Ab initio calculations on (OH)4 defects in α-quartz

Lin, Jingxiang; Payne, Mike C.; Heine, Volker; McConnell, J. D. C.

doi:10.1007/bf00203145

Cited by 30 publications

(25 citation statements)

References 21 publications

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“…Just as importantly, it permits analytic forces to be computed allowing for relaxation of the ions to their minimum energy configuration. This type of calculation has been used to study defect energies in MgO 18 , OH groups as substitutional defects in quartz 19 , reconstruction of the silicon (111) surface 20 and dissociation of Cl 2 at a silicon surface 21 . One difficulty in using plane-wave pseudopotential methods for oxides has been that the tightly-bound oxygen 2p electrons require a high energy cutoff making the calculations expensive to perform.…”

mentioning

confidence: 99%

Water chemisorption and reconstruction of the MgO surface

Refson¹,

Wogelius²,

Fraser³

et al. 1995

Phys. Rev. B

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158

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The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol −1 . We conclude that protonation stabilizes the otherwise unstable (111) Magnesium oxide has long provided a prototype for the study of surface structure and chemical reactions of oxides. Naturally occurring MgO, known by its mineral name of periclase, is not a common crustal mineral, but its simple structure makes it an excellent example for the investigation of mineral surface chemistry.Reactions at mineral surfaces are responsible for much of the chemical change which occurs in the Earth's crust. Weathering reactions control the erosion of rocks and the consequent evolution of surface topography thus providing an opposing mechanism to the more dramatic process of mountain building. Aqueous reactions in sedimentary basins are responsible for the diagenetic processes which transform unconsolidated sediments into rocks. In this work we study the nature of a simple mineral surface when exposed to an aqueous environment and the chemical interaction of water with that surface. This is both a prerequisite to studying the interaction with aqueous solutions and a tractable first step towards ligand-exchange reactions in more complex silicate minerals.We have performed experiments on single-crystals of MgO prepared with high-quality (001) faces which were reacted with acidic solutions. The experiments and results are reported in detail elsewhere 1 , the main feature being the development of an altered surface layer. Elastic Recoil Detection Analysis (ERDA)2 shows protonation to a depth of 900Å with a H/Mg ratio close to 2 giving a probable chemical composition of magnesium hydroxide. Indeed brucite (the mineralogical name for Mg(OH) 2 ) is the most common alteration product of periclase in the natural environment 4 and well-crystallized intergrowths of brucite on periclase have been reported 5 . The initial stage in the reaction is hydroxylation of the surface. MgO has the cubic rocksalt structure with (001) cleavage planes. This is the most stable surface and is the only one seen experimentally 6 . The simplest possibility for a hydroxylated surface is obtained by dissociating a water molecule and placing the OH group above each magnesium ion and the H above each oxygen of the (001) surface (see Fig. 1a) as postulated by Coluccia et al.7 . Some striking hydroxylation experiments were reported by Jones et al. who studied surface roughening on (001) faces of nanocrystalline MgO in a transmission electron microscope 8 . The remarkable affinity of MgO for water is demonstrated by their in situ observation of hydration-induced surface roughening over 10 minutes under vacuum with P H 2 O < 10 −5 Pa. The presence of surface hydroxyl groups on MgO powde...

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mentioning

confidence: 99%

Water chemisorption and reconstruction of the MgO surface

Refson¹,

Wogelius²,

Fraser³

et al. 1995

Phys. Rev. B

Self Cite

245

158

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“…The d-component of the Ge pseudopotential is generated in a similar way to match smoothly around the core radius of 1.217 Å to a screened potential that is constructed by carrying out an all-electron calculation on the free Ge atom in the same reference configuration and is chosen to be the local for the KleinmanBylander representation. Although Si and Ge pseudopotential is constructed using only local density approximation (LDA) it has shown to be highly transferable over the required energy ranges from neutral atoms [31], dimers [32], silicon oxide defects [32], and silicon surface reconstruction [33]. For the ultrasoft pseudopotential of H it is generated with an atomic reference electron configuration 1s 1 of core radius r c,s ϭ 0.423 Å, augmentation radius of r aug,s ϭ 0.423 Å for the construction of the pseudized augmentation function, and reference energy of s,1 ϭ Ϫ6.394 eV.…”

Section: Methodsmentioning

confidence: 99%

Density functional study of XH₄ (XSi and Ge) reactivity upon dissociative adsorption onto the Si(100) surface

Lin

Lee

2003

Int J of Quantum Chemistry

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ABSTRACT:Total energy calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) and ultrasoft pseudopotential approximation and an analysis tool of atom-resolved density of states (ADOS) have been used to investigate (1) the energetic profiles for the possible initial dissociative adsorption of XH 4 (XASi and Ge) onto the Si(100)O(2 ϫ 2) surface to evaluate their reactivity and (2) the effect of surface electronic states of Si(100)O(2 ϫ 2) on gaseous molecular precursors XH 4 (XASi and Ge) during initial dissociative adsorption to understand the factors governing their reactivity. Our calculated lower-energy barrier for initial dissociative adsorption of GeH 4 is due to the forming of stronger bond of SiOH between H within GeH 4 and buckled-down Si atom on the Si(100)O(2 ϫ 2) surface accompanying the larger extent of unbuckling of the buckled SiASi dimer on the Si(100)O(2 ϫ 2) surface at the transition state. Our evaluated better reactivity for GeH 4 than SiH 4 (a factor of around 14.6) is slightly larger than observed higher reactivity for GeH 4 than SiH 4 (a factor of between 2 and 5 depending on the incident kinetic energy) employed supersonic molecular bean techniques. Finally, our calculated ADOS indicate that the surface electronic states of buckled SiASi dimer on the Si(100)O(2 ϫ 2) surface energetically favorably participate in the transition state during GeH 4 initial dissociative adsorption to reduce the energy barrier, i.e., enhance its reactivity, in comparison with SiH 4 initial dissociative adsorption onto the Si(100)O(2 ϫ 2) surface under the same reaction conditions.

