Concentration Of Cation Vacancies Research Articles

Dramatic effects of excess oxygen on physical properties and crystal structure of La 0.95Sr 0.05MnO y single crystal

We have examined the effects of excess oxygen on the magnetization, electrical transport properties and crystal structure of La 0.95Sr 0.05MnO y . An antiferromagnetic insulator phase confirmed in the as-grown crystal at low temperatures changed to a ferromagnetic insulator phase after the introduction of excess oxygen. The ferromagnetic transition temperature, T C , systematically increased with increasing oxygen content, y , due to an increase in the mean valence of manganese. However, the T C of crystals with excess oxygen was lower than that of the La 1− x Sr x MnO 3.00 having identical manganese valence. This is considered to be a result of the competition among the mean valence of manganese, the concentration of cation vacancies, and the mean ionic radius of cations at the A-site.

Highly cation-deficient manganese perovskite, La 1− xMn 1− yO 3 with x= y

Oxidative ( δ>0) nonstoichiometry in the perovskite ‘LaMnO 3+ δ ’ has been known to be manifested not with O interstitials but rather with cation vacancies of equal amounts at the two cation sites, La and Mn, i.e. La 1− x Mn 1− y O 3 with x= y. Here, we report the fabrication of samples with record-high cation-vacancy concentrations ( x>0.12 or δ>0.4) by means of a variety of high-pressure oxygenation techniques. Linear (negative) dependence of the cell volume on x was observed within the whole x range investigated, down to 56.9 Å 3 (per formula unit) for a sample oxygenated at 5 GPa and 1100 °C using Ag 2O 2 as an excess oxygen source. With increasing degree of cation deficiency in La 1− x Mn 1− x O 3, the ferromagnetic transition temperature was found to follow a bell shape with respect to x exhibiting a maximum of ∼250 K about x≈0.1. For moderately oxygenated samples large magnetoresistance effect was evidenced.

The impact of microwave radiation on radiative recombination of CdS

The paper considers the impact of a powerful short-term microwave radiation ( f = 2.45 GHz) on defect states of CdS monocrystals with the aid of investigation of the luminescence spectra in a domain of 0.6–2.5 μm at 77 K. It has been found experimentally that the microwave processing affects the luminescence characteristics of the crystals because of a change in concentration of cation vacancies (centers of photosensitivity) and in concentration of centers of fast (nonradiative) recombination. The results of the investigations point to the prospects in applying the microwave radiation in technological processes for the control of photosensitivity and of the luminescence spectra of cadmium chalcogenide.

Role of Chloride Ion in Passivity Breakdown on Iron and Nickel

The effects of on the electronic properties of the n-type passive film on Fe and the p-type passive film on Ni in pH 8.5 buffer solution were investigated using photocurrent spectroscopy and Mott–Schottky analysis in order to elucidate the mechanism of passivity breakdown on metals. Metastable pitting events were observed in the potentiostatic current transients (chronoamperograms) for Fe measured at when 0.1 M of NaCl was added to the solution, demonstrating that induces passivity breakdown. However, the Mott–Schottky analysis revealed chloride ion, present at a concentration of 0.10 M, did not change the concentration of oxygen vacancies in the passive film on Fe, indicating that the oxygen vacancy is not responsible for the passivity breakdown. It was observed by Mott–Schottky analysis that as the concentration of in the solution increased, the concentration of cation vacancy in the passive film on Ni also increased, which is in concert with the view that metal vacancies are responsible for the breakdown of passivity on nickel. Finally, by using electrochemical impedance spectroscopy to interrogate the point defect generation and annihilation reactions that occur at the metal/film and film/solution interfaces, we show that the observed increase in cation vacancy concentration in the passive film on Ni is due to chloride-catalyzed ejection of cations from the film/solution interface. These findings are inconsistent with chloride-catalyzed film dissolution and chloride penetration mechanisms for passivity breakdown, but they are entirely consistent with cation vacancy generation at the barrier layer/solution interface and subsequent cation vacancy condensation at the metal/barrier layer interface, as postulated in the point defect model.

Diffusion coefficient measurements of La 2/3− xLi 3 xTiO 3 using neutron radiography

Lithium tracer diffusion experiments using neutron radiography (NR) technique were carried out on the perovskite-type structured lithium ion conductors La 2/3− x Li 3 x TiO 3 in the temperature range from 150 to 400 °C. Isotope ( 6Li) concentration profiles in the annealed samples consisting of 7Li were measured by NR, from which the tracer diffusion coefficients of lithium ions were calculated. Lower lithium concentration samples showed higher diffusion coefficients and vice versa, which was consistent with the assumption that the diffusion coefficient is proportional to the cation vacancy concentration. On the other hand, the activation energies of lithium ion diffusion grow larger with the lithium content. In addition, correlation factors were discussed comparing with the diffusion coefficients obtained from the conductivity data.

