Non-stoichiometric Variations Research Articles

Sorption and continuous filtration of heavy metals and radionuclides using novel nano-Farringtonite: Mechanisms delineation using EXAFS

Simultaneous removal of a wide range of toxic heavy metal cations and potential radionuclides from water bodies and their continuous filtration with a single low-cost and eco-friendly material represents several scientific merits. Herein, for the first time, we report the simple and straightforward wet-chemical synthesis of novel nano-Farringtonite (FAR) composed of magnesium and phosphate ions. Further, the potential of known alternate nano-hydroxyapatite (HAP) and novel engineered nano-FAR including their non-stoichiometric variations was evaluated for the removal of mimicking radionuclide (Sr2+) and heavy metals (Cd2+, and Zn2+) from water bodies. Non-stoichiometric FAR (ns-FAR) have shown multifold higher contaminant removal capacities than HAP, i.e., Sr2+≈ 85 mg/g vs 49.5 mg/g, Cd2+≈ 560 mg/g vs 98.5 mg/g, and Zn2+ = 489 mg/g vs 62 mg/g in batch mode. NsFAR showed complete removal of Cd2+ and Zn2+ with <20% and 0% recovery, respectively in three consecutive sorption-recovery cycles, probing towards permanent incorporation of these ions. Spectroscopic analysis and extended x-ray absorption fine structure (EXAFS) spectroscopy fitting confirmed ion exchange and crystal incorporation as probable removal mechanisms. The high ionic potential of Mg2+ allows easy ion exchange with +2 charged metal toxins of varying ionic radius at both Mg1 and Mg2 sites of FAR. nsFAR showed instantaneous separation of these cations in continuous column mode with >2,00,000 L/kg of water filtration capacity (at 1 mg/L), justifying the adsorbent's candidature in water purification applications.

Infiltration of Lanthanum Doped Ceria into Nickel-Zirconia Anodes for Direct Butane Utilization in Solid Oxide Fuel Cells

Ni-Gd doped ceria (Ni-GDC) anodes exhibit high durability during direct butane utilization in solid oxide fuel cells (SOFCs). However, the cells containing Ni-GDC anodes showed less performance compared to those containing conventional Ni-Y stabilized zirconia (Ni-YSZ) anodes, when zirconia-ceria solid solution formed at the interface between the YSZ electrolyte and Ni-GDC anode during co-sintering at high temperature in anode-supported SOFCs. The formation of the solid solution increases electrochemical impedance at low frequencies due to oxygen nonstoichiometric variation process. To prevent the formation of this solution, La doped ceria (LDC) was infiltrated into the Ni-YSZ anode after sintering. The maximum power density of the cell containing Ni-YSZ anode with LDC infiltration (Ni-YSZ+LDC) was higher than those containing Ni-YSZ and Ni-GDC anodes. Distribution of relaxation times (DRT) analysis revealed that the polarization resistance of fuel oxidation process decreased, and the oxygen nonstoichiometric variation process was not observed after LDC infiltration. Power was continuously generated over 100 h using butane fuel for the Ni-YSZ+LDC. No carbon deposition was observed after the durability test, because the LDC nanoparticles enhanced the catalytic activity toward carbon oxidation. The LDC infiltration is an method to obtain the comparability of performance and durability for direct butane utilization.

Effect of Native Defects on Transport Properties in Non-Stoichiometric CoSb₃.

