Magnesium Vacancies Research Articles

Non-stoichiometry in MgAl2O4 spinel

Stoichiometric magnesium aluminate spinel, MgAl2O4, contains equimolar proportions of Al2O3 and MgO. Spinel can, however, exhibit significant deviations from this stoichiometric composition. There is considerable disagreement concerning which species compensate for either excess Al2O3 or MgO non-stoichiometry. Here, we use empirical and quantum mechanical (density functional theory) atomistic simulation techniques to investigate the defect chemistry accommodating non-stoichiometry. The incorporation of excess Al2O3 was found to be a lower energy process than the solution of excess MgO. Elevated magnesium and aluminium cation vacancy defect concentrations are predicted in Al2O3 rich spinels, whilst MgO excess is facilitated by a combination of oxygen vacancy and magnesium interstitial defects.

Experimental study of the incorporation of Li, Sc, Al and other trace elements into olivine

We performed a series of synthesis experiments at 1 atm pressure to investigate the substitution mechanisms of 1+ and 3+ ions into olivine. Forsterite crystals were grown from bulk compositions that contained the element of interest (e.g. Li) and different amounts of additional single trace elements. By working at constant (major element) liquid composition and temperature we eliminated all compositional effects other than those due to the trace elements. Mineral–melt pairs were then analysed to determine the compositional-dependence of the partition coefficient ( D), which corresponds to [ element ] mineral [ element ] glass , and where [element] refers to weight concentration of the element in the respective phase. We find that Li forms a stable coupled substitution with Sc and, at above ∼500 ppm Sc in the crystal, Li + and Sc 3+ ions form an ordered neutral complex ([LiSc]). This complex dissociates at lower trace element concentrations and a second, concentration-independent, mechanism begins to dominate. This second solution mechanism is most likely 2Li + ⇔ Mg 2+ where one of the Li atoms is in an interstitial position in the crystal lattice. Natural olivines show Li contents slightly greater than Sc (on an atomic basis), indicating that both substitution mechanisms are significant. Unlike Sc, Al does not appear to form a stable complex with Li in the olivine structure. Sodium is highly incompatible in olivine with D Na Fo – melt of ∼0.00015–0.03. Olivine–liquid partitioning of Na + is independent of Sc 3+ or Al 3+ concentration. This indicates that the coupled substitution of Na + with any 3+ ions is unlikely. Instead, the relevant substitution mechanism appears to be 2Na + ⇔ Mg 2+. Although independent of 3+ ion concentration, D Na Fo – melt is inversely correlated with the Li concentration of both melts and crystals, implying that Na competes (unsuccessfully) with Li to replace Mg in the olivine structure. Aluminium is highly incompatible in forsterite D Al Fo – melt = 0.006 ± 0.0005 n = 7 . Values D Al Fo – melt of are similar for all phase pairs synthesised from starting materials containing between 10 and 100,000 ppm Al. This suggests that Al is principally incorporated in forsterite by replacing one Mg and one Si atom Mg Mg x + Si Si x ↔ Al Mg • + Al Si ′ , where the Al atoms on octahedral (Mg) and tetrahedral (Si) sites are dissociated from one another. The incorporation of gallium into forsterite is influenced by the presence of Li. Where Li concentration in the crystal is much greater than that of Ga (on an atomic basis) we find an excellent correlation between D Ga Fo – melt and melt Li content. This relationship indicates that Ga 3+ and Li + replace 2Mg 2+ on octahedral sites and that the Ga and Li atoms are, like Sc and Li, strongly associated in the crystal structure. The mechanism by which scandium is incorporated into forsterite is strongly governed by the presence Li. As discussed above, ordered Li Mg ′ Sc Mg • complexes form readily in forsterite in Li-rich experiments. Under Li-absent but Sc-rich conditions (Sc in the crystal >∼500 ppm), D Sc Fo – melt is proportional to the concentration of Sc in the melt. This indicates that Sc incorporation is charge-balanced by the formation of magnesium vacancies 3 Mg Mg x ↔ 2 Sc Mg • + V Mg ″ , and that both species are associated Sc Mg • V Mg ″ . At lower Sc concentrations (<500 ppm in the crystal), the concentration-dependence of partitioning indicates that the Sc Mg • V Mg ″ complexes dissociate. Our results demonstrate that partitioning of 1+ and 3+ ions into olivine is complex and involves a range of point defects which yield strongly composition-dependent crystal–melt partition coefficients. Since physical and chemical properties of natural olivine, such as diffusion of 6Li and 7Li and H 2O solubility, depend on the concentrations of the defects identified in this study, our results provide an important insight into how determining substitution mechanisms can improve our understanding of large-scale mantle processes and properties.

