Maria Alexandra Gomez

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Verified

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• PhD
• Professor at Mount Holyoke College

The three most commonly used centrality measures in network theory have been adapted to consider ion conduction time rather than the number of steps. Flow-IN centrality highlights sites with the largest flow of ions from the nearest neighbor sites. Return-flow centrality highlights sites with a fast rate of first returns for the conducting ion. Flo...

Yttrium-doped barium zirconate is one of the fastest solid-state proton conductors. While previous studies suggest that proton–tuples move as pairs in yttrium-doped barium zirconate, a systematic catalog of possible close proton–tuple moves is missing. Such a catalog is essential to simulating dual proton conduction effects. Density functional theo...

Understanding the relationship between the acceptor dopant size and proton conductivity in barium zirconate, BaZrO3, is important for maximizing efficiency in this promising fuel cell material. While proton conduction pathways with larger YZr′ and smaller AlZr′ defects have been explored, proton pathways with ScZr′, a defect of comparable size to t...

Acceptor-doped barium zirconate is one of the most promising proton conducting materials for stationary hydrogen fuel cells. Dopant-defect proton traps shape the proton conduction landscape. Inspired by findings that oxygen vacancies may decrease trapping near some dopant defects ( Chem. Mater. 2018, 30, 4919−4925), the effect of oxygen vacancies o...

Proton conduction is an important property for fuel cell electrolytes. The search for molecular details on proton transport is an ongoing quest. Here, we show that in hydrated yttrium doped barium zirconate using X-ray and neutron diffraction that protons tend to localize near the dopant yttrium as a conjugated superstructure. The proton jump time...

Y-doped BaZrO3 is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in such a material. Geometry optimisations of 100 structures with a 12.5% dopant concentration allowed us to identify...

Inspired by significant local distortions found near vacancies in a neutron pair distribution function analysis study (G. King et al., Inorg. Chem. 2012, 51, 13060) of Sr2MSbO5.5 (M = Ca and Sr), this computational study finds minimum‐energy structures with these and related distortions using density functional theory (DFT) with the Perdew‐Burke‐Er...

Y-doped BaZrO$_3$ is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in such a material. Geometry optimisations of 100 structures with a 12.5% dopant concentration allowed us to ident...

Acceptor doped perovskites show promise as fuel cell proton conducting ceramic membranes and continue to be rapidly developed. Proton conduction in barium zirconate with 6.25% ordered yttrium doping at the zirconium site is explored. The dopant increases octahedral distortions in the lattice similar to our earlier studied 12.5% dopant system though...

Kinetic Monte Carlo (KMC) and graph searches show that proton conduction limiting barriers and trajectories in BaZr0.875 Y0.125 O3 are affected by the presence of other protons. At 1000 K, KMC limiting conduction barriers increase from 0.39 eV to 0.45 eV as the proton number is increased. The proton-proton radial distribution begins to rise at 2 Å...

A centrality measure based on the time of first returns rather than the number of steps is developed and applied to finding proton traps and access points to proton highways in the doped perovskite oxides: AZr0.875D0.125O3, where A is Ba or Sr and the dopant D is Y or Al. The high centrality region near the dopant is wider in the SrZrO3 systems tha...

A combination of electronic structure methods, graph theory, and kinetic Monte Carlo (KMC) shows that doping BaZrO 3 with aluminum instead of yttrium at the Zr site changes the long range proton conduction pathways by altering local geometry. When the zirconium ion is replaced by the smaller aluminum ion, there is a small contraction which decrease...

Dynamic programming is used to find periodic proton conduction paths in pseudo-cubic and orthorhombic perovskites. Periodic paths through a large periodic simulation box are compared to periodic paths through a smaller simulation box and pathways stitched from small simulation box periodic and aperiodic paths. For both size systems in the 900–1300...

Doping orthorhombic SrZrO(3) at 12.5% of the Zr sites with Al(3+) leads to a local squaring of the lattice, while doping with larger Y(3+) increases local octahedral distortions. Proton activation energy barriers and transition state theory prefactors are calculated. The wide range of intra-, inter-, and rotational barriers suggest that a comprehen...