The effect of native defects originated by a non-stoichiometric variation of composition in CoSb3 on I-V curves and Hall effect was investigated. Hysteretic and a non-linear behavior of the I-V curves at cryogenic temperatures were observed; the non-linear behavior originated from the Poole-Frenkel effect, a field-dependent ionization mechanism that lowers Coulomb barriers and increases emission of charge carriers, and the hysteresis was attributed to the drastic decrease of specific heat which produces Joule heating at cryogenic temperatures. CoSb3 is a narrow gap semiconductor and slight variation in the synthesis process can lead to either n- or p-type conduction. The Sb-deficient CoSb3 presented an n-type conduction. Using a single parabolic model and assuming only acoustic-phonon scattering the charge transport properties were calculated at 300 K. From this model, a carrier concentration of 1.18 × 1018 cm−3 and a Hall factor of 1.18 were calculated. The low mobility of charge carriers, 19.11 cm2/V·s, and the high effective mass of the electrons, 0.66 m0, caused a high resistivity value of 2.75 × 10−3 Ω·m. The calculated Lorenz factor was 1.50 × 10−8 V2/K2, which represents a decrease of 38% over the degenerate limit value (2.44 × 10−8 V2/K2).

Chemical disorder as an engineering tool for spin polarization inMn3Ga-based Heusler systems

Our study highlights spin-polarization mechanisms in metals, by focusing on the mobilities of conducting electrons with different spins instead of their quantities. Here, we engineer electron mobility by applying chemical disorder induced by non-stoichiometric variations. As a practical example, we discuss the scheme that establishes such variations in tetragonal Mn3Ga Heusler material. We justify this approach using first-principles calculations of the spin-projected conductivity components based on the Kubo-Greenwood formalism. It follows that, in majority of the cases, even a small substitution of some other transition element instead of Mn may lead to a substantial increase in spin-polarization along the tetragonal axis.

Oxygen Non-Stoichiometry, Defect Equilibrium and Electrical Properties of La0.8Sr0.2Sc0.2Mn0.8O3±δ

Non-stoichiometric variation of oxygen content in La0.8Sr0.2Sc0.2Mn0.8O3±δ were determined by means of coulometric titration as a function of oxygen partial pressure p(O2) and temperature. La0.8Sr0.2Sc0.2Mn0.8O3±δ oxides shows oxygen excess and oxygen deficient composition depending on p(O2). Random defect model was constructed based on the experimental oxygen non-stoichiometry. The non-stoichiometric variation of oxygen content of La0.8Sr0.2Sc0.2Mn0.8O3±δ can be explained by the defect equilibrium model. Thermodynamic quantities such as partial molar enthalpy and partial molar entropy were calculated using Gibbs-Hemholtz equation. Electrical conductivity measurements were performed as a function of oxygen partial pressure and temperature, which shows a typical p-type behavior.

Theoretical and experimental study of non-stoichiometric SrRuO&lt;sub&gt;3&lt;/sub&gt; : a role of oxygen vacancies in electron correlation effects

We present a theoretical and experimental study of the electronic structure of SrRuO3 after introduction of vacancies. Theoretically, the modeling of non-stoichiometric variations of SrRuO3 was performed by removing appropriate atoms or their combinations, whereas in the experiments the annealing treatment at moderate temperatures in ultra-high vacuum was applied. At comparison of the obtained results allows us to draw an important conclusion that the formation of oxygen vacancies may be closely related to the manifestation of strong correlation effects in SrRuO3.

Theoretical and experimental study of non-stoichiometric SrRuO&lt;sub&gt;3&lt;/sub&gt; : a role of oxygen vacancies in electron correlation effects

We present a theoretical and experimental study of the electronic structure of SrRuO3 after introduction of vacancies. Theoretically, the modeling of non-stoichiometric variations of SrRuO3 was performed by removing appropriate atoms or their combinations, whereas in the experiments the annealing treatment at moderate temperatures in ultra-high vacuum was applied. At comparison of the obtained results allows us to draw an important conclusion that the formation of oxygen vacancies may be closely related to the manifestation of strong correlation effects in SrRuO3.

