Negative Mixing Energy Research Articles

First Principles' Calculations of CO Adsorption on Bimetallic Nickel-Iron Clusters

Abstract: Density functional theory (DFT) calculations using the Generalized Gradient Approximation (GGA) functional are performed to investigate the structural, energetic and magnetic properties of bimetallic alloyed 〖〖Ni〗_n Fe〗_(N-n) isomers (N = 2-6 and N = 43, n = 0 to N). The negative mixing energies indicate the stability of these clusters as compared to Ni and Fe bare clusters. All clusters exhibit high spin ground states, with a significant decrease in the magnetization per atom for Fe19Ni24 cluster with the iron core. The adsorption energies of CO on the most stable isomers of alloyed clusters are lower than those on Fe clusters with the same structure. This behavior indicates a decrease of CO poisoning and an increase of methanol oxidation reaction (MOR) activity upon alloying. Keywords: Ni-Fe bimetallic clusters, DFT, CO adsorption, Methanol oxidation reaction, Catalytic activity.

Nanoclusters and nanoalloys of group 13 elements (B, Al, and Ga): benchmarking of methods and analysis of their structures and energies.

We investigated the structural and energetic properties of nanoclusters and nanoalloys composed of group 13 elements (B, Al, and Ga) up to a cluster size of 12. We conducted a comprehensive benchmark analysis of density functional and post-Hartree-Fock methods to identify efficient and accurate approaches for studying these systems using our benchmark dataset (BAlGa16) consisting of sixteen dimers and trimers. We compared different density functionals and post-Hartree-Fock methods using bond length and binding energy as parameters. B2PLYP closely follows CCSD(T) for geometry optimization, while REVPBE, BPBE, and PBE show cost-accuracy balanced performances. MRACPF was used as the reference for benchmarking energies, with NEVPT2 being the most accurate method, followed by CCSD(T) and DLPNO-CCSD(T). M06 and range-separated hybrid functionals perform well. Based on a cost-accuracy analysis, we recommend M06/def2-SVP as the preferred method. Additionally, we explored the structural evolution of pure, binary, and ternary clusters of group 13 elements up to 12 atoms, uncovering global and local minima. Ga clusters exhibited more rectangular faces compared to the predominantly trigonal faces of B and Al clusters. Binary clusters showed B in center positions, while Ga preferred outer positions, confirming the higher cohesion of B. The most favorable size of binary clusters (12) exhibited similar compositions of Al and Ga atoms. Compositions with 16.67-40% B, 16.67-60% Al, and 20-50% Ga were estimated to have negative mixing energies, indicating their relative stability.

Alternative algorithms for simultaneous modeling of ordering and intermediate compound growth during reactive diffusion

Standard Monte Carlo algorithms with constant pair interactions within the first coordination shell are usually quite effective for the description of ordering, tracer diffusion, chemical diffusion (but without Kirkendall shift effect) within any already existing ordered intermediate phase. Yet, one encounters big problems if one tries to simulate the formation and growth of ordered intermediate phase or, especially, simultaneous formation and growth of several intermediate phases during interdiffusion. Two algorithms providing solution of this problem, are discussed. The first algorithm is known (but not widely used for reactive growth) – account of interactions in the two coordination shells with negative mixing energy in the first coordination shell and positive mixing energy in the second one. The second algorithm is new: interactions only within the first coordination shell of each atom but depending on the local composition within the cluster around interacting atoms. Both algorithms are shown to provide formation and growth of the ordered intermediate phases with rather narrow concentration ranges, growing with time (after some initial period) according to parabolic law.

Structural and Electronic Properties of Binary Clusters SimGen (m + n = 6–13)

Using genetic algorithm combined with density functional theory calculations, we performed an unbiased global search for the most-stable structures of binary clusters SimGen with size s = m + n from 6 to 13. Further, we studied the structural and electronic properties of SimGen clusters using the B3LYP and CCSD(T) methods coupled with 6-311G + (d) basis set. For s = 6-12, SimGes-m clusters exhibit similar geometries to Sis and Ges clusters, respectively. However, for s = 13, the geometries of SimGes-m clusters fall into five completely different patterns. The negative mixing energies of SimGes-m clusters indicate that they possess higher energetic stability than Sis and Ges clusters. Among all clusters investigated, Si₂Ge₄, Si₂Ge5, Si₂Ge6, Si6Ge₃, Si5Ge5, Si7Ge₄, Si₃Ge9, and Si8Ge5 clusters have the relatively lower mixing energies and thus the highest energetic stabilities. Moreover, the Si₂Ge₄, Si₂Ge6, Si5Ge5, Si7Ge₄, and Si8Ge5 clusters with higher HOMO-LOMO gaps should have higher chemical stabilities than the same-sized Sis and Ges clusters. The Si5Ge5 cluster has a higher ionization potential than Si10 and Ge10. At the size s = 13, the geometry with the highest symmetry has the highest energetic and chemical stabilities.