When BaZrO(3) is doped with Y in 12.5% of Zr sites, density functional theory with the PBE functional predicts octahedral distortions within a cubic phase yielding a greater variety of proton binding sites than undoped BaZrO(3). Proton binding sites, transition states, and normal modes are found and used to calculate transition state theory rate co...

Quantum Monte Carlo has found a new application far afield from the solution of the Schrodinger equation for many-body problems. In this paper the authors report a method for solving multidimensional optimization problems that is superior, in many cases, to other methods such as conjugate gradient methods, the simplex method, direction-set methods,...

In this study, the effect of adsorption of a single hydrogen atom upon the properties of Ni and Pd clusters of up to ten atoms was investigated. The required potential energies for the interactions of the atoms within the clusters were determined with the embeddedatom method, a semi-empirical method with parameters fit to data for bulk systems, giv...

In the present paper, the authors focus on proton conduction pathways in a cubic perovskite KTaO(3) and an orthorhombic perovskite SrZrO(3). Density functional theory with a generalized gradient approximation is used to find proton binding sites. The nudged elastic band method is used to find transition states between minima. With this potential en...

Plateaus in water adsorption isotherms on hydroxylated BeO surfaces suggest significant differences between the hydroxylated (1 0 0) and (0 0 1) surface structures and reactivities. Density functional theory structures and energies clarify these differences. Using relaxed surface energies, a Wulff construction yields a prism crystal shape exposing...

A mixed quantum/classical density matrix approximation is derived. The density matrix makes use of quantum subsystem vibrational wave functions. The diagonal of the density matrix can be used as an equilibrium distribution in Monte Carlo simulations. The approximate distribution compares well with the path integral distribution for a model system....

Proton conducting oxide ceramics have shown potential for use in fuel cell technologies. Understanding the energy pathways for proton conduction could help us design more efficient fuel cell materials. This paper describes how octahedral tilting affects the relative energies of proton binding sites, transition states, and conduction pathways in cub...

The hydroxide anion plays an essential role in many chemical and biochemical reactions. But a molecular-scale description of its hydration state, and hence also its transport, in water is currently controversial. The statistical mechanical quasichemical theory of solutions suggests that HO.[H2O]3(-) is the predominant species in the aqueous phase u...

The structures for the cubic forms of CaTiO3, CaZrO3, BaTiO3 and BaZrO3 are found. Even when cubic symmetry is enforced, the calcium perovskites exhibit distortions characteristic of orthorhombic phases while the barium perovskites remain undistorted. These distortions in the calcium perovskites give rise to different proton binding sites facilitat...

The HO−(aq) ion participates in myriad aqueous phase chemical processes of biological and chemical interest. A molecularly valid description of its hydration state, currently poorly understood, is a natural prerequisite to modeling chemical transformations involving HO−(aq). Here it is shown that the statistical mechanical quasi-chemical theory of...

The hydroxide anion plays an essential role in many chemical and biochemical reactions. But a molecular-scale description of its hydration state, and hence also its transport, in water is currently controversial. The statistical mechanical quasi-chemical theory of solutions suggests that HO[H2O]3- is the predominant species in the aqueous phase und...

Quasi-chemical theory and electronic structure results on inner-sphere H(H2O)n+ clusters are used to discuss the absolute hydration free energy of H+(aq). It is noted that this quantity is not thermodynamically measurable, and this leads to some relative misalignment of available tables of absolute hydration free energies of ions in water. The simp...

We develop a quasi-chemical theory for the study of packing thermodynamics in dense liquids. The situation of hard-core interactions is addressed by considering the binding of solvent molecules to a precisely defined `cavity' in order to assess the probability that the `cavity' is entirely evacuated. The primitive quasi-chemical approximation corre...

Proton transfer plays a pivotal role in chemical and biological systems. Previous studies suggest that the quantum nature of the proton must be considered to understand proton transfer. Since small molecules such as solvated protons appear in many systems, we are developing techniques to find compact wave functions for molecular subsystems within l...

Trapping of tritium on polymers with specific functional groups was investigated as a means of treating waste streams containing low levels of tritium. Chemical exchange of tritium with hydrogen on the functional group was used as the mechanism for trapping. The polymers tested include Aurorez polybenzimidazole resin beads, Chelex 100 resin beads,...

ely understood. Bernal and Fowler originally proposed the existence of a connected hydrogen bond pathway such as seen in Fig. 1. An excess proton on one side of this pathway causes a sequence of proton shifts and results in an excess proton in the other side of the chain. Fig. 1. A proton on one side of the chain causes a sequence of proton shifts....