Thermodynamic Properties, Defect Analysis, and Electrical Conductivity of the La0.8Sr0.2ScxMn1-xO3−δ Infiltrated into YSZ Scaffolds

La0.8Sr0.2ScxMn1-xO3−δ–yttria stabilized zirconia composite anodes were synthesized by infiltration and investigated as alternative anodes for intermediate solid oxide fuel cells. Non-stoichiometric variation of oxygen content and electrical conductivity in La0.8Sr0.2ScxMn1-xO3−δ–yttria stabilized zirconia composite was studied by high temperature coulometric titration at 923–1023 K. La0.8Sr0.2ScxMn1-xO3−δ shows an oxygen deficient composition depending on the oxygen partial pressure, temperature, and Sc content. A defect chemistry model is presented by taking into consideration the interaction of randomly distributed defects, . The proposed defect model fits well with experimental non-stoichiometric data. The interaction among the defects for vacancy formation is strongly influenced by the amount of dopant, Sc content, and temperature. The thermodynamic properties, oxidation enthalpy ΔH and entropy ΔS, were obtained from the oxygen isotherms. The electrical conductivities measured by the 4-probe method as a function of oxygen partial pressure and temperature were high enough for composite anode application.

Defect chemical and statistical thermodynamic studies on oxygen nonstoichiometric Nd 2 − xSr xNiO 4 + δ

In order to elucidate how oxygen content changes in Nd 2 − x Sr x NiO 4 + δ ( x = 0, 0.2, 0.4), defect chemical and statistical thermodynamic analyses were carried out. The relationship among δ, P(O 2), and T were analyzed by a defect equilibrium model. Since Nd 2 − x Sr x NiO 4 + δ shows metal like band conduction at high temperatures, chemical potential of hole is expressed by the integration of the Fermi-Dirac distribution function and the density of state. The nonstoichiometric variation of oxygen content in Nd 2 − x Sr x NiO 4 + δ can be explained by the defect equilibrium model with a regular solution approximation. Partial molar entropy and partial molar enthalpy of oxygen are calculated from the nonstoichiometric data and Gibbs–Helmholtz equation. The relationship among defect structure, defect equilibrium, and thermodynamic quantities is elucidated by the statistical thermodynamic model. Thermodynamic quantities are calculated by the statistical thermodynamic model with the results of defect chemical analysis and compared with those obtained from experimental results. Thermodynamic quantities calculated by the statistical thermodynamic model can explain rough tendency of those obtained from the δ– T– P(O 2) relationship.

Oxygen nonstoichiometry and chemical stability of Nd 2−xSr xNiO 4+δ

Nonstoichiometric variation of oxygen content in Nd 2− x Sr x NiO 4+ δ ( x=0, 0.2, 0.4) and decomposition P(O 2) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O 2) range from 10 −20 to 1 bar. Nd 2− x Sr x NiO 4+ δ shows the oxygen excess and the oxygen deficient composition depending on P(O 2), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La 2− x Sr x NiO 4+ δ : Nd 2− x Sr x NiO 4+ δ show more oxygen excess than La 2− x Sr x NiO 4+ δ in the higher P(O 2) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd 2− x Sr x NiO 4+ δ in the oxygen excess region is much smaller than that of La 2− x Sr x NiO 4+ δ . The oxygen nonstoichiometric behavior of Nd 2− x Sr x NiO 4+ δ is more ideal-solution-like than that of La 2− x Sr x NiO 4+ δ .

Oxygen nonstoichiometry and defect equilibrium in La 2 − xSr xNiO 4 + δ

Nonstoichiometric variation of oxygen content in La 2 − x Sr x NiO 4 + δ ( x = 0, 0.1, 0.2, 0.3, 0.4) and decomposition P(O 2) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range between 873 and 1173 K and the P(O 2) range between 10 − 20 and 1 bar. La 2 − x Sr x NiO 4 + δ showed the oxygen excess and the oxygen deficient compositions depending on P(O 2), temperature, and the Sr content. The value of partial molar enthalpy of oxygen approaches zero as δ increases in the oxygen excess region, which indicate that the interstitial oxygen formation reaction is suppressed as δ increase. The relationship between δ and log P(O 2) were analyzed by two types of defect equilibrium models. One is a localized electron model, and the other is a delocalized electron model. Both models can well explain the oxygen nonstoichiometry of La 2 − x Sr x NiO 4 + δ with a regular solution approximation.

