Magnesium Vacancies Research Articles

Fast-neutron-induced and as-grown structural defects in magnesium aluminate spinel crystals with different stoichiometry

Several hole-type paramagnetic defects (a hole localized at a regular oxygen ion near charged structural defect/defects) have been revealed in fast-neutron-irradiated MgO⋅2.5Al2O3 single crystals using the EPR method. Three of them (a dominant V1 and small amount of V2 and V22) were recently revealed in a neutron-irradiated stoichiometric MgAl2O4, while a novel V4 center, ascribed to a hole in a form of O− ion nearby a complex of a magnesium vacancy and a positively charged antisite defect (V4 ≡ O–‑VMg‑Al|Mg), was created by fast neutrons only in a nonstoichiometric single crystal. The pulse annealing of the EPR signal of these centers was compared to that of radiation induced optical absorption in the same crystals. The EPR and optical characteristics of the holes localized near as-grown structural defects in X-ray irradiated 1:1 and 1:2.5 Mg‑Al spinel crystals were compared as well. The tentative scenario of the thermal annealing process of neutron-induced defects (hole-type and complementary electron F-type ones) in MgO⋅2.5Al2O3 was considered.

Энергетическое обоснование образования димеров иттербия в монокристаллах форстерита

AbstractA structural computer simulation of ytterbium-containing forsterite crystals has been carried out. Atomistic simulation is performed using the GULP 4.1 software package (General Utility Lattice Program). Different mechanisms of ytterbium dissolution in forsterite crystals are considered, and the dissolution energies of isolated defects, as well as charged and neutral clusters of various configurations, are calculated. The results of the calculation manifest that the formation of ytterbium clusters with a magnesium vacancy gives a significant gain in the dissolution energy. The formation of neutral clusters (dimers) in the M1 site (Yb_Mg1ν_Mg1Yb_Mg1) leads to an energy reduction by 1.7 eV compared with the statistical distribution of defects. As a result of simulation, it is shown the energetic substantiation of the formation of ytterbium dimers in forsterite crystals. A model of the most energetically favorable center in the M1 site is proposed—a dimer, consisting of a pair of trivalent ytterbium ions with a magnesium vacancy between them, forming a chain parallel to the crystallographic axis c .

Structure and spin glass behavior in La0.77Mg0.23-x□xMnO3 (0 ≤ x ≤ 0.2) manganites

We have synthesized the vacancy manganites La0.77Mg0.23-x□xMnO3 powders, with x = 0, 0.05, 0.1, 0.15 and 0.2 by the solid state reaction method. X-ray diffraction analysis using Rietveld refinement has proven that all the compounds under investigation crystallize in the orthorhombic structure with Pnma space group. The result of Rietveld refinements shows that the magnesium vacancies modify the structural parameters such as the MnO bond length, the MnOMn angles and the volume. Zero-field-cooled and Field-cooled thermomagnetic curves and the thermal variation of AC-susceptibility show a paramagnetic-ferromagnetic transition for both compounds and exhibit the onset of a spin glass-like state at lower temperatures. The increase of the magnesium-vacancy content lowered the Curie magnetic transition temperature (TC) from TC = 140.82 K for x = 0 to TC = 128.22 K for x = 0.2. The irreversibility temperature observed from the ZFC and FC magnetization splitting is also modified from 131.65 K for x = 0 –118.78 K for x = 0.20. The irreversibility is associated to the presence of ferromagnetic clusters and spin-glass phase like behavior.

Effect of the interfacial O and Mg vacancies on electronic structure and transport properties of the FeRh/MgO/FeRh (0 0 1) magnetic tunnel junction: DFT calculations