First principles study of bimetallic Ni13-nAgn nano-clusters (n = 0-13): Structural, mixing, electronic, and magnetic properties.

Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni13-nAgn nano-clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg12 alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with the Ag environment causes reduction of Ni moment.

Spatial modulation of the composition of a binary liquid near a repulsive wall

When a binary liquid is confined by a strongly repulsive wall, the local density is depleted near the wall and an interface similar to that between the liquid and its vapor is formed. This analogy suggests that the composition of the binary liquid near this interface should exhibit spatial modulation similar to that near a liquid-vapor interface even if the interactions of the wall with the two components of the liquid are the same. The Guggenheim adsorption relation quantifies the concentrations of two components of a binary mixture near a liquid-vapor interface and qualitatively states that the majority (minority) component enriches the interface for negative (positive) mixing energy if the surface tensions of the two components are not very different. From molecular dynamics simulations of binary mixtures with different compositions and interactions we find that the Guggenheim relation is qualitatively satisfied at wall-induced interfaces for systems with negative mixing energy at all state points considered. For systems with positive mixing energy, this relation is found to be qualitatively valid at low densities, while it is violated at state points with high density where correlations in the liquid are strong. This observation is validated by a calculation of the density profiles of the two components of the mixture using density functional theory with the Ramakrishnan-Yussouff free-energy functional. Possible reasons for the violation of the Guggenheim relation are discussed.

First-principles prediction of phase stability and crystal structures in Li-Zn and Na-Zn mixed-metal borohydrides

We use a combination of first-principles density functional theory (DFT) calculations and the recently developed prototype electrostatic ground state (PEGS) method to predict low-energy crystal structures and study phase stability of Li-Zn and Na-Zn mixed-metal borohydride compounds [i.e., NaZn(BH${}_{4}$)${}_{3}$, NaZn${}_{2}$(BH${}_{4}$)${}_{5}$, LiZn(BH${}_{4}$)${}_{3}$, and LiZn${}_{2}$(BH${}_{4}$)${}_{5}$]. We find the following: (i) DFT $+$ PEGS successfully predicts low-energy structures in these mixed-metal borohydride systems. (ii) DFT calculations show negative mixing energies in both the Li-Zn and Na-Zn borohydride systems, consistent with the observation of mixed-metal ordering in these systems. (iii) Our DFT calculations of the recently reported experimental crystal structures of NaZn${}_{2}$(BH${}_{4}$)${}_{5}$ and NaZn(BH${}_{4}$)${}_{3}$ show that the former has a negative mixing energy, while the latter has a positive mixing energy. (iv) Using the PEGS approach, we predict a new crystal structure of NaZn(BH${}_{4}$)${}_{3}$ with negative mixing energy and find that the experimental structure of NaZn${}_{2}$(BH${}_{4}$)${}_{5}$ and the PEGS obtained structure of NaZn(BH${}_{4}$)${}_{3}$ lie on the ground state convex hull. (v) In the Li-Zn borohydride system, we have used the PEGS $+$ DFT approach to predict a stable crystal structure of new, previously unobserved stoichiometry, LiZn(BH${}_{4}$)${}_{3}$. As a consequence of this predicted low-energy compound, DFT calculations of the experimentally reported structure of LiZn${}_{2}$(BH${}_{4}$)${}_{5}$ show that it is unstable with respect to decomposition into LiZn(BH${}_{4}$)${}_{3}$ $+$ Zn(BH${}_{4}$)${}_{2}$. (vi) In addition, we also elucidate the ground state crystal structure of NaBH${}_{4}$, and confirm that reorientation of (BH${}_{4}$)${}^{\ensuremath{-}}$ units is the driving force behind the order-disorder phase transition in NaBH${}_{4}$. (vii) Finally, we predict a new low-energy crystal structure of Zn(BH${}_{4}$)${}_{2}$, and illustrate its similarities with the crystal structure of Mg(BH${}_{4}$)${}_{2}$.