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“…These authors also show that the activation energy for the migration of water molecules in the amorphous structure of SiOs (Hagon et al, 1987) is likely to be more closely related to that for the migration of water molecules in dislocation cores in quartz, a point which is of relevance later. Recent ab initio calculations (Lin et al, 1994) using full periodic boundary conditions rather than a simple cluster configuration indicate that the minimum insertion energy for the water molecule in quartz is 88.15 kJ/mole with respect to well separated water molecules at 0 K.…”

Section: Interstitial Water Moleculesmentioning

confidence: 99%

“…While these may provide a source of water it is not clear that molecular water in this form plays any direct role in the development of the abundant dislocations necessary to the weakening process in quartz. On the basis of electron-optical studies McLaren et al (1983) suggested that small defects in synthetic wet quartz, showing strong electron-optical strain contrast, were bubbles of high pressure fluid which are responsible for the volume increase of the quartz crystal on heating, but this conclusion has recently been seriously challenged (Lin et al, 1994;McConnell et al, 1995), and these defects are now regarded as planar rafts of aggregated [4HIsi defects formed on precipitation. Two aspects of the behaviour of fluid inclusions in quartz are of direct interest in the present discussion.…”

Section: Macroscopic Fluid Inclusionsmentioning

confidence: 99%

“…Since it has very important implications in the present analysis the concentration of interstitial water molecules in quartz was calculated as a function of temperature and pressure using the ab initio results of Lin et al (1994) and a simple but adequate model for the thermodynamic properties of the interstitial water molecule. By far the most important contribution to the partial molar Gibbs free energy of the interstitial water is the P V term associated with its confinement in the limited space available within the c-axis channel of the a-quartz structure.…”

Section: Interstitial Water Moleculesmentioning

confidence: 99%

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Ab initiostudies on water related species in quartz and their role in weakening under stress

McConnell

1997

Phase Transitions

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Ab initio calculations on the modes of incorporation of water in quartz suggest that, apart from the incidental inclusion of molecular water as fluid inclusions, the most prevalent mode of uptake of water in a-quartz is structural and involves the replacement of Si by four protons in a process formally described as the [4H]si substitution. Existing ub initio computer calculations were used as a basis on which to determine the equilibrium concentration of [4H]si defects in quartz as a function of relevant temperatures and pressures. These calculations indicate that [4H] si concentrations are in the range lo-'-10-4/Si atom increasing rapidly with increasing water pressure at constant temperature, while above 300°C the defect concentration decreases monotonically with increasing temperature in all cases, implying that the defects must react on heating to produce molecular water. Ab inirio results have also been used to establish the concentration of interstitial water molecules in quartz under the same conditions. In this case the concentrations are vanishingly small and of the order of IO-''/Si atom at 500°C and l.OGPa, implying that the diffusion coefficient for water on interstitial sites in quartz must also be extremely low.It has recently become apparent that the results of the ab initio based calculations for the concentration of both [4H]si defects and interstitial water molecules are directly relevant to the weakening behaviour in wet quartz on heating. Since the reaction in which [4H]si defects produce molecular water on heating involves a very substantial volume increase and, since the diffusion coefficient for the migration of molecular water on interstitial sites is now known to be prohibitively small, excess water produced on heating cannot escape from the crystal which therefore behaves locally as a. totally closed system, at least within the time frame of conventional laboratory experiments. Here the reaction of [4HIsi defects to give molecular water on heating leads to the development of water bubbles with the concomitant growth of edge dislocations with b = $u(1120), the dislocations taking up the material transfer from the bubble volume in the closed system. At concentrations of [4HIsi defects of the order of IOOppm, the development of a dislocation densities of the order of 108/cm2 can be shown to be reasonable, a figure which is also confirmed from existing electron optical studies of heated wet quartz.

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Ab initio calculations on (OH)4 defects in α-quartz

Cited by 30 publications

References 21 publications

Water chemisorption and reconstruction of the MgO surface

Water chemisorption and reconstruction of the MgO surface

Density functional study of XH₄ (XSi and Ge) reactivity upon dissociative adsorption onto the Si(100) surface

Ab initiostudies on water related species in quartz and their role in weakening under stress

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Ab initio calculations on (OH)4 defects in α-quartz

Cited by 30 publications

References 21 publications

Water chemisorption and reconstruction of the MgO surface

Water chemisorption and reconstruction of the MgO surface

Density functional study of XH4 (XSi and Ge) reactivity upon dissociative adsorption onto the Si(100) surface

Ab initiostudies on water related species in quartz and their role in weakening under stress

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Density functional study of XH₄ (XSi and Ge) reactivity upon dissociative adsorption onto the Si(100) surface