The electronic, magnetic and transport properties of oxygen or magnesium vacancies at the FeRh/MgO/FeRh (001) magnetic tunnel junction are studied within first principles. Configurations with one O or Mg vacancy per C(2×2) surface unit cell, which is located in the MgO interfacial layers, are investigated. We observed that the O and Mg vacancies defect have a very little influence on the magnetic state of the spacer. Very interestingly, the Fe atoms exhibit an enhanced magnetic moment in the case of Mg-vacancy, this latter was found to decrease in the case of O-vacancy. The variations in the spin polarization and magnetic moment values for Fe and Rh atoms at the interface were found to be larger in presence of Mg vacancy. An analysis of the charge densities of our systems was also performed; large variations in the Mg-vacancy system were observed. This affects more the t2g states of the interfacial Fe atom. Moreover, we present an ab initio calculated transmission and I-V characteristics for FeRh/MgO/FeRh (001) magnetic tunnel junction and we compare results to those of O and Mg-vacancy at the interface using the TRANSIESTA code, which combines the DFT electronic structure calculations with the non-equilibrium Green function formalism (NEGF) for transport properties. The results show that the zero-bias minority spin transmission is much larger than the majority spin transmission for all structures. In all systems and for all magnetic configurations, minority spin currents are higher than majority spin ones, this means that transport properties are, mainly, determined by minority spin channel.

Dimer self-organization of impurity ytterbium ions in synthetic forsterite single crystals

Paramagnetic centers formed by impurity Yb3+ ions in synthetic forsterite (Mg2SiO4) grown by the Czochralski technique are studied by X-band CW and pulsed EPR spectroscopy. These centers are single ions substituting magnesium in two different crystallographic positions denoted М1 and М2, and dimer associates formed by two Yb3+ ions in nearby positions М1. It is established that there is a pronounced mechanism favoring self-organization of ytterbium ions in dimer associates during the crystal growth, and the mechanism of the spin–spin coupling between ytterbium ions in the associate has predominantly a dipole–dipole character, which makes it possible to control the energy of the spin–spin interaction by changing the orientation of the external magnetic field. The structural computer simulation of cluster ytterbium centers in forsterite crystals is carried out by the method of interatomic potentials using the GULP 4.0.1 code (General Utility Lattice Program). It is established that the formation of dimer associates in the form of a chain parallel to the crystallographic axis consisting of two ytterbium ions with a magnesium vacancy between them is the most energetically favorable for ytterbium ions substituting magnesium in the position М1.

Effect of oxygen and magnesium vacancies on the d0 magnetism of C-monodoped MgO using ab initio method

The energetics, electronic structure and magnetism of C-monodoped MgO are investigated using first-principles GGA + U calculations. The results reveal that C induces magnetism in MgO when doped at the O and interstitial sites but becomes nonmagnetic when doped at Mg site. The calculated defect formation energies show that C prefers the Mg site as compared with the O and interstitial sites under O-rich condition. The interaction of native defects (Mg and O vacancies) with dopant C is well studied in detail and their magnetic coupling are discussed in terms of electronic structures and spin densities. It is found that the native defect play an important role on the magnetism formation of C-monodoped MgO system. In agreement with previous experimental works, the Mg vacancy mostly can promote the magnetism formation of C-anion-doped MgO system, and the O vacancy mostly can suppress the magnetism formation. However, when the native defect and dopant C are nearest neighbor, the Mg vacancy will weaken the magnetism formation and the O vacancy will completely suppress the magnetism formation. For C doped the interstitial site, in presence of Mg vacancy completely suppresses but not promotes the magnetism formation. For the C doped the interstitial site in presence of O vacancy, the defect system has lower defect formation energy compared with the one in absence of O vacancy and still holds stable magnetism. Our results indicate that the magnetism of MgO with composite defect is a common outcome of Hund's coupling, hybridization, Jahn-Teller distortion and compensating effect.

Pressure dependence of electrical conductivity in forsterite

AbstractElectrical conductivity of dry forsterite has been measured in muli‐anvil apparatus to investigate the pressure dependence of ionic conduction in forsterite. The starting materials for the conductivity experiments were a synthetic forsterite single crystal and a sintered forsterite aggregate synthesized from oxide mixture. Electrical conductivities were measured at 3.5, 6.7, 9.6, 12.1, and 14.9 GPa between 1300 and 2100 K. In the measured temperature range, the conductivity of single crystal forsterite decreases in the order of [001], [010], and [100]. In all cases, the conductivity decreases with increasing pressure and then becomes nearly constant for [100] and [001] and slightly increases above 7 GPa for [010] orientations and a polycrystalline forsterite sample. Pressure dependence of forsterite conductivity was considered as a change of the dominant conduction mechanism composed of migration of both magnesium and oxygen vacancies in forsterite. The activation energy (ΔE) and activation volume (ΔV) for ionic conduction due to migration of Mg vacancy were 1.8–2.7 eV and 5–19 cm3/mol, respectively, and for that due to O vacancy were 2.2–3.1 eV and −1.1 to 0.3 cm3/mol, respectively. The olivine conductivity model combined with small polaron conduction suggests that the most part of the upper mantle is controlled by ionic conduction rather than small polaron conduction. The previously observed negative pressure dependence of the conductivity of olivine with low iron content (Fo90) can be explained by ionic conduction due to migration of Mg vacancies, which has a large positive activation volume.