Raman spectroscopic evidence for the most stable water/ethanol dimer and for the negative mixing energy in cold water/ethanol trimers

Spontaneous Raman scattering in supersonic jet expansions is used to prove that the mixed dimer of ethanol and water (corresponding to a volume fraction of 79% ethanol in the liquid) prefers ethanol in a gauche conformation as the hydrogen bond acceptor. This represents a particularly simple case of adaptive aggregation. Furthermore, it is shown experimentally that the isolated cold trimer built from one ethanol and two waters (corresponding to 64% ethanol in the liquid) has a significantly negative excess enthalpy, in line with the thermodynamic bulk observation at room temperature.

Regular solution model as applied to the interaction of halides in melt

Binary solid-liquid phase diagrams of halides with a common anion and a known mixing energy of the components in melt are analyzed by the regular solution model. The solid-liquid phase diagrams of eutectic systems with a negative mixing energy of molten components (i.e., systems with complex formation in melt) are satisfactorily fitted by this model.

First-principles aluminum database: Energetics of binary Al alloys and compounds

Using an extensive series of first-principles density functional calculations, we have constructed a ``first-principles aluminum database'' of thermodynamic properties of binary Al-based alloys with 26 different solute elements, $X$. In all cases, first-principles results are critically compared to experimental data for observed Al-rich ordered compounds, dilute impurities in Al, disordered $\mathrm{Al}\text{\ensuremath{-}}X$ solid solutions, and pure elements, $X$, in a variety of structure types. We find the following: (i) In all $\mathrm{Al}\text{\ensuremath{-}}X$ systems, first-principles formation enthalpies of ordered compounds are in excellent agreement with experimental data. Impurity energetics for $X$ in Al also agree rather well with thermodynamically assessed values from the COST507 database. (ii) Formation enthalpies of ordered compounds and energies of dilute impurities for elements from the $3d$ series are most negative at the beginning and the end of the series, and reach a maximum near the middle of the series for Cr. The ordering tendency decreases dramatically in the Al-Cu system with filled $d$ bands. (iii) The special quasirandom structure approach has been used to obtain mixing energies of disordered solid solutions across the whole composition range for all systems. We find that mixing energies follow the same general trend across the $3d$ series as the ordered and impurity formation enthalpies. Asymmetry in the mixing energies is also similar in all systems, giving less negative mixing energies for Al-rich compositions. (iv) Calculation of the solubility enthalpy, which is the difference in the formation energy per solute atom between the ordered and dilute solid solution phases, shows that the observed low solubility in $\mathrm{Al}\text{\ensuremath{-}}X$ systems is due to very negative values of the ordered formation enthalpies in comparison with those for the dilute solid solution.

Mixed Monolayers of Amphiphilic Cyclodextrins and Phospholipids: I. Miscibility under Dynamic Conditions of Compression

The miscibility of per-(6-amino-2,3-di-O-hexyl) β-CD hydrochloride salt abbreviated NH3-β-CD-OC6 with 1,2 dipalmitoyl, 3-sn-phosphatidyl choline (DPPC) and 1,2 dipalmitoyl, 3-sn-phosphatidic acid (DPPA) and of per-(6-dodecanoylamino-6-deoxy) β-CD abbreviated C11CONH-β-CD with DPPC has been assessed by surface pressure–area experiments under dynamic conditions. The determination of their mixing behavior has been approached by thermodynamic characterization derived from a comparison of the composition dependence of collapse pressure, for various contents of two components in monolayers, with that predicted from a model of ideal mixing. Component miscibility has additionally been addressed by the use of the Smaby–Brockman state equation for liquid-expanded monolayers which characterizes interaction between monolayer components using the water activity coefficient. While for both NH3-β-CD-OC6–DPPC and NH3-β-CD-OC6–DPPA systems the average surface areas followed the additivity rule, for the latter system surface pressures at collapse significantly deviated from ideality. The occurrence of an interaction at the level of oppositely charged polar groups of NH3-β-CD-OC6 and DPPA was also demonstrated by the nonideal composition dependence of the water activity coefficient (f1) characterizing the interfacial water. The mixing energies and interaction parameters at collapse pressures were calculated using the Joos approach. Positive values of these factors indicated that NH3-β-CD-OC6 and DPPA mixed films were thermodynamically less stable than the films in which ideal mixing of components occurred. The mixing of NH3-β-CD-OC6 with DPPC appeared to be almost ideal. In the case of the C11CONH-β-CD–DPPC system, analysis according to both the Joos and Goodrich approaches showed the occurrence of an important interaction which resulted in negative mixing energies characteristic of thermodynamically stable mixed films.