Complex topology rather than complex membership is a determinant of protein dosage sensitivity

The ‘balance hypothesis' predicts that non-stoichiometric variations in concentrations of proteins participating in complexes should be deleterious. As a corollary, heterozygous deletions and overexpression of protein complex members should have measurable fitness effects. However, genome-wide studies of heterozygous deletions in Saccharomyces cerevisiae and overexpression have been unable to unambiguously relate complex membership to dosage sensitivity. We test the hypothesis that it is not complex membership alone but rather the topology of interactions within a complex that is a predictor of dosage sensitivity. We develop a model that uses the law of mass action to consider how complex formation might be affected by varying protein concentrations given a protein's topological positioning within the complex. Although we find little evidence for combinatorial inhibition of complex formation playing a major role in overexpression phenotypes, consistent with previous results, we show significant correlations between predicted sensitivity of complex formation to protein concentrations and both heterozygous deletion fitness and protein abundance noise levels. Our model suggests a mechanism for dosage sensitivity and provides testable predictions for the effect of alterations in protein abundance noise.

Nonstoichiometry Effects in SrTiO3 Ceramics Assessed by Transmission Electron Microscopy

Strontium titanate (SrTiO3, ST) has a perovskite type structure that is cubic at room temperature, but transforms into a tetragonal one at 105K. At very low temperatures, ST exhibits an extremely large dielectric permittivity and piezoelectric and superconducting characteristics. ST finds applications in tunable microwave devices, due to a dependence of its dielectric response on the electric field and low microwave losses. ST electrical properties are strongly dependent on grain boundaries features and directly influenced by grain size distribution. It was found in our previous studies that a small variation in the stoichiometry of ST has a significant effect on the grain size of the sintered ceramic and related electrical properties: increased grain size and dielectric permittivity values have been reported for Ti excess compositions whereas Sr excess caused a decrease of grain size and of the dielectric permittivity. The tailoring of the dielectric properties by small non-stoichiometric variations in ST needs, however, a full understanding of its effects on the microstructure, phases structure and on the structure / composition of the grain boundaries.

Thermodynamic Behaviour of Arsenic and Antimony in Copper Matte Smelting - A Novel Mathematical Synthesis

The conventional treatises which assumed the solute forms of monatomic As and Sb in molten mattes yielded the activity coefficients which varied erratically between about 0.1 to 100 within narrow nonstoichiometric variations of ± 0.5% S. In contrast, the present work demonstrates mathematically that arsenic and antimony dissolve in copper matte as short range orders of AsCu3 and SbCu2 at matte smelting temperatures. The standard heats of formation have been newly assessed to be ∆Ho298 = -75.8, -25.9 and -28.1 kJ/mole for AsCu3, SbCu2 and SbCu3, respectively where the last two values are considerably more negative than any data previously reported. Based on Nernst's distribution law, regularity and quasichemicality models as well as by stepwise numerical integrations of volatilization test data in the literature, the Raoultian activity coefficients of AsCu3 and SbCu2 at infinite dilution have been assessed to be 32±10 and 20±2, respectively in high grade CuS0.5-mattes containing 70 to 80 mass% Cu at 1473 K. The temporal progress of arsenic and antimony volatilization in industrial copper matte converting was computer-simulated by adopting the presently established activity coefficients of AsCu3 and SbCu2. The results were compared with the commercial operating data. Unexpectedly, the computer simulation model recommends lowering the converting temperature if a higher elimination by volatilization of arsenic is desired.La présente étude démontre mathématiquement que l'arsenic et l'antimoine ont tendance à former des quasi-molécules d'AsCu3 et de SbCu2 dans la matte fondue à 1400-1550 K. Des nouvelles valeurs d'enthalpie standard de formation (∆Ho298) pour AsCu3, SbCu2 et SbCu3 sont présentées ici, soit -75.8, -25.9, et -28.1 kJ/mole respectivement. Ces valeurs pour SbCu2 et SbCu3 sont beaucoup plus négatives que celles déjà publiées dans la littérature. En se basant sur la loi de distribution de Nernst ou sur les modèles de solutions régulières et quasi-chimiques, ainsi qu'en effectuant des intégrations numériques graduelles des données expérimentales de la littérature concernant la volatilisation, des valeurs de 32±10 et 20±2 ont été obtenues comme coefficients respectifs d'activité à dilution infinie d'AsCu3 et de SbCu2 dans la matte de CuS0.5 contenant 70-80 % Cu à 1473 K. Ces résultats contredisent la théorie conventionnelle qui suppose que l'arsenic et l'antimoine se dissolvent dans la matte sous forme de soluté monoatomique, d'où les coefficients d'activité qui varieraient irrégulièrement de 0.1 à 100 environ, avec une petite variation en soufre de ± 0.5 % dans la matte. En utilisant les nouveaux coefficients d'activité d'AsCu3 et de SbCu2, un modèle informatique a été construit afin de simuler la volatilisation de l'arsenic et de l'antimoine dans un convertisseur industriel en fonction du temps de soufflage. Contrairement à la pratique courante, le présent modèle préconise d'abaisser la température de soufflage, si une élimination plus élevée de l'arsenic est voulue.

