Effect Of Transition Metal Impurities Research Articles

Combination Effect of Transition Metal Impurities on Oxygen Vacancy Formation Energetics in TiO2

Combination Effect of Transition Metal Impurities on Oxygen Vacancy Formation Energetics in TiO2

Understanding the Effects of Transition Metal Impurities on Nickel (oxy) Hydroxide Electrocatalysts in Alkaline Media

Nickel-based materials are widely used as critical components of electrochemical energy devices, including alkaline electrolyzers, capacitors, and alkaline batteries. These materials are often preferred because of their availability and low cost compared to precious metals, which makes them excellent candidates for accomplishing the decarbonization goals and electrifying our society. However, several studies have demonstrated that the incorporation of trace impurities from commercial alkaline electrolytes induces significant changes in the intrinsic activity, pseudocapacitance, and stability of Ni-based materials, resulting in erroneous descriptions of their performance. Thus, having a better understanding of the effects of these impurities will lead to practical ways to prevent or control their impact on electrochemical energy devices.In this work, we study the incorporation of transition metals (i.e., Fe, Co, Cu) that form insoluble hydroxides in alkaline media over nickel (oxy) hydroxide electrocatalysts. We systematically examine how these trace impurities modify the OER activity, stability, and the pseudocapacitive properties of NiOOH. As depicted in Figure 1, KOH electrolytes containing trace impurities of Fe and Co induce significant changes in the OER onset potential of Ni(OH)2/NiOOH electrocatalysts, according to the irreversible charge increase measured through coulovoltammetry (QV) curves. Moreover, distinct redox peaks can be seen for each scenario. After aging in purified KOH electrolyte (Fig. 1a), the cyclic voltammogram (CV) of Ni(OH)2/NiOOH exhibits four oxidation peaks that can be ascribed to a mixture of both α and β structural phases (Fig. 1b) while aging in Fe-unpurified KOH promotes the α phase and shifts the redox peak anodically (Fig. 1c). Aging in Co-unpurified KOH promotes a more ordered β structural phase without changing the OER activity significantly (Fig. 1d), which is desirable for pseudocapacitive materials. Additional experiments examining the OER activity, stability, and capacitance provide valuable insights into these impurities, and specific strategies to prevent and minimize their effects will be discussed. Overall, this work illustrates the importance of tracking trace metal impurities in alkaline electrolytes to minimize undesirable effects in electrochemical energy devices. Figure 1