The effect of TiO2 on the magnetic power losses and electrical resistivity of polycrystalline MnZn-ferrites

In this paper the mechanism through which TiO2 affects the magnetic power losses of iron excess polycrystalline MnZn-ferrites is investigated. It has been found that TiO2, on being diluted in the bulk of the grains, promotes the homogeneous accumulation of calcium and silicon along the grain boundaries, by providing high specific resistivity properties to the resulting microstructure. This calcium and silicon segregation enhancement is attributed to the increased cation vacancy concentrations associated with the incorporation of tetravalent Ti in the spinel lattice. The creation of cation vacancies through a dopant such as Ti does not have the same effect as the creation of cation vacancies through control of the partial pressure of oxygen during firing. The latter is believed to promote calcium and silicon accumulation at the pores and the triple points rather than along the grain boundaries, resulting thus in microstructures with reduced specific resistivity.

Oscillations in the thickness dependences of the room-temperature Seebeck coefficient in SnTe thin films

The dependences of the Seebeck coefficient S on the thickness ( d = 5–110 nm) of SnTe thin films grown by thermal evaporation in vacuum on (001)KCl substrates were obtained at room temperature. Distinct oscillations in the S( d)-dependences were observed and attributed to the size quantization effect in SnTe thin films. The monotonic component of the d-dependences of S decreases with increasing thickness. In this connection it is suggested that the equilibrium concentration of non-stoichiometric cation vacancies depends on the SnTe film thickness.

Thermal expansion of troilite and pyrrhotite determined byin situcooling (873 to 373 K) neutron powder diffraction measurements

AbstractThe thermal expansion coefficients for natural troilite, FeS, Ni-rich pyrrhotite, Fe0.84Ni0.11S, and Ni-poor pyrrhotite, Fe0.87Ni0.02S, were measured during cooling by in situ neutron powder diffraction over the temperature range 873–373 K. Between 873 and 573 K, the mean thermal expansion coefficients for the three compositions are 7.4(3)×10−5{FeS}, 8.0(4)×10−5{Fe0.84Ni0.11S} and 8.5(4)×10−5K−1{Fe0.87Ni0.02S}. Below 573 down to 373 K, the first two increase considerably to 14.1(7)×10−5{FeS} and 9.3(5)×10−5{Fe0.84Ni0.11S} while the latter sample shows no significant variation, 8.4(5)×10−5K−1. Below 573 K, the thermal expansion is highly anisotropic, with Δa/100 K−1ranging from 0.89(9)% {FeS} to 0.48(12)% {Fe0.87Ni0.02S} while Δc/100 K−1ranges from −0.39(11)% {FeS} to −0.13(2)% {Fe0.87Ni0.02S}.Upon cooling through 573 K, troilite and pyrrhotite undergo a transition where the FeS6octahedra distort and in the case of pyrrhotite, cation-vacancy clustering occurs. The thermal expansion coefficients are bigger for low cation-vacancy concentrations and decrease as the pyrrhotites become less stoichiometric. This indicates that the thermal expansion in these minerals is damped by vacancy ordering or clustering. The thermal expansion coefficients for troilite and pyrrhotite are amongst the largest reported for sulphide minerals and their role in the formation of ore textures is discussed briefly.

Two‐Fold Diffusion Kinetics of Oxygen Re‐Equilibration in Donor‐Doped BaTiO3

Oxygen (nonstoichiometry) re‐equilibration kinetics was examined, via a conductivity relaxation method, on 1 m/o donor (La)‐doped BaTiO3 (nominal composition, Ba0.99La0.01Ti0.9975O3) against oxygen partial pressure in the range of −16<log (PO2/ atm)≤ 0 at 1200°C. The kinetics has been found to be onefold with a single relaxation time or twofold with two different relaxation times depending on oxygen activity. It is attributed to the relative contributions depending on oxygen activity of fast relaxation of the oxygen sublattice and sluggish relaxation of the cation sublattices: their revelations are determined by the ratio of oxygen vacancy to cation vacancy (say, ) concentrations and their relaxation times. Their chemical diffusion coefficients have been extracted from the relaxation times asanddepending on oxygen activity, which are in good agreement with the reported values. Defect diffusivities are subsequently determined by using the thermodynamic factors of component O and Ti, which have been evaluated from the equilibrium conductivity isotherm, asand.

