Formation Of Cation Vacancies Research Articles

Enhancement of the hole-induced d0-ferromagnetism in ZnO through compensated donor–acceptor complexes: a first-principles study

The presence of spontaneous magnetization in transparent conducting oxides with sufficient holes is an intrinsic property of the wide-band-gap semiconductors. In this paper, to enhance the hole-induced d0-ferromagnetism of transparent conducting oxides, we choose ZnO as a prototype material and study the hole-induced d0-ferromagnetism in ZnO by first-principles methods. Our calculations show that the hole-induced d0-ferromagnetism is limited by the spontaneous formation of intrinsic cation vacancies or extrinsic acceptor doping. To solve this problem, we propose an effective passivated codoping approach to reduce the formation energies of the acceptors by doping the host with passive donor–acceptor complexes. It is demonstrated that the formation energies and polarization energies of VZn or NO in ZnO can be reduced significantly by doping GaZn+NO complexes, while leaving the magnetic moment in these supercells almost unchanged, which finally improve hole-induced d0-ferromagnetism in the host. In principle, the suggested approach to enhance the hole-induced d0-ferromagnetism can also be applied to other transparent conducting oxides.

Ab initio study the effects of Si and Mg dopants on point defects and Y diffusion in YAG

The effects of Si and Mg dopants on point defects and Y diffusion in YAG are studied by the ab inito DFT calculations. Introduction of Si in YAG could largely decrease the formation energies of cation vacancies. It will improve the concentration of cation vacancies and enhance the lattice diffusion. Mg dopant has the effect of reducing O vacancy formation energies. In the codoping condition, the Si and Mg ions shows agglomeration in YAG. It indicates that using both MgO and SiO2 as the sintering aids would form MgO–SiO2 liquid phase, which could enhance the densification rate. The calculated electronic structures show that the Si or Mg dopant does not introduce defect state in the band gap of YAG. The Si shows a strong covalent property and Mg shows a strong ionic property in YAG. The charged states almost have no effect on local structure, electronic structure and effective charge of the dopants. We also studied the effect of Si and Mg dopants on Y diffusion via vacancy mechanism on the Y, Al(tet) and Al(oct) sublattices. The results show that Si and Mg dopants only slightly decrease the barrier energies of Y diffusion, indicating Si and Mg dopants enhance Y diffusion mainly through promoting the formation of cation vacancy or liquid phase.

Elastic moduli of η‐Ta2N3, a tough self‐healing material, via laser ultrasonics

AbstractThe elastic moduli of the dense polycrystalline oxygen‐bearing η‐Ta2N3, a novel hard and tough high‐pressure (HP) material, were measured using the laser ultrasonic technique. The bulk modulus was determined to be B0 = 281(15) GPa which is only ∼11% below that from HP compression measurements. Our value of the shear modulus G0 = 123(2) GPa is below those ones predicted theoretically for model structures. The discrepancies in G0 could be due to a substitution of an‐ ions and the formation of cation vacancies in η‐Ta2N3. Self‐healing behaviour of η‐Ta2N3 by mechanical polishing was observed and confirmed by two independent experimental methods. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures

We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.

Effect of CO2 partial pressure on the cathode lithiation in molten carbonate fuel cells

Abstract Nickel cathode is transformed to lithiated nickel oxide by oxidation and lithiation during the conditioning process for molten carbonate fuel cells. In the lithiation process, the amount of lithium inserted into nickel oxide depends on the oxygen and CO 2 composition and this affects the performance of nickel cathode. In this paper, CO 2 interruption technique was applied to investigate the effects of CO 2 interruption on the lithiation of nickel oxide. During the CO 2 interruption for 24 h in cathode operating at 20 mA/cm 2 , the carbonate ion in electrolyte was decomposed into oxygen and CO 2 . With the additional oxygen on cathode surface, Ni 2+ is oxidized to Ni 3+ with formation of cation vacancy in NiO. The lithium content of cathode increased from 3.0 at.% to 17.4 at.% (over-lithiation) and hence LiNiO 2 phase was formed in the cathode. Cathode surface area is increased by a decrease in NiO particle size with the formation of micropores. The morphological change in cathode enhanced its electrochemical performance in the single cell. Cell voltage of the single cell that has been subjected to CO 2 interruption at 120 mA/cm 2 was enhanced by 300 mV, due primarily to the reduction in the internal resistance from 2.0 to 0.8 Ω cm 2 and also in the charge transfer resistance from 3.0 to 1.1 Ω cm 2 .

