Hybrid-exchange Density Functional Theory Research Articles

Stability of the AlF 3[formula omitted] surface in H 2O and HF environments: An investigation using hybrid density functional theory and atomistic thermodynamics

Thermodynamic calculations based on hybrid-exchange density functional theory are used to predict the surface structure and stability of α-AlF 3 ( 0 1 1 ¯ 2 ) in the presence of gaseous HF and H 2O environments. The clean stoichiometric α-AlF 3 ( 0 1 1 ¯ 2 ) is predicted to be Lewis acidic. However, under most reaction conditions this surface is unstable with respect to the adsorption of hydroxyl ions. This is consistent with experimental observations. It is predicted that the surface containing no hydroxyl ions could only be realised at high temperatures and under unrealistically dry conditions.

A quantum-mechanical study of the vinyl fluoride adsorbed on the rutile TiO 2(1 1 0) surface

The adsorption of vinyl fluoride on the rutile TiO 2(1 1 0) surface has been simulated, on the basis of a recently proposed experimental model, using hybrid-exchange density functional theory. Different surface coverages have been considered and the lateral interaction between adsorbed vinyl fluoride molecules has been quantified through a simple model of nearest and next nearest neighbouring molecules. The vibrational frequencies of the adsorbed molecule have been calculated and are found to be in excellent agreement with those observed providing support for the proposed adsorption model. The effect of the adsorption on the electronic structure of the molecule and the surface have been characterised by computing electrostatic potential maps and the local density of states.

Composition and Structure of the α-AlF3(0001) Surface

Strong Lewis acid catalysts are widely used in a variety of industrial processes including Cl/F exchange reactions. Aluminum fluorides (AlF3) have great potential for use in such reactions. Despite the importance of the surface in the catalytic process little is known about the detailed atomic scale structure of AlF3 surfaces. In the current study we employ state of the art surface thermodynamics calculations based on hybrid-exchange density functional theory to predict the composition and structure of the basal plane surface of alpha-AlF3 for the first time. We examine four possible terminations of the alpha-AlF3 (0001) surface and demonstrate that the surface is terminated by a layer containing two fluorine atoms per cell at all realistic fluorine partial pressures. The fluorine ions in the outermost layer of the material reconstruct to mask the Al3+ ion from the external gas phase and consequently we would expect this surface to be inactive as a Lewis acid catalyst in line with experimental observation.

Hybrid density functional study of organic magnetic crystals: bi-metallic CrIII cyanides and rhombohedral C60

Periodic hybrid-exchange density functional theory calculations have been used to investigate the magnetic properties of two classes of organic magnets, namely the bi-metallic CrIII cyanides and the polymerized rhombohedral C60 fullerenes (Rh-C60). For the systems KMII[CrIII(CN)6] with M II=V, Mn, Ni and CrIII[CrIII(CN)6], the magnetic ordering energies, Mulliken populations, and spin density plots are reported for the optimized geometries. The qualitative nature of the magnetic coupling mechanism is consistent with that observed in previous unrestricted Hartree–Fock calculations, but the coupling energies computed here are significantly higher. The increased coupling is found to be a result of both changes in the geometry and the electronic structure resulting from the more reliable treatment of electronic exchange and correlation effects. The existence of long-range coupling between local spin moments is investigated in three different defective Rh-C60 structures: (i) a previously proposed prototype structure, where an atom is removed from the C60 cage; (ii) a related structure in which vacancies in nearby cages are brought closer together in pairs; and (iii) a structure where the intra-fullerene bond between the two inter-fullerene bonds is broken spontaneously after applying isotropic pressure to one layer of the Rh-C60 structure. All of these structures are characterized by low flat spin polarized bands at the Fermi edge and localized spin moments around the defects, but no evidence of long-range magnetic coupling is found.

Experimental and Theoretical Study of a Surface Stabilized Monolayer Phase of Nickel Oxide on Pd(100)

A surface stabilized monolayer phase of nickel oxide, c(4 x 2)-Ni(3)O(4), has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybrid-exchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.