The Thermophysical Properties of Solid Solutions of CaLa2S4-La2S3System

The thermal conductivity coefficient in the temperature range from 275 to 450 K and the coefficient of thermal expansion in the range from 300 to 900 K are experimentally determined for solid solutions of the CaLa2S4-La2S3 system. The mechanisms of heat transfer in CaLa2S4- La2S3 samples in the investigated temperature range are discussed, as well as the factors which define the complex concentration dependence of thermal conductivity coefficient. The correlation is treated between the value and temperature dependence of the coefficient of thermal expansion and the variation of the interatomic bond force in the case of variation of the concentration of cation vacancies in the investigated crystals.

Effect of H+ on Fe–Mg interdiffusion in olivine, (Fe,Mg)2SiO4

To quantify the effect of hydrogen on the kinetics of Fe–Mg interdiffusion in olivine, diffusion couples composed of crystals with Mg∕(Mg+Fe) ratios of 0.91 and 0.83 were annealed under water-saturated conditions at T=1373K and P=300MPa. With fO2 buffered at the Ni–NiO phase boundary, fH2O≈300MPa. The resulting interdiffusivity, D̃Fe–Mg≈1×10−16m2∕s, is approximately one order of magnitude larger than that measured under anhydrous conditions. The enhancement in diffusivity in the presence of water results from an increase in the concentration of cation vacancies associated with the introduction of high concentrations of protons as point defects into the olivine structure.

Resistive donor-doped SrTiO 3 sensors: I, basic model for a fast sensor response

In contrast to dense donor-doped SrTiO 3 (STO) ceramics and single crystals, porous fine grained thick films reveal a surprisingly fast resistivity response in reply to a change of the oxygen partial pressure ( pO 2) within some 10 ms even at temperatures below 900 °C. Although this unexpected behavior was attributed to a grain boundary effect, the resistivity versus pO 2 plot shows a similar characteristic as found for dense ceramics or single crystals, respectively. This observation suggests that cation vacancies, which significantly influence the electrical behavior of the bulk, but which are known to equilibrate only at highest temperatures, may also play a key role in the formation of resistive grain boundaries in the moderate temperature range. For mobility reasons, a change in the cation vacancy concentration might then be limited to a few monolayers on each side of the interface and a very high interface density of defect states is needed to explain the drastic resistivity change of the sensor observed in the experiment. We propose a generalized point defect model that involves the formation of a near-interface space charge region. The latter is found to affect the local defect equilibria significantly. As a consequence, the concentration of each defect type differs from the bulk value. The fast sensor response might then originate from a space charge induced increase of the cation vacancy concentration situated only near the interface that exceeds the value predicted from electroneutral defect considerations by 2 orders of magnitude. The local formation of cation vacancies causes a strong depletion of electrons. The space charge itself is formed due to the difference in mobilities of ionic and electronic species.

On the Nature of Spontaneous Rectangular and Square Hysteresis Loops in Ferrites

The conditions necessary for producing rectangular- and square-loop ferrites are inferred from analysis of new and earlier reported experimental data. The effect of microinhomogeneities on the properties of such ferrites is investigated. It is shown that α-Fe2O3 inclusions, local tetragonal distortions, and linear microinhomogeneities have an adverse effect on their magnetic properties and that the high values of Ht/Hc in all rectangular-loop ferrites are due to aggregated microinhomogeneities containing increased concentrations of cation vacancies compared to the host material. The origin of such inhomogeneities is discussed.