Something from Nothing: Enhancing Electrochemical Charge Storage with Cation Vacancies

The performance of electrochemical energy storage devices (e.g., batteries and electrochemical capacitors) is largely determined by the physicochemical properties of the active electrode materials, such as the thermodynamic potential associated with the charge-storage reaction, ion-storage capacity, and long-term electrochemical stability. In the case of mixed ion/electron-conducting metal oxides that undergo cation-insertion reactions, the presence of cation vacancies in the lattice structure can enhance one or more of these technical parameters without resorting to a drastic change in material composition. Examples of this enhancement include the charge-storage properties of certain cation-deficient oxides such as γ-MnO2 and γ-Fe2O3 relative to their defect-free analogues. The optimal cation-vacancy fraction is both material- and application-dependent because cation vacancies enhance some materials properties at the expense of others, potentially affecting electronic conductivity or thermal stability. Although the advantages of structural cation vacancies have been known since at least the mid-1980s, only a handful of research groups have purposefully integrated cation vacancies into active electrode materials to enhance device performance. Three protocols are available for the incorporation of cation vacancies into transition metal oxides to improve performance in both aqueous and nonaqueous energy storage. Through a processing approach, researchers induce point defects in conventional oxides using traditional solid-state-ionics techniques that treat the oxide under appropriate atmospheric conditions with a driving force such as temperature. In a synthetic approach, substitutional doping of a highly oxidized cation into a metal-oxide framework can significantly increase cation-vacancy content and corresponding charge-storage capacity. In a scaling approach, electrode materials that are expressed in morphologies with high surface areas, such as aerogels, contain more defects because the increased fraction of surface sites favors the formation of cation vacancies. In this Account, we review studies of cation-deficient electrode materials from the literature and our laboratory, focusing on transition metal oxides and the impact cation vacancies have on electrochemical performance. We also discuss the challenges and limitations of these defective structures and their promise as battery materials.

Effects of Mg substitution on microstructure and electrical properties of NiMn2−xMgxO4 NTC ceramics

Abstract

Correlation between anionic substitution and structural properties in AlCr(O xN 1−x) coatings deposited by lateral rotating cathode arc PVD

The influence of oxygen content on the properties of cathodic arc-deposited AlCr(O xN 1−x) coatings has been studied. All samples were prepared in a nitrogen-rich mixture of N 2 and O 2 at 550 °C using lateral rotating arc cathodes (LARC) technology together with a pulsed bias voltage. The obtained coatings were characterized by various techniques including XRD, EPMA, TEM, pin-on-disk wear tests and nanoindentation. The results obtained allow to classify the coatings into three groups with respect to their microstructure, mechanical properties and oxygen content, x. For the first group of samples with x ≤ 0.6, single-phase films of (Al,Cr)O xN 1-x with fcc lattice were obtained, with well-developed columnar structure and a hardness of 30 to 33 GPa. In the second group, a diffuse columnar structure was observed while the fcc lattice was still present despite the large proportion of oxygen, 0.6 < x ≤ 0.97, and the observed hardness decreased to 25 GPa. No amorphous phase was detected in this group as confirmed by TEM. The simulation of XRD patterns of nitride lattices with oxygen incorporation allowed to suggest the formation of cation vacancies in the structure of the investigated oxynitride coatings. The third group is formed by coatings with x > 0.97, where a well-crystalline α-(Al,Cr) 2O 3 corundum phase was observed and the hardness increased again to 28 GPa. Our results indicate that the second group of coatings is metastable and after heat treatment transforms to a composite of cubic oxynitride and corundum oxide. Both friction and wear of samples from the entire investigated compositional range were studied at room temperature and 600 °C. The low wear rates observed for the oxynitride coatings underline their potential for use in turning and milling applications.