Thermodynamics and kinetics of the Schottky defect at terraces and steps on the MgO(001) surface

The Schottky defects at both the flat MgO(001) surface and the monatomic-step edge have been investigated by equilibrium molecular dynamics simulations. The formation enthalpy as a function of the distance between the Mg and O vacancies that form a Schottky defect have been calculated and discussed. We conclude that a step edge is a stable location for a vacancy. The migration mechanism has been elucidated and an intermediate state has been identified. The associated activation enthalpies have been determined in the 700-1100 K temperature range. Both magnesium and oxygen vacancies at the surface are very mobile and can play a role during the crystal growth.

Nature of Defect States in Nitrogen-Doped MgO

The nature of nitrogen-doped MgO, N-MgO, is investigated by density functional theory (DFT) calculations using both periodic supercells and emebedded cluster models. The electronic structure and the spin properties, in particular, the hyperfine coupling constants, of substitutional for O and interstitial N species are determined by using a hybrid exchange-correlation functional in order to account for the self-interaction problem in DFT. Both substitutional and interstitial N introduce magnetic impurities in bulk MgO and generate new energy levels at about 0.5−1.7 eV above the top of the MgO valence band. We have also considered the simultaneous presence of neutral magnesium vacancies (VMg) and neutral oxygen vacancies (VO) in the material, and we found that a charge transfer can occur between the N-impurity states and these intrinsic defects. In particular, a substitutional nitrogen behaves as an electron donor in the presence of VMg and as an electron acceptor in the presence of VO centers. The occurren...

Electrical conductivity measurements of periclase under high pressure and high temperature

Abstract The electrical conductivity of sintered polycrystalline periclase has been measured at 20 GPa and temperatures up to 1700 K with a KAWAI-type multi-anvil apparatus. Arrhenius plot of the electrical conductivity shows two linear regions, implying a change of charge transport mechanism with temperature in this experiment. At temperatures below 700 K small polaron conduction dominates, with an activation energy of 0.2 eV. At higher temperatures, the activation energy is determined to be 1.18 eV, which is much higher than 0.2 eV at low temperature. Our observations suggest the high-temperature conduction is attributed to a large polaron process with magnesium vacancy trapping hole on oxygen.

Electrical conductivity of synthetic iron-bearing olivine

The electrical conduction in synthetic, dry polycrystalline, iron-bearing olivine (Fo90) was investigated as a first-order approach to the electrical conductivity in the upper mantle. This fundamental study is of great importance to better understand the charge-transport mechanisms seen in olivine. Conduction processes in synthetic samples are not influenced by a complex geological history in contrast to conductivity in natural olivine. The experiments show that the apparent activation energy for conductivity for Fo90 is 230 kJ mol−1. In currently accepted defect modeling, natural and synthetic olivine requires a mechanism involving small polaron formation (Fe· Mg and magnesium vacancies (V Mg) as the dominant diffusing species to explain a fO 2 1/6 relation to electrical conduction. Here, Fo90 shows no contribution of small polarons to conductivity at temperatures between 1,000 and 1,200°C and almost no dependence on fO2. Instead, under reducing conditions magnesium vacancies (and electrons) appear to be the major charge carriers.