Oxygen stoichiometry, ferromagnetism, and transport properties ofLa2−xNiMnO6+δ

Nominal La 2 NiMnO 6 prepared by solid-state reaction in air in accordance with earlier reports is shown to contain excess oxygen as well as the coexistence of two ferromagnetic phases of comparable Curie temperatures, onemonoclinic and the other rhombohedral. As originally predicted, ordering of Ni 2 + and Mn 4 + ions gives ferromagnetic Ni 2 + -O-Mn 4 + interactions and transforms the orthorhombic Pbnm space group to monoclinic P2 1 In with β90° and the rhombohedral R3c space group to R3m or R3. Synthesis by the Pechini method in Ar, air, and O 2 atmospheres under different thermal treatments also consistently gave O 6 + δ ; the lowest δ=0.05(1) was attained for a single P2 1 /n phase reacted under Ar at 1350°C. Lowering the mean A-site atomic radius in La 2 - x □ x NiMnO 6 and Nd 2 NiMnO 6 also stabilizes the monoclinic phase, and near oxygen stoichiometry was attained in La 2 - x □ x NiMnO 6 for x0.09. Excess oxygen is accommodated in the perovskite structure by the creation of cation vacancies, and it is shown that lanthanum vacancies create deep three-hole acceptor traps. Comparison with the double perovskite La 2 CoMnO 6 and La 2 NiRuO 6 + δ versus La 2 CoRuO 6 signals that stabilization of lanthanum vacancies is associated with a Ni 3 + /Ni 2 + redox couple that is stabilized by a counter octahedral-site cation M having a strong covalent component to its M-O bonding. It is therefore proposed that in the presence of Ni 2 + , but not Co 2 + , a lanthanum vacancy is stabilized by the formation of two holes trapped deeply in molecular orbitals of an O 1 2 cluster of the oxygen atoms that neighbor a lanthanum vacancy. Transport data also indicate a lowering of the separation of the Mn 4 + /Mn 3 + and Ni 3 + /Ni 2 + redox couples from E g ≥0.8 eV to E g 0.3 eV in the ordered Ni 2 + , Mn 4 + array. This lowering and a motional enthalpy ΔH m n 0.1 eV of electrons is attributed to locally cooperative Jahn-Teller deformations of low-spin Ni 3 + and high-spin Mn 3 + octahedral sites. The magnetization M(5 K, 50 kOe) is lowered by both local atomic disorder and the formation of antiphase boundaries. It is shown that a prolonged anneal at 800 °C reduces the local atomic disorder, but it does not remove the antiphase boundaries. Synthetic strategies to increase the magnetization must be designed to reduce the concentration of antiphase boundaries and cation vacancies as well as the atomic disorder.

P-Type Partial Conductivity of Donor(La)-Doped BaTiO3

P-type partial conductivity has been determined on donor (La Ba • )-doped BaTiO3 in full thermodynamic equilibrium state at a fixed temperature of 1200°C: For the nominal compositions of Ba0.99La0.01Ti0.9975O3, Ba0.99La0.01TiO3 and Ba0.985La0.01TiO3, the p-type conductivity is found to vary with oxygen activity as σp = (σm/2)(a O 2/a O 2*)+1/4 with σm ≈ 0.01 S cm−1 and a O 2* ≈ 32, 120, 310, respectively, in the a O 2 region where conventionally the electronic conductivity varies as σ ∝ aO 2 −1/4 and hence, the doped donors are believed to be compensated by cation vacancies (say, [LaBa •] ≈ 4[V″Prime;Ti]). This experimental fact supports that in the vicinity of the stoichiometric composition of the system which falls approximately at a O 2 = a O 2*, while cation vacancy concentration is fixed by the donor concentration, oxygen vacancy concentration in the minority is also essentially fixed, thus, keeping the activity of TiO2 (or BaO) fixed. It is consequently suggested that donor-doped BaTiO3 contains a second phase even in its stoichiometric regime.

Mixed conductivity and stability of A-site-deficient Sr(Fe,Ti)O 3-δ perovskites

Deficiency in the A sublattice of perovskite-type Sr1–yFe0.8Ti0.2O3–δ (y=0–0.06) leads to suppression of oxygen-vacancy ordering and to increasing oxygen ionic conductivity, unit cell volume, thermal expansion, and stability in CO2-containing atmospheres. The total electrical conductivity, predominantly p-type electronic in air, decreases with increasing A-site deficiency at 300–700 K and is essentially independent of the cation vacancy concentration at higher temperatures. Oxygen ion transference numbers for Sr1–yFe0.8Ti0.2O3–δ in air, estimated from the faradaic efficiency and oxygen permeation data, vary in the range from 0.002 to 0.015 at 1073–1223 K, increasing with temperature. The maximum ionic conductivity was observed for Sr0.97Fe0.8Ti0.2O3–δ ceramics. In the system Sr0.97Fe1–xTixO3–δ (x=0.1–0.6), thermal expansion and electron-hole conductivity both decrease with x. Moderate additions of titanium (up to 20%) in Sr0.97(Fe,Ti)O3–δ result in higher ionic conductivity and lower activation energy for ionic transport, owing to disordering in the oxygen sublattice; further doping decreases the ionic conduction. It was shown that time degradation of the oxygen permeability, characteristic of Sr(Fe,Ti)O3–δ membranes and resulting from partial ordering processes, can be reduced by cycling of the oxygen pressure at the membrane permeate side. Thermal expansion coefficients of Sr1–yTi1–xFexO3–δ (x=0.10–0.60, y=0–0.06) in air are in the range (11.7–16.5)×10–6 K–1 at 350–750 K and (16.6–31.1)×10–6 K–1 at 750–1050 K.