This letter considers several physical arguments about contributions to hydrophobic hydration of inert gases, constructs default models to test them within information theories, and gives information theory predictions using those default models with moment information drawn from simulation of liquid water. Tested physical features include: packing...

Within the broad class of metal-hydrogen systems, clusters are of particular importance. Their high surface to volume ratio makes them ideal candidates for catalytic applications. Surface and bulk studies have shown that transport and vibrational spectroscopy of hydrogen are very sensitive to substrate structure. The wide variety of geometries exhi...

This paper investigates Monte Carlo techniques for construction of compact wave functions for the internal atomic motion of the D3O+ ion. The polarization force field models of Stillinger et al. and of Ojamae et al. were used. Initial pair product wave functions were obtained from the asymptotic high temperature many-body density matrix after contr...

To assess the importance of coupling to electron-hole pair (ehp) excitations for molecular sticking, scattering, and diffusion dynamics at metal surfaces, simulations of the CO/Cu(100) system were performed using the “molecular dynamics with electronic frictions” method. Over a range of incident translational energies, energy losses to ehp excitati...

Using a combination of ground state, equilibrium, and dynamical Monte Carlo methods, we examine the role of hydrogen-hydrogen interactions on selected structural and time-dependent properties of hydrogen containing metal clusters. Equilibrium simulations include studies of the classical and quantum-mechanical geometries and energetics for embedded...

There has been great interest recently in simulation of proton exchange events in liquid water. The obvious limitations of rigid water models have led to a long and continuing history of flexible and dissociative water models for simulation of aqueous systems. These models have been used in numerous classical and several quantum mechanical simulati...

Using both classical and quantum mechanical Monte Carlo methods, a number of properties are investigated for a single hydrogen atom adsorbed on palladium and nickel clusters. In particular, the geometries, the preferred binding sites, site specific hydrogen normal mode frequencies, and finite temperature effects in clusters from two to ten metal at...

Quantum annealing is a new method for finding extrema of multidimensional functions. Based on an extension of classical, simulated annealing, this approach appears robust with respect to avoiding local minima. Further, unlike some of its predecessors, it does not require an approximation to a wavefunction. In this paper, we apply the technique to t...

Stochastic dynamics simulations of vibrational relaxation rates are reported for a carbon monoxide molecule adsorbed on the (100) face of copper. A recently developed ‘‘molecular dynamics with electronic friction’’ scheme that self‐consistently incorporates both phonon and nonadiabatic electron–hole (e‐h) pair mechanisms of energy dissipation is em...

Recent experiments on the H/Ni(111) system have demonstrated that high-resolution electron-energy-loss spectra of subsurface absorbate species can be observed. We report here molecular-dynamics simulations for both the H/Ni(111) and H/Pd(111) systems. The necessary atomic forces are obtained from embedded atom method (EAM) potentials. From such cal...

Implicit dielectric solvent models have been developed in an effort to funnel computational resources to the solvated molecules of interest. More recently, the quasi-chemical theory developed by L. R. Pratt and S. Rempe provided a systematic way of including the most important solvent contributions to calculations of the chemical potential for the...

Many doped perovskite oxides exhibit proton conduction when exposed to water vapor. Conduction through the perovskite lattice involves a transfer step and a rotation step. Although the size of the perovskite is thought to influence which step is rate limiting, there is conflicting experimental data as to whether transfer or rotation is the rate-det...

The inverse isotope effect in macrobicyclic amines is being studied. Experimentally, the proton initially forms a weak bond with the nitrogen at an external site, and then undergoes isomerization to a stronger internal site. In certain size cages, this isomerization rate is 20for a deuteron than a proton. Results of minimum energy structure and tra...

Information theory offers a useful tool for studying the hydrophobic effect. This project characterizes a hard sphere default model for use in information theories of the hydrophobic effect. Monte Carlo runs were performed to produce isobaric/isothermal distributions of hard spheres at various densities. From the distributions, the probabilities of...