectively, in T-ZrO2 when compared to the gas phase frequencies. This is an important characteristic for guiding future experimental efforts to detect and identify hydrogen defects in T-ZrO2. The insights presented in this work advance our predictive understanding of the degradation behavior of T-ZrO2 as a corrosion resistant passive layer, as a gate dielectric and in biomedical applications.