Optical and fluorescence properties of MgO nanoparticles in micellar solution of hydroxyethyl laurdimonium chloride

The critical micelle concentration of hydroxyethyl laurdimonium chloride (HY) is estimated in this Letter. MgO exhibited substantial absorbance and fluorescence in presence of HY micelles. The optical properties of MgO were utilized to estimate the critical micelle concentration of a surfactant. The signals of spectra were assigned and explained in terms of electron transitions produced by magnesium and oxygen vacancies of different coordinations. The CMC values of HY evaluated by absorbance, fluorescence and specific conductivity were found to be 9.8, 8.0 and 2.7mM, respectively. The synthesized MgO nanoparticles were characterized by XRD, TEM, UV and fluorescence spectroscopy.

The Mechanism of Intragranular Acicular Ferrite Nucleation Induced by Mg-Al-O Inclusions

The features of inclusion and microstructure for carbon structural steel containing Mg-Al-O inclusions were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). It can be seen that, in Mg-Al-O inclusions, the elements of Mn, Si, and S coexist, and their central mole ratio of Mg/Al varies in a wide range. This value for most inclusions is larger than 0.5, which suggests the formation of solid solution between MgAl2O4and MgO. After etching, the typical microstructure of intragranular acicular ferrites is observed, which is due to the nucleation effect induced by Mg-Al-O inclusions. From the SEM-EDS mapping images, it is found that the element of sulfur accumulates on the periphery of nucleation inclusion. Moreover, line EDS analysis hints that Mn-depletion zone (MDZ) exists in steel matrix, which is adjacent to the complex inclusion. Combined with the theoretical analysis, this phenomenon can be explained by the absorption of Mn due to the magnesium vacancy in MgAl2O4, and this MDZ promotes the nucleation of intragranular acicular ferrite. Through statistical analysis of SEM images for microstructure, the probabilistic nature of inducing nucleation effect is revealed. These results may be helpful to clarify the nature of oxide metallurgy.

The importance of defining chemical potentials, substitution mechanisms and solubility in trace element diffusion studies: the case of Zr and Hf in olivine