Influence of Sn 4+ and Sn 4+/Mg 2+ doping on structural features and visible absorption properties of α-Fe 2O 3 hematite

Pure, Sn-doped and Mg/Sn co-doped α-Fe 2O 3 hematite samples were synthesized by precipitation process. Fe 2O 3 is the most popular red mineral pigment which is used largely in traditional ceramics, tar and concrete. The compounds were characterized by powder X-ray diffraction (XRD), scanning transmission electronic microscopy (energy dispersive X-ray cartography), Mössbauer spectroscopy, magnetic investigations versus temperature and visible–NIR spectroscopy. Both 57Fe and 119Sn Mössbauer analyses combined with rietveld XRD refinements are the ideal techniques to characterize tin–iron oxides. Hence, thanks to these techniques it was shown how the synthesis temperature influences directly the grain size and the dopants concentration limit which can be incorporated into the host hematite matrix. The stabilization of these tetravalent and divalent dopants into the hematite framework leads to reduce the crystal growth and to limit the (AF) ordering due to the formation of cationic vacancies. The study of the Morin magnetic transition emphasizes this demonstration. In a second part, the influence of the dopants incorporation on the material color was investigated in order to show which key parameters allow improving the red color saturation of iron oxides. In order to improve the red color of the hematites, it was shown that the introduction of cationic vacancies—limiting the octahedral distortion thanks to the interruption of the dissymmetric metal–metal orbital coupling—is the key point. Vacancies are created by Sn 4+, doping for an increase of the introduced Sn 4+ concentration; it acts to the detriment of the color saturation.

Defect studies with positrons: What could we learn on III-nitride heterostructures?

We have applied positron annihilation spectroscopy to study 400 – 500 nm InGaN-based LED structures, as well as InGaN and AlGaN materials with varying In and Al contents. We find that the effect of adding In to GaN on the annihilation parameters obeys the Vegard's law, while in the case of AlGaN the possible effect of Al is completely screened by efficient formation of cation vacancies. The results obtained in the InGaN LED structures are indistinguishable from defect-free GaN, suggesting that the positrons annihilate preferentially in the barriers of the MQW system.

Cation-site intrinsic defects in Zn-doped CdTe

The properties of the cation vacancy and the Te antisite, two dominant defects in CdTe and Cd1-xZnxTe alloys grown in Te-rich conditions, are examined using first-principles calculations. First, the structure, electronic levels, and migration paths of V-Cd and Te-Cd in CdTe are studied in detail. Additionally, we analyze the evolution of the stability and electronic properties in Cd1-xZnxTe alloys, taking into account both the role of alloying in the position of the ionization levels and its effects on the equilibrium concentration of those two defects. It is shown that the formation of cation vacancies becomes progressively more favorable as x increases, whereas Te antisites become less stable, backing the trend towards p-type conductivity in dilute Cd1-xZnxTe.

Effect of the oxygen excess on the properties of weakly doped La1−x Ca x MnO3 lanthanum manganites

The effect of an oxygen excess δ on the magnetic and electrical properties of La1−x Ca x MnO3+δ (x=0.10–0.15) has been studied over wide ranges of temperatures and magnetic fields. As δ increases, the magnetic ordering temperature Tcdecreases by 70–90 K, the magnetoresistance increases (the electrical resistivity decreases by a factor of up to 104 in a field of 9 T), and the effective moment μeff of the paramagnetic susceptibility substantially exceeds the theoretical value at temperatures two to four times higher than T c and undergoes a jump, just as the activation energy of electrical resistivity, at T∼270 K. These results are attributed to the formation of cation vacancies, the localization of electrons in their vicinity with the subsequent formation of magnetic clusters, tunneling (or hopping) of carriers among them, changes in the sizes of clusters with variations in the temperature and magnetic field strength, the onset of frustrations initiated by the competition among different types of exchange interaction, and the dependence of the cluster parameters on the annealing conditions. Annealing of the oxygen-excess samples at high temperatures in vacuum (above 1100°C) restores the samples to a nearly initial state with the magnetic and magnetotransport properties characteristic of weakly doped manganites, as a result of the removal of cation vacancies.