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr2+ and Cr3+ replacing magnesium ions at octahedral positions in Mg2SiO4: Cr and Mg2SiO4: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \( P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr2+ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \( P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \( P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr2+ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr3+ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr3+ and Li+ ions. Doping with lithium increases the concentration of Cr3+ ions and, thus, decreases the fraction of Cr2+ and Cr4+ ions in the total content of chromium centers.

Vacancies in magnesium silicide – stoichiometric vacancies preferred?

AbstractMagnesium silicide (Mg2Si) in its calcium fluorite (CaF2) structure is a hardly investigated material up to now. However, it may play an important role in upcoming new materials for photovoltaic applications due to its semiconducting properties and the fact that it is consisting of non‐poisonous elements. On the other hand metastable phases of Mg2Si play an extremely important role in the hardening of 6xxx (AlMgSi‐based) aluminum alloys. Hence, it seems worth investigating the formation energy of vacancies in Mg2Si. In this paper we calculate the formation energy for vacancies on the different sublattices using the ab‐initio method SIESTA. While the values for the silicon (1.04 eV) and the magnesium vacancy (1.74 eV) lie in the expected range, the formation energy of the di‐vacancy and the stoichiometic tri‐vacancy turn out to be just 0.68 eV and 0.78 eV, respectively. This is smaller than the value for both mono‐vacancies. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A computational study of magnesium point defects and diffusion in forsterite

We have studied the formation and migration of point defects within the magnesium sub-lattice in forsterite using a combination of empirical and quantum mechanical modelling methodologies. Empirical models based on a parameterised force field coupled to a high throughput grid computing infrastructure allow rapid evaluation of a very large number of possible defect configurations. An embedded cluster approach reveals more accurate estimates of defect energetics for the most important defect configurations. Considering all defects in their minimum energy, equilibrium positions, we find that the lowest energy intrinsic defect is the magnesium Frenkel type, where a magnesium atom moves from the M1 site to form a split interstitial defect. This defect has 2 four-co-ordinated magnesium atoms located outside opposite triangular faces of an otherwise vacant M1 octahedron. The split interstitial defect is more stable than regular interstitials where magnesium is located in either of the two structurally vacant octahedral sites in the hexagonally close packed oxygen lattice. M1 vacancies are also found to form when iron(II) oxidises to iron(III). The energy of the defects away from the equilibrium positions allows the energy barrier to diffusion to be calculated. We have considered the migration of both magnesium vacancies and interstitials and find that vacancies are more mobile. When the contribution from the formation energy of the defects is included we arrive at activation energies for vacancy diffusion that are in agreement with experiment.

The effect of magnesium vacancies on the π intraband scattering in MgxB2 as determined by point contact Andreev reflection

In order to introduce structural defects into the π band with minimal distortions in the σ band, bulk samples of varying magnesium content were prepared. Point contact Andreev reflection measurements reveal that the density of states of the π band and the behavior of the energy gaps in field imply a relative increase of π band to σ band scattering with increasing magnesium deficiency. The results are consistent with the observed increase of the low temperature Hc2 value in magnesium deficient MgB2.

Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism

Infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure. Two mechanisms occur in the system MgO‐SiO2‐H2O associated with silicon and magnesium vacancies, respectively. A third mechanism is associated with trivalent cation substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is the one most prevalent in natural olivines from the spinel‐peridotite facies of the Earth's upper mantle, is associated with Ti4+ [Berry et al., 2005]. Here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure MgO‐SiO2‐H2O system, and possible hydrogen‐bearing and hydrogen‐free point defects in Ti4+‐doped forsterite. Calculated structures are used to compare the predicted orientation of the O‐H bonds with the experimentally determined polarization. The energies are used to discuss how different regimes of chemical environment, temperature (T), pressure (P), and both water content and water fugacity impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable. We find that given the presence of Ti impurities, the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism leads to the water‐mediated formation of silicon vacancies. As silicon is known to be the most slowly diffusing species in olivine, this provides a credible explanation of the observed water weakening effect in olivine.