The diffusion, substitution mechanism and solubility limits of Zr and Hf in synthetic forsterite (Mg2SiO4) and San Carlos olivine (Mg0.9Fe0.1)2SiO4 have been investigated between 1,200 and 1,500 °C as a function of the chemical potentials of the components in the system MgO(FeO)–SiO2–ZrO2(HfO2). The effect of oxygen fugacity and crystallographic orientation were also investigated. The solubilities of Zr in forsterite are highest and diffusion fastest when the coexisting three-phase source assemblage includes ZrSiO4 (zircon) or HfSiO4 (hafnon), and lower and slower, respectively, when the source assemblage includes MgO (periclase). This indicates that Zr and Hf substitute on the octahedral sites in olivine, charge balanced by magnesium vacancies. Diffusion is anisotropic, with rates along the crystal axes increasing in the order a < b < c. The generalized diffusion relationship as a function of chemical activity (as \(a_{{{\text{SiO}}_{2} }}\)), orientation and temperature is: \(logD_{\text{Zr}} = \frac{1}{4}loga_{{{\text{SiO}}_{2} }} + logD_{0} - \left( {\frac{{368 \pm 17\;{\text{kJ}}\;{\text{mol}}^{ - 1} }}{{2.303\;{\text{RT}}}}} \right)\) where the values of log D 0 are −3.8(±0.5), −3.4(±0.5) and −3.1(±0.5) along the a, b and c axes, respectively. Most experiments were conducted in air (fO2 = 10−0.68 bars), but one at fO2 = 10−11.2 bars at 1,400 °C shows no resolvable effect of oxygen fugacity on Zr diffusion. Hf is slightly more soluble in olivine than Zr, but diffuses slightly slower. Diffusivities of Zr in experiments in San Carlos olivine at 1,400 °C, fO2 = 10−6.6 bars are similar to those in forsterite at the same conditions, showing that the controls on diffusivities are adequately captured by the simple system (nominally iron-free) experiments. Diffusivities are in good agreement with those measured by Spandler and O’Neill (Contrib Miner Petrol 159:791–818, 2010) in San Carlos olivine using silicate melt as the source at 1,300 °C, and fall within the range of most measurements of Fe–Mg inter-diffusion in olivine at this temperature. Forsterite–melt partitioning experiments in the CaO–MgO–Al2O3–SiO2–ZrO2/HfO2 show that the interface concentrations from the diffusion experiments represent true equilibrium solubilities. Another test of internal consistency is that the ratios of the interface concentrations between experiments buffered by Mg2SiO4 + Mg2Si2O6 + ZrSiO4 or Mg2SiO4 + ZrSiO4 + ZrO2 (high silica activity) to those buffered by Mg2SiO4 + MgO + ZrO2 (low silica activity) agree well with the ratios calculated from thermodynamic data. This study highlights the importance of buffering chemical potentials in diffusion experiments to provide constraints on the interface diffusant concentrations and hence validate the assumption of interface equilibrium.

Simulation of forsterite protonation by the interatomic potential method

The interatomic potential method in an ionic approximation was used to model the protonation of forsterite crystals. The formation of isolated OH− groups in iron-free and iron-bearing crystals and neutral clusters of protonated cation vacancies was considered. It was shown that the presence of trivalent impurities may significantly facilitate protonation processes owing to their reduction to a divalent state or formation of clusters with cation vacancies. In most cases, charge balancing of hydrogen-bearing defects by magnesium vacancies is energetically favorable over that involving silicon vacancies.

Site-specific hydrogen diffusion rates in forsterite

Synthetic hydroxylated titanium-bearing and undoped MgO-buffered forsterite crystals were heated at atmospheric pressure in air at temperatures from 800 to 1200 °C to determine hydrogen diffusion as a function of the type of point-defect mechanism by which the hydroxyl is incorporated. OH-stretching bands, measured by infrared spectroscopy, were assigned to the four substitution mechanisms: Ti-clinohumite point defects, silicon and magnesium vacancies, and point defects associated with trivalent cations. In the experiments at 800 to 1000 °C, hydrous defects related to trivalent cations and Mg-vacancies disappear first in the Ti-doped forsterite, followed by the hydrous defects related to Ti and a fraction of the hydrated Si-vacancies. Measured bulk diffusion coefficients for hydrogen exchange related to the latter process are one order of magnitude slower than previously reported and with higher activation energy (296±14 kJmol−1). After the elimination of the defects related to Ti, the hydroxyl concentration related to Si-vacancies remained constant for the duration of the experiments. This observation is in agreement with the very slow diffusivity of hydrogen in Si-vacancies measured in the undoped MgO-buffered dehydroxylation experiments at higher temperatures (from 1000 to 1200 °C), which were ∼3 orders of magnitude slower than previously reported at 1000 °C, and with an activation energy of 461±11 kJmol−1. Hydrogen diffusion in forsterite is far more complex than previously assumed. This complexity can be used to gain more information on the timescales of the processes causing olivine to lose its structural “water”, but quantitative modeling will require not only the knowledge of the intrinsic diffusivities of the different hydrous defects but also their relative proportions and the possible reactions between them.