Oxygen self-diffusion in single-crystal MgO: secondary-ion mass spectrometric analysis with comparison of results from gas–solid and solid–solid exchange

Self-diffusion coefficients for 18O in single-crystal MgO have been determined from a novel specimen comprising an epitaxial layer of high-purity Mg 18O upon a single crystal substrate of normal MgO. Heating the specimen in air produced a gas–solid exchange gradient at the sample surface as 18O in the epitaxial layer exchanged with 16O in air. A solid–solid interdiffusion gradient was produced between the substrate crystal and the 18O-enriched epitaxial layer. SIMS analysis of gas–solid exchange gradients prepared in the temperature range 1000–1650 °C provided diffusion coefficients that could be described as D/( m 2 s −1)= 1.8 −1.1 +2.9 ×10 −10 exp[−(3.24±0.13) eV/kT] . Interdiffusion gradients produced by annealing at 1100 and 1200 °C yielded the self-diffusion coefficients that were comparable to those obtained from gas–solid exchange, indicating that the surface exchange reaction is fast enough. The results are interpreted in terms of a defect model in which oxygen diffusion occurs by an interstitial type of defect as a result of suppression of anion vacancy concentration by large concentrations of extrinsic cation vacancies.

The Debye temperature and Grüneisen parameter of the CaLa2S4-La2S3 system

The thermal expansion coefficient of solid solutions in the CaLa2S4-La2S3 system at a temperature of 300 K is investigated experimentally. The Debye temperature, the Gruneisen parameter, and the isothermal compressibility coefficient of solid solutions in the system under investigation are determined from the experimental thermal expansion coefficient. It is demonstrated that, upon substitution of calcium ions for cation vacancies in La2S3, the Debye temperature decreases, the isothermal compressibility coefficient increases, and the Gruneisen parameter remains constant for all compositions in the CaLa2S4-La2S3 system. A correlation between the ionic radii of Ca2+ and La3+, the concentration of cation vacancies, and the rigidity of the lattice, on the one hand, and the Debye temperature, the Gruneisen parameter, and the isothermal compressibility coefficient, on the other, is revealed for the studied samples.

Passivity breakdown of Al 2024-T3 alloy in chloride solutions: a test of the point defect model

Cyclic voltammograms of Al 2024-T3 alloy samples, in deaerated aqueous chloride solutions of various concentrations, have been recorded and analysed. A linear dependence between the breakdown potential, E b, and the square root of the scan rate, ν 1/2, was found for all chloride concentrations and also for synthetic seawater leading to α values between 0.41 and 0.38. The breakdown potential at zero scan rate, E b (ν=0) , showed also a good linear dependence on the logarithm of the activity of the chloride anions. From the slope of the straight line a value of 3.29 was obtained for the ratio ( ξ/ J m) from which a ξ value less than or equal to 3×10 15 cation vacancies cm −2 was calculated. Average critical concentration of cation vacancies on Al and γ-Al 2O 3 computed from geometric arguments has showed excellent agreement with those computed from experimental data and analytical equations derived from the Point defect model (PDM).

Electronic Properties of Mixed-Valence Manganates: The Role of Mn Substitutional Defects

Single-phase perovskites in the solid solution series La0.7+yA0.3-yMn1-xMxO3 (with 0.00 ≤ x ≤ 0.10; A = Sr2+, M = Cu2+, Zn2+, Sc3+, Cr3+, Co3+, and Ga3+; A = Ba2+, M = Cu2+, Zn2+, and Sc3+) have been prepared via the acetic acid solutions freeze-drying method. This soft procedure makes possible strict stoichiometric control, and the synthetic variables allow one to maintain a constant proportion of Mn4+ (ca. 32%) in the 47 compounds prepared. In this way, the concentration of cationic vacancies at A and B sites is practically negligible in all cases. X-ray powder diffraction patterns corresponding to the 47 compounds have been completely indexed with rhombohedral perovskite cells. The crystal structures have been refined in space group R̄3c, in the hexagonal setting, from room-temperature data. The variation with x of a set of structural parameters (rhombohedral cell volume, rhombohedral cell angle, and B−O and A−O bond lengths) has been considered as a function of the mean sizes of cations at A and B sites. Manganates in these series present colossal magnetoresistance. The values of the critical temperature, Tc, for the paramagnetic−ferromagnetic transition in La0.7+yA0.3-yMn1-xMxO3 exhibit three different patterns, which clearly appear related to the mean size of cations at B sites. This fact has been interpreted by considering the variation of the electronic contribution to Tc with the structural disorder introduced by the presence of cations with different sizes at B sites.