Defect processes in MgAl 2O 4 spinel

In perfect normal MgAl 2O 4 spinel the Mg 2+ ions occupy tetrahedral 8a sites and Al 3+ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.

First-principles study of the interfacial structures ofAu∕MgO(001)

By using a first-principles method, the theoretical analysis of six probable $\mathrm{Au}∕\mathrm{Mg}(001)$ interface models points out that two of them with oxygen or magnesium vacancies in the interface are most stable. It is found in this work that besides O or Mg vacancies having to exist in this interface, the stability of an interface depends deeply on the atomic configuration of interface. Such a configuration in which each Au atom in the upper Au layer of interface bonds with each O atom in the MgO under layer is theoretically considered to be most stable. Nevertheless, these results need to be confirmed by experiments.

Simulations of cascades in pure and alumina doped magnesia

Atomic scale molecular dynamics simulations of displacement cascades are carried out in an initially defect-free MgO lattice and in a lattice where 0.2% of the Mg2+ ions have been replaced by Al3+ ions, charge compensated by magnesium vacancies. Results show that the properties of the cascade in the perfect lattice exhibit considerable anisotropy with respect to the direction of the incident primary knock-on atom (PKA). We find that a different anisotropy of equivalent magnitude occurs as a consequence of the (random) distribution of defects in the vicinity of the cascade. This suggests that the details of damage production during a cascade event can depend sensitively on the distribution of dopants and impurity atoms around the PKA.

SEO3: A new model of olivine electrical conductivity

SUMMARY Standard Electrical Olivine 3 (SEO3) is a new model of electrical conductivity for olivine as a function of temperature and oxygen fugacity, and is based on a previous study of point defect behaviour in a dunite. That study produced quantitative estimates of mobility and concentration for small polarons and magnesium vacancies as a function of temperature and oxygen fucacity. Unlike earlier SO1 and SO2 models of olivine conductivity, the dunite sample was buffered by pyroxene, and avoided alteration at high temperature. The low-temperature extrapolation produced by SEO3 is in good agreement with SO2, while the high-temperature extrapolation is significantly more conductive, which is in general agreement with many field studies that show the Earth’s upper mantle to be more conductive than predicted by SO2 for reasonable temperatures. Plausible variations in mantle oxygen fugacity correspond to about half an order of magnitude variation in electrical conductivity, equivalent to about 100 degrees in temperature.

Investigating the role of cationic vacancy on the MgO (0 0 1) defect surface: Embedded cluster models study

The adsorption and dissociation of Cl 2 at different magnesium vacancy sites (V, V − or V 2− center) of the MgO (0 0 1) defect surface have been investigated using density functional theory coupled with embedded cluster models. It is rather interesting to find that the magnesium vacancy sites at the edge or corner exhibit quite high surface reactivity toward the dissociation of Cl 2. The dissociation process proceeds without energy barrier as reflected by the calculated potential energy curve. Whereas, the case is different for the magnesium vacancy sites at the terrace, i.e., no surface reactivity toward the dissociation of Cl 2. Meanwhile, the results herein are compared to those at “electron-rich” oxygen vacancy sites. The present findings illustrate that the low-coordinated magnesium vacancy sites on the MgO (0 0 1) surface should be attached to importance in the catalysis community because they can act as active centres in adsorption and dissociation processes of technological importance. We believe that these results would be particularly beneficial for understanding the chemical reactivity of MgO defect surfaces.