The effect of non-stoichiometry on the high-temperature behaviour of MgAl2O4 spinel

AbstractThe effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect spinel with composition Mg0.4Al2.4☐0.2O4 was investigated by X-ray diffraction, in situ, at temperatures up to 1273 K. No evidence of symmetry violations from the Fd3m evenat the highest temperature were noted. The volume thermal expansion is markedly less than that of stoichiometric MgAl2O4 spinel, regardless of the degree of inversion. The u oxygen atomic coordinate remains constant throughout the temperature range investigated, with the M–O an dT–O bond lengths showing identical rates of expansivities. An analysis of the evolution of polyhedral volumes with temperature indicates that at 1273 K the octahedron inflates by 0.099 Å3 and the tetrahedron by 0.056 Å3. The expansion of the octahedron is significantly greater than in stoichiometric MgAl2O4 spinel, whereas the expansion of the tetrahedron is similar. The results demonstrate that an excess of Al in the spinel structure accompanied by the formation of cation vacancies strongly affect an important thermodynamic property, in this case, thermal expansion. Such an effect must be considered for those phases stable inthe Earth’s mantle where 4–5 wt.% Al2O3 is thought to be present.

Effects of non-stoichiometry on the spinel structure at high pressure: Implications for Earth’s mantle mineralogy

A non-stoichiometric sample of spinel with composition T(Mg 0.4Al 0.6) M(Al 1.8□ 0.2)O 4 was investigated by single-crystal X-ray diffraction in situ up to about 8.7 GPa using a diamond anvil cell. The P( V) data were fitted using a third-order Birch–Murnaghan equation of state and the unit-cell volume V 0, the bulk modulus K T0 and its first pressure derivative K′ were refined simultaneously providing the following coefficients: V 0 = 510.34(6) Å 3, K T0 = 171(2) GPa, K′ = 7.3(6). This K T0 value represents the lowest ever found for spinel crystal structures. Comparing our data with a stoichiometric and natural MgAl 2O 4 (pure composition) we observe a decrease in K T0 by about 11.5% and a strong increase in K′ by about 33%. These results demonstrate how an excess of Al accompanied by the formation of significant cation vacancies at octahedral site strongly affects the thermodynamic properties of spinel structure. If we consider that the estimated mantle composition is characterized by 3–5% of Al 2O 3 this could imply an Mg/Al substitution with possible formation of cation vacancies. The results of our study indicate that geodynamic models should take into account the potential effect of Mg/Al substitution on the incompressibility of the main mantle-forming minerals (olivine, wadsleyite, ringwoodite, Mg-perovskite).

Molecular Dynamics Study of Oxygen Vacancy Diffusion in BaTiO&lt;sub&gt;3&lt;/sub&gt; Doped with Rare Earth Ions

Atomistic mechanisms that restrain diffusion of oxygen vacancies in BaTiO3 doped with rare earth ions as donors were analyzed using molecular dynamics simulation. It was confirmed that formation of cation vacancies and reduction of lattice volume are sources of resistance for the diffusion. The cation vacancies trap the oxygen vacancies at the nearby O2- sites by an attractive force associated with Coulombic interaction. In contrast, the rare earth ions repel the oxygen vacancies, which migrate via O2- sites, and accelerate the diffusion. This is one of the factors that determine the restraint behavior of the diffusion, which depends on the type of rare earth ion in BaTiO3-based materials.

Effect of cation deficiency on the structure and properties of layered lanthanum barium ferrocuprate

Cation-deficient LaBaCuFeO5+δ samples have been synthesized, and their unit cell parameters have been determined. The thermal expansion, electrical conductivity, and thermo emf of these samples have been measured in air in the temperature range 300–1100 K. The oxides, like LaBaCuFeO5+δ, are p-type semiconductors. The lanthanum deficiency leads to an increase in the size of the LaBaCuFeO5+δ unit cell and has practically no effect on its thermo-and electrophysical properties. The formation of cationic vacancies in the-[Ba(Cu,Fe)2O5]∞-blocks of the LaBaCuFeO5+δ phase leads to the tetragonal distortion of its cubic structure and a decrease in the thermal expansion coefficient and conductivity of the samples. The decrease in conductivity is mainly caused by an increase in the energy of transfer of charge carriers, holes, in the-(Cu,Fe)O2-layers of LaBaCuFeO5+δ.