High-pressure phases ofFeTiO3from first principles

The structural, elastic, and electronic properties of the high-pressure $\mathrm{Fe}\mathrm{Ti}{\mathrm{O}}_{3}$ polymorphs and their relative phase stability under pressure are investigated using hybrid exchange density functional theory. For the currently known phases the computed structural and elastic parameters are in good agreement with those observed, as is the predicted stability of the phases. The transformation of the perovskite polymorph to a new high-pressure polymorph with space group $Cmcm$ is predicted to occur at 44 GPa at 0 K.

Identification of possible Lewis acid sites on the β-AlF3(100) surface: an ab initio total energy study

Strong Lewis acid catalysts are widely used in a variety of industrial processes including Cl/F exchange reactions. Aluminium fluorides (AlF3) have great potential for use in such reactions. Despite the importance of the surface in the catalytic process little is known about the detailed atomic scale structure of AlF3 surfaces. In the current study we employ state of the art total energy calculations based on hybrid-exchange density functional theory to predict the composition and structure of the (100) surface of beta-AlF3 for the first time. We examine six possible terminations of the beta-AlF3 (100) surface and demonstrate that there are two relatively low energy terminations that result in the formation of under co-ordinated Al3+ ions at the surface. Such under co-ordinated ions are expected to be strong Lewis acid sites. This is the first ab initio determination of the atomic scale structure of such sites on the surface of beta-AlF3.

Magnetic properties of polymerizedC60: The influence of defects and hydrogen

A combination of reactive force field molecular dynamics and hybrid-exchange density functional theory (DFT) generates a defective structure of $\mathrm{Rh}\text{\ensuremath{-}}{\mathrm{C}}_{60}$ possessing an inter-cage link. Hybrid-exchange DFT is used within periodic boundary conditions to investigate the long-range magnetic coupling between the resulting defects. Inelastic neutron scattering experiments highlight the presence of hydrogen chemically bonded to carbon in the magnetic samples. A simple spin model previously applied to studies of planar conjugated $\ensuremath{\pi}$ electron systems is used to illustrate the mechanism through which chemically bonded hydrogen leads to a ferromagnetic ground state for this system.

Spin singlet formation in MgTi2O4: evidence of a helical dimerization pattern.

The transition-metal spinel MgTi2O4 undergoes a metal-insulator (M-I) transition on cooling below T(M-I)=260 K. A sharp reduction of the magnetic susceptibility below T(M-I) suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of MgTi2O4, which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi2O4 is an orbitally ordered band insulator.

Vibrational and electronic spectra of alkali halides doped with CrO 2-4 ions : S.C. Jain, A.V.R. Warrier and S.K. Agarwal, Solid State Physics Laboratory, Lucknow Road, Delhi-7, India

<dm:abstracts xmlns:dm="http://www.elsevier.com/xml/dm/dtd"><ce:abstract xmlns:ce="http://www.elsevier.com/xml/common/dtd" view="all" class="author" id="aep-abstract-id10"><ce:section-title>Publisher Summary</ce:section-title><ce:abstract-sec view="all" id="aep-abstract-sec-id11"><ce:simple-para id="fsabs020" view="all">This chapter elaborates the magnetic properties of novel carbon allotropes. The polyhedral molecules of fullerene, C<ce:inf loc="post">60</ce:inf>, undergo cross-linking reactions when annealed at pressures to form a variety of crystalline polymers with layered structures. These polymerized fullerenes (poly-Cm) demonstrate ferromagnetic behavior only when prepared in a relatively narrow range of synthesis conditions. This indicates that the process of formation of a ferromagnetic phase may be related to the occurrence of the defects due to cage opening. It is also feasible that the weak bonds between polyhedra are readily broken, and can act as the spin centers. The possibility of magnetic centers arising in defective polyhedral cages was examined in a study that modeled <ce:italic>poly</ce:italic>-C<ce:inf loc="post">60</ce:inf> using a combination of reactive force field molecular dynamics and hybrid-exchange density functional theory. The calculations showed that it was particularly favorable for hydrogen to bind at one of the three types of spin centre in this particular defect, and that the perturbation introduced by hydrogen quenched the antiferro coupling pathway selectively, leading to ferromagnetism with moment.</ce:simple-para></ce:abstract-sec></ce:abstract></dm:abstracts>