Infrared signatures of OH-defects in wadsleyite: A first-principles study

The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (β-Mg2SiO4) are studied, at 0 and 15 GPa, by first-principles calculations based on density functional theory (DFT). Four types of OH-defects are considered: fully protonated magnesium vacancies, fully protonated silicon vacancies, silicon vacancies compensated by a magnesium cation and two protons, and OH-defects associated with the migration of a silicon cation to a normally vacant site, as reported by Kudoh and Inoue (1999). The results suggest that the main absorption band constituted by a doublet (3326 and 3360 cm-1) corresponds to at least two types of OH-defects involving M3 vacancies with protonation of the O1-type O atoms along the O1...O4 edges. The main contribution of the less intense band at 3581 cm-1 is likely related to the partial protonation of a silicon vacancy (protonation of the O3-type oxygen) associated with the migration of the silicon cation to the Si2 site. This assignment is consistent with several experimental constraints: wavenumber and pleochroism of infrared OH-stretching bands, pressure-dependence of the band wavenumber, evidence from X‑ray diffraction of magnesium vacancies in M3 site, and increase of the b/a axial ratio with water content. The integrated absorption coefficients of the corresponding OH-defects are also calculated and thus complement the set of data obtained previously for forsterite and ringwoodite. Absorption coefficients of wadsleyite computed at 0 and 15 GPa indicate that for a precise quantification of the hydrogen content in in situ experiments, one must consider higher absorption coefficients than those determined at 0 GPa after quench. It is also shown that a single theoretical relation can account for the three Mg2SiO4 polymorphs at 0 GPa: Kint = 278.7 ± 18.1 (3810 ± 465 - x), where Kint is the integrated molar absorption coefficient of the OH stretching modes and x is the average wavenumber in cm-1. Absorption coefficients are significantly lower than the general calibrations, the use of which would lead to an underestimation of the water concentrations

Self-organized ferromagnetic nanowires in MgO-based magnetic tunnel junctions

The focus of this study is to examine the distribution of defects and defect-induced properties in MgO-based magnetic tunnel junctions (MTJs). To this end, first-principles calculations were performed to estimate the electronic structures and total energies of MgO with various defects by using the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional. From connections drawn between the calculated results and previously reported experimental data, we propose that self-organized ferromagnetic nanowires of magnesium vacancies can be formed in MgO-based MTJs. This self-organization may provide the foundation for a comprehensive understanding of the conductivity, tunnel barriers and quantum oscillations of MgO-based MTJs. Further experimental verification is needed before firm conclusions can be drawn.

(Invited) Optical and Magnetic Properties of Defective MgO Microcrystals

Micrometer-sized MgO crystals are prepared by solid phase reaction between Mg and B2O3. The MgO microcrystals exhibit efficient photoluminescence (PL) emissions derived from oxygen vacancies, showing broadband laser emission at room temperature. Also, room temperature ferromagnetism is observed in the lasing MgO microcrystals. Ferromagnetic coupling between the surface magnesium vacancies via the surface low-coordinated oxygen atoms is suggested to explain the observed magnetic behavior.

Ultrasounds and microwave-assisted synthesis of mesoporous hectorites

Stable mesoporous delaminated and silicate-bearing hectorites were synthesized at short times (1–2h) varying the pH of the hydrothermal treatment (7 and 10) and using microwaves and ultrasounds during the synthesis. The use of microwaves, ultrasounds and aging pH of 10 improved hectorite crystallinity. Additionally, longer times of ultrasounds involved a better incorporation of magnesium and lithium cations in the octahedral sheet for the delaminated hectorites prepared a pH 7. Delaminated hectorites showed higher BET areas (536–603m2/g) than silicate-bearing hectorites, due to delamination and probably to the presence of magnesium vacancies in the octahedral sheet. The formation of vacancies was favored with the use of microwaves and at short times of ultrasounds. This agrees with the C.E.C. values and the low amounts of lithium obtained for delaminated samples.