Space charge characterisation by EDS microanalysis in spinel MgAl 2O 4

It was already showed that spinel presents a grain boundary sliding deformation accommodated by diffusion during creep at high temperature [Béclin, F., Duclos, R., Crampon, J. and Valin, F., Microstructural superplastic deformation in MgO·Al 2O 3 spinel. Acta Metall. Mater. 1995, 43, 2753–2760; Addad, A., Etude de la plasticité haute température de matériaux céramiques spinelle-zircone. Ph.D. thesis, Laboratoire de Structure et Propriétés de l’Etat solide, Villeneuve d’Ascq, 1995]. A space charge layer at grain boundary in ionic ceramics can explain the observed interface reaction controlling diffusion at low stress (less than 60 MPa). The nonstoichiometric area due to ionic defects should create an electrostatic potential between the grain surface and inside grain. The purpose of this work is to study the grain boundary region stoichiometry to confirm this theory. Fine-grained spinel (average grain size under micron), which presents an interface reaction at low stresses, is studied by TEM nano-scale microanalysis. The cation ratio Al/Mg variation is well described. It varies from 2.34 at grain boundary to 2.10 at 100 nm inside the grain. The ratio Al/O shows no variation across the grain boundary and suggests that the stoichiometry defect observed is due to an excess of magnesium vacancies located at the grain boundary.

Conversion of N2O to N2 on MgO (001) surface with vacancy: A DFT study

AbstractThe adsorption and decomposition of N2O at regular and defect sites of MgO (001) surface have been studied using cluster models embedded in a large array of point charges (PCs) by DFT/B3LYP method. The results indicate that the MgO (001) surface with oxygen vacancies exhibits high catalytic reactivity toward N2O adsorptive‐decomposition. It is different from the regular MgO surface or the surface with magnesium vacancies. Much elongation of O‐N bond of N2O after adsorption at oxygen vacancy site with O end down shows that O‐N bond has been broken with concurrent production of N2, leaving a regular site instead of the original oxygen vacancy site (F center). The MgO (001) surface with magnesium vacancies hardly exhibits catalytic reactivity. It can be concluded that N2O dissociation likely occurs at oxygen vacancy sites of MgO (001) surface, which is consistent with the generally accepted viewpoint in the experiments. The potential energy surface (PES) reflects that the dissociation process of N2O does not virtually need to surmount a given energy barrier.

S Adsorption at Regular and Defect Sites of the MgO (001) Surface: Cluster Model Study at DFT Level

We have studied the adsorption of sulfur at regular and defect sites of the MgO (001) surface using cluster models embedded in a large array of point charges by the density functional method. The calculated results indicate that it is a chemical adsorption regarding sulfur at both the regular site and the defect site of the MgO (001) surface. Especially for sulfur adsorbed at different oxygen vacancy sites (F, F + and F 2+ centers) and different magnesium vacancy sites (V, V - and V 2- centers), it has very large adsorption energies, which reflects the fact that the MgO (001) surface with the vacancies is an excellent adsorbent for sulfur adsorption. Besides, we find that the adsorbed sulfur is almost inserted into the lattice for sulfur adsorbed at the magnesium vacancy site of the MgO (001) surface. The adsorption energy of sulfur on the MgO (001) surface with magnesium vacancies is much larger when compared to that on the MgO (001) surface with oxygen vacancies. At the same time, it is also found that the S behaves as an electron acceptor except that it is adsorbed at the magnesium vacancy site behaving as an electron donor.

THEORETICAL STUDY OF N2O ADSORPTION AND DECOMPOSITION AT REGULAR AND DEFECT SITES OF MgO (001) SURFACE

The adsorption of N 2 O at regular and defect sites of MgO (001) surface has been studied systematically using cluster models embedded in a large array of point charges by density functional method. The calculated results show that the MgO (001) surface with oxygen vacancies exhibits high catalytic reactivity toward N 2 O adsorptive-decomposition at variance with the regular MgO surface or the surface with magnesium vacancies. Much elongation of O–N bond of N 2 O after adsorption at oxygen vacancy site with O end of N 2 O down indicates that O–N bond has been broken with concurrent production of N 2, leaving a regular site instead of the original oxygen vacancy site (F center). Besides, the MgO (001) surface with magnesium vacancies hardly exhibits catalytic reactivity. It can be concluded that N 2 O dissociation is likely occurred at MgO (001) surface oxygen vacancy sites, which is consistent with the generally accepted viewpoint in the experiment.