Structure and electrical and transport properties of cation-deficient samples of perovskite ferrocuprates RBaCuFeO5 + δ (R = Y, La)

The structure, thermal expansion, and electrical properties of cation-deficient ferrocuprates RBaCuFeO5 + δ (R = Y, La), which are p-type semiconductors, are investigated. The linear thermal expansion coefficient of the yttrium-barium ferrocuprate decreases upon formation of vacancies in the A sublattice (Y, Ba), while the unit cell parameters remain practically unchanged upon the formation of vacancies in different sublattices (Y, Ba, Cu/Fe) of the crystal structure. From analyzing the results of the thermopower measurements, it is assumed that the YBaCuFeO5 + δ ferrocuprate is actually a “nanocomposite” consisting of nanoregions of the Y2Cu2O5 and BaFeO3 − δ phases. The lanthanum deficiency brings about an increase in the LaBaCuFeO5 + δ unit cell while leaving very nearly unaffected its linear thermal expansion coefficient and electrical properties. The formation of cation vacancies in extended [Ba(Cu,Fe)2O5] blocks of the LaBaCuFeO5 + δ phase gives rise to tetragonal distortion of its cubic structure, a decrease in the linear thermal expansion coefficient, and an increase in the electrical resistivity of the samples, the latter resulting primarily from an increase in the carrier transport energy in (Cu,Fe)O2 layers of LaBaCuFeO5 + δ.

Influence of A-site nonstoichiometry on sintering, microstructure and electrical properties of (Bi 0.5Na 0.5)TiO 3 ceramics

Lead-free (Bi 0.5Na 0.5) 1+ x TiO 3 ceramics ( x = −0.02, −0.01, −0.005, 0, 0.005 and 0.01) were prepared by ordinary sintering. The effect of A-site stoichiometry on the densification, microstructure, dielectric properties, high-temperature impedances, and piezoelectric properties was explored. It was found that the high conductivity of (Bi 0.5Na 0.5)TiO 3 (BNT) ceramics should be mainly attributed to the formation of A-site cation vacancies during sintering. Improved physical and electrical properties can be achieved in the sample with A-site cation excess. The control of the stoichiometry proves to be an effective way to improve BNT ceramics for possible application.

Characterization of potassium lithium niobate (KLN) ceramic system

Abstract The room temperature stability region of the tetragonal tungsten bronze-type potassium lithium niobate (K 3 Li 2 Nb 5 O 15 , KLN) has been determined by X-ray phase analysis on ceramic samples synthesized by solid state reaction in the composition range of [K 2 O] = 28–33 mol%, [Li 2 O] = 12.5–20.5 mol% and [Nb 2 O 5 ] = 50.5–55.5 mol%. Lattice parameters and temperature dependence of the dielectric constants were measured on single phase samples. The axial a / c ratio of the cell parameters, the density and the Curie temperature of the ferroelectric samples have been found predominantly characteristic for the Nb 2 O 5 content, varying progressively between 3.11–3.14, 4.431–4.596 g/cm 3 and 492–460 °C, respectively. The results are discussed from the points of view of defect chemistry and phase stability. The concentration of intrinsic defects formed primarily by antisite Nb atoms at Li sites has been estimated on the basis of “alkali cation vacancy formation model”.

Preparation and electrical properties of Fe xCu 0.10Ni 0.66Mn 2.24− xO 4 (0 ≤ x ≤ 0.90) NTC ceramics

In this paper, the dense Fe x Cu 0.10Ni 0.66Mn 2.24− x O 4 (0 ≤ x ≤ 0.90) ceramics with accurate stoichiometry are prepared by the route of solid-state coordination reaction. The effect of Fe addition on the electrical properties of the ceramics has been investigated by powder X-ray diffraction (XRD), electrical measurement and thermogravimetric analysis. With Fe content x increasing from 0 to 0.45, the resistivity remains almost unchanged, whereas the thermal constant B decreases drastically. With a further increase in x from 0.45 to 0.90, both the resistivity and the thermal constant B increase. With a rise in x from 0 to 0.9, the aging coefficient drops from 19.9 to 0.6%. The remarkable improvement of the electrical stability can be attributed to the mechanism that substitution of Fe 3+ for Mn 2+ and Mn 3+ restrains the formation of cation vacancies in spinel lattice, thus the modification of cation distribution and the consequent aging under thermal stress is greatly alleviated.