Magnetic dilution method upon solving the problem of electron structure of lanthanum gallate doped with chromium, strontium, and magnesium

Magnetic properties of lanthanum gallate doped with chromium, strontium, and magnesium demonstrating electronic–ionic conductivity were studied taking various ratios between the contents of Cr and diamagnetic elements. In the context of the diluted solution model invoking Heizenberg–Dirac–van Vleck theory the fractions of clusters and single chromium atoms were calculated for the series of LaCrxGa1-xO3-δ, La1-0.2xSr0.2xCrxGa1-xO3-δ, La1-0.5xSr0.5xCrxGa1-xO3-δ, LaCrxGa1-1.2xMg0.2xO3-δ, LaCrxGa1-1.5xMg0.5xO3-δ and La1-0.2xSr0.2xCrxGa1-1.2xMg0.2xO3-δ solid solutions (0.01<x<0.10). A substantial influence of the nature and concentration of heterovalent diamagnetic substituent was shown on the quantitative and qualitative composition of chromium atoms clusters incorporating strontium, magnesium, and oxygen vacancies, the clustering directly depending on the Cr:A ratio (A–Sr, Mg).

Diffusion of aluminium in MgO from first principles

We have calculated the diffusivity of aluminium in periclase, MgO, under pressures relevant to deep planetary interiors from first principles. We reconcile differences between experimental migration enthalpies and those obtained with previous theoretical studies by finding a lower energy saddle point for the aluminium atom migration. Previous studies did not recognise a bifurcation at the saddle point. We also explain differences between experimental and theoretical binding enthalpies of an aluminium with a magnesium vacancy. We find that binding enthalpies continuously increase with decreasing aluminium concentrations, such that the difference between experimental and theoretical binding energies can be attributed to differing concentrations. We also find that binding energies increase with pressure as the permittivity decreases. Aluminium therefore not only causes extrinsic vacancy formation but also binds some of them, effectively removing them for magnesium diffusion. We discuss the implications for how other 3+ ions affect diffusion in oxides and silicates.

Isolated Defects in Sr2MgSi2O7: A DFT Study

Abstract The modifications in the crystal and electronic structure due to the Eu2+ luminescent centre and isolated vacancies in the Sr2MgSi2O7(:Eu2+) persistent luminescence material were calculated using the density functional theory (DFT). The exact role of defects in the storage of excitation energy is currently not well understood. They can be formed e.g. due to charge compensation and preparation conditions. The probability of vacancy formation was studied using the total energy of the defect containing host. The most significant structural modifications due to defects in the environment of the Eu2+ luminescent centre were found with the introduction of the strontium vacancy. Electron traps were created by Eu2+ and the strontium as well as oxygen vacancy, whereas the strontium, magnesium and silicon vacancies created also shallow hole traps in the material. The electron traps close to the conduction band can contribute to the persistent luminescence efficiency since they are readily bleached by the thermal energy at room temperature. However, too shallow or deep traps can decrease this efficiency. The role of the intrinsic defects and dopants in the energy storage of the persistent luminescence materials was discussed. PACS: Type pacs here, separated by semicolons.

Calculations of Li adsorption and diffusion on MgO(100) in comparison to Ca

Density-functional theory calculations are used to study the adsorption and growth of small Li clusters on MgO(100). The binding of Li is found to be similar to that of Ca in many respects. Monomers bind to oxygen sites on the terrace and diffuse at room temperature to defect sites. A range of defect sites will bind monomers with increasing energies from oxygen vacancies, steps, kinks, divacancies, and magnesium vacancies, as well as to peroxo species on step edges. The binding strength correlates with the amount of Bader charge transferred from the adsorbing metal to the substrate. Small clusters are found to be highly mobile on the surface at room temperature. In two cases, we have found qualitative differences in the binding energies of Ca and Li which lead to different growth modes. First, Ca binds less strongly to charged oxygen vacancy defects (${\text{F}}^{+}$ centers) than it does on the terrace whereas Li forms a bond with the electron in the vacancy, and is trapped at room temperature. Second, the gas phase Ca dimer distance is longer than the O-O distance on the MgO surface so that epitaxial Ca islands are strained whereas Li is not. These two differences have a profound effect on the growth of clusters. At room temperature, a Ca monomer will not trap at defect sites occupied by another Ca monomer. Thus, Ca atoms diffuse on the terrace until the defects are saturated before nucleating three-dimensional islands. On the other hand, two-dimensional epitaxial Li island nucleate at Li-bound defects. These calculations explain the lower initial heat of adsorption of Li and higher surface coverage as compared to Ca in adsorption microcalorimetry measurements by Farmer et al. [J. Am. Chem. Soc. 131, 3098 (2009)].