The structure of non-stoichiometric phases in the Sb 2 S 3 —PbS system close to Sb 2 S 3

High-resolution electron microscopy has been used to investigate the structure of the defect phases produced when melts of Sb 2 S 3 and PbS with compositions in the range of 0-23 mol% PbS are cooled. Chains of four square pyramidal SbS 5 groups that occur in Sb 2 S 3 crystals are found to be lengthened by incorporation of PbS into the crystalline matrix of the non-stoichiometric Sb 2 S 3 . This is a new way of accommodating a non-stoichiometric variation of composition. At low PbS concentrations these longer chains aggregate in small disordered volumes of crystal to form clusters. At higher concentrations, lamellae of new structures occur, which intergrow on either {101} or {100} planes of Sb 2 S 3 . The {101} material is structurally related to the phase cannizzarite and the {100} material is related to galena. These intergrowths can either be ordered, producing new homologous series of phases, or disordered, resulting in operationally non-stoichiometric phases.

The microstructure of slightly substoichiometric NbO 2

The microstructure of NbO 2 in samples of niobium oxides with bulk compositions lying between NbO and NbO 2 has been investigated by electron microscopy and X-ray diffraction. In oxygendeficient samples NbO 2 possesses a complex microstructure which has been interpreted as being due to unmixing of two slightly substoichiometric forms of the oxide. One of these is of the rutile structure while the other was found to have a different symmetry characterized by the space group P4 1. The composition range of the NbO 2 phase was found to be at most from NbO 1.99 to NbO 2.034. No evidence was found to suggest that any nonstoichiometric variation in composition was accommodated by the presence of crystallographic shear planes.

Czochralski-grown proustite and related compounds

The Czochralski technique as applied to synthesis of single-crystal proustite Ag 3AsS 3, pyrargyrite Ag 3SbS 3, and related compounds is described. Nucleation of the crystal was accomplished by a cold-finger technique using an iridium rod. The melt was covered with a disk to decrease evaporation losses and to segregate the chemically-formed dross from the liquid-solid interface. In addition, a non-stoichiometric variation of composition, involving excesses of the more volatile components, was used to lengthen the life of the melt. Crystals were pulled as fast as 75 mm/hour but best quality material, i.e. material without cellular structure, was obtained using growth rates of approximately 2 mm/hr. Ingots as large as 2.4 cm in diameter were grown without core effects at slow pulling rates. Initial experiments indicated a strong tendency for the heaxagonal crystal to grow in the c-direction, with a lesser preference for 60° orientation. Large single crystals of good quality were obtained and optical properties were investigated in the spectral range 0.5 to 25 μm on X-cut and Z-cut plates. Proustite crystals with resistivities of approximately 10 8 Ω-cm were obtained.