Switching the O–O bond-formation mechanism by controlling water activity

The O–O bond-formation mechanism in water oxidation is still ambiguous even now. In this issue of Chem, Wang and co-workers prove that this mechanism on a Co-based electrocatalyst can switch from a radical coupling pathway to a water nucleophilic attack pathway when the applied potential is lifted. The O–O bond-formation mechanism in water oxidation is still ambiguous even now. In this issue of Chem, Wang and co-workers prove that this mechanism on a Co-based electrocatalyst can switch from a radical coupling pathway to a water nucleophilic attack pathway when the applied potential is lifted. The oxygen evolution reaction (OER) is of paramount significance in the area of solar energy conversion. It is the initial step of photosynthesis to produce dioxygen for supporting aerobic life and to liberate electrons and protons as solar energy carriers. Meanwhile, in the area of next-generation energy revolution based on electrochemistry, it is the key anode reaction of a series of energy-related small-molecule activations (hydrogen production, carbon dioxide reduction, nitrogen reduction, and so forth) driven by sustainable electricity.1Zhang X.-P. Wang H.-Y. Zheng H. Zhang W. Cao R. O–O bond formation mechanisms during the oxygen evolution reaction over synthetic molecular catalysts.Chin. J. Catal. 2021; 42: 1253-1268Crossref Scopus (44) Google Scholar The OER mechanism is very complicated because of its multiple and diverse proton-coupled electron-transfer fundamental steps, including the kinetically challenging O–O bond-formation step. With decades of research, the O–O bond-formation mechanism on the oxygen-evolving center (OEC) in nature is still under debate. This important and challenging process has inspired chemists to develop a wealth of catalytic motifs in order to understand its reaction mechanism. However, catalytic O–O bond formation is still not fully understood on homogeneous molecular catalysts with well-defined structures—not to mention on heterogeneous systems with more uncertainties of the structures of surface catalytic centers.2Zhang X.-P. Chandra A. Lee Y.-M. Cao R. Ray K. Nam W. Transition metal-mediated O-O bond formation and activation in chemistry and biology.Chem. Soc. Rev. 2021; 50: 4804-4811Crossref PubMed Google Scholar In this issue of Chem, Wang and co-workers probe the O–O bond-formation mechanism on heterogeneous Co-based electrocatalysts by innovatively changing the water activity in water-in-salt electrolytes.3Lang C. Li J. Yang K.R. Wang Y. He D. Thorne J.E. Croslow S. Dong Q. Zhao Y. Prostko G. et al.Observation of a potential-dependent switch of water oxidation mechanism on Co-oxide-based catalysts.Chem. 2021; 7: 2101-2117https://doi.org/10.1016/j.chempr.2021.03.015Abstract Full Text Full Text PDF Scopus (16) Google Scholar They discovered that the water-oxidation mechanism might switch from an intramolecular oxygen radical coupling (RC) mechanism at low potentials to a predominant water nucleophilic-attack acid-base (AB) mechanism at higher potentials. Specifically, the resting state of the catalyst under water-oxidation potential is the HO–Co(III)–(μ-O)2–Co(IV)–OH intermediate. The hydroxide has a partial radical character to undergo coupling to form hydroperoxide via the RC mechanism at relatively low potentials. At higher potentials, the above-mentioned intermediate can be further oxidized into the O=Co(IV)–(μ-O)2–Co(IV)–OH motif. The terminal electrophilic oxo of O=Co(IV)– can participate in the water nucleophilic-attack reaction to form the O–O bond through an AB mechanism. In addition, the authors believe that the activation energy barrier of the AB mechanism is a function of applied electrode potential given that one of the substances (water) has a large dipole moment, and its dynamics at electrified interfaces could strongly depend on the electrode potential. The RC pathway is thermodynamically favored over the AB pathway under low potentials, whereas the formation of O=Co(IV)– and the polarization of water at higher potentials cause predominance of the AB pathway. To elucidate the switch of the O–O bond-formation mechanism at different potentials, the authors cleverly controlled the water activity by using water-in-salt electrolytes. They adjusted the water activity from 1 to 0.83 by changing the NaNO3 concentrations in a 0.1 M KPi buffer at neutral pH. They observed a decrease in the water-oxidation rate as the water activity decreased in a potential range of 1.61–1.71 V (versus reversible hydrogen electrode [RHE]; all potentials refer to RHE hereafter). The water-oxidation rate decreased by a factor of ∼1.2 at 1.615 V when the water activity decreased from 1 to 0.83. Interestingly, this rate-suppression factor dramatically increased to ∼4.3 at 1.71 V. The distinctive suppression factors indicate that water molecules are less likely to be involved in the rate-determining step (RDS) of water oxidation at low potentials, whereas water participates in the RDS as a substance at high potentials. This observation corroborates the switch of O–O bond formation from the intramolecular oxygen RC pathway to the water nucleophilic-attack AB pathway. The authors also tested the kinetic isotope effect (KIE) on water oxidation in heavy water with KIE values of ∼2 and ∼4.2 at 1.625 and 1.71 V, respectively. The O–O bond formation from the RC pathway was insensitive to the H/D substitution, but the AB pathway was highly likely to involve the transfer of protons in the RDS. This KIE experiment consolidates their conclusion that the OER mechanism switches at different potentials. In addition, the authors reported the surface-enhanced infrared absorption spectroscopy (SEIRAS) of the catalysts on Au substrate with isotopic labeling experiments to insinuate the presence of Co superoxide intermediate under water-oxidation potentials, which supported their proposed catalytic cycles to a certain extent. In addition to evaluating how altering the activity of water affects O–O bond formation, the authors also performed comprehensive and well-designed control experiments to tightly bridge their findings and conclusions. They excluded any irreversible changes, the effects of limited mass transport, the effects of Na+ from the salts, and the effects of transition-metal impurities on catalytic activity in the tested system; they confirmed that the high concentration of salts did not significantly affect the local pH or block the catalytic sites at the interface; and they also showed a near-unity faradic efficiency of water oxidation to dioxygen. The exact experimental design and the rigorous analysis of experimental data in heterogeneous electrocatalytic water oxidation are extremely important because this is a complicated reaction happening at an intricate interface. Unlike in homogeneous systems, where the catalyst structures are relatively well defined, the surface structures of heterogeneous electrocatalysts are very ambiguous.4Yang X. Wang Y. Li C.M. Wang D. Mechanisms of water oxidation on heterogeneous catalyst surfaces.Nano Res. 2021; https://doi.org/10.1007/s12274-021-3607-5Crossref Scopus (21) Google Scholar The reaction rates of heterogeneous electrocatalysis could be affected by a variety of factors, and the interferences should be eliminated when the origins of the activities are assigned.5Trotochaud L. Young S.L. Ranney J.K. Boettcher S.W. Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation.J. Am. Chem. Soc. 2014; 136: 6744-6753Crossref PubMed Scopus (2130) Google Scholar The following aspects are recommended in heterogeneous electrocatalysis regarding mechanism studies. First, it is better to find a catalyst with a specific surface structure, and this structure should be relatively stable under catalytic conditions. Second, to exclude any possible interference, catalyst platforms for comparison studies with limited but straightforward differences are favored over counterparts with many physical varieties. Third, rigorous experimental behaviors should be followed and special attention should be paid to experimental details so as to include any possible minor factors that could be easily ignored but might have considerable impact on catalytic performance.6Wei C. Rao R.R. Peng J.Y. Huang B.T. Stephens I.E.L. Risch M. Xu Z.C.J. Shao-Horn Y. Recommended practices and benchmark activity for hydrogen and oxygen electrocatalysis in water splitting and fuel cells.Adv. Mater. 2019; 31: 1806296Crossref PubMed Scopus (541) Google Scholar Particularly, the switch of the O–O bond-formation mechanism at different potentials on the tested Co-based electrocatalysts might have some inherent connections with the OEC bearing a Mn-based complex in nature. Unlike early transition metals (Ti, V, and Cr) that can form very stable M=O units or late transition metals (Ni and Cu) that can only theoretically form unstable M=O structures, the metal-oxo motif from Mn, Fe, and Co can switch between two valence tautomers in the form of Mn+1=O2– and Mn–O•–.2Zhang X.-P. Chandra A. Lee Y.-M. Cao R. Ray K. Nam W. Transition metal-mediated O-O bond formation and activation in chemistry and biology.Chem. Soc. Rev. 2021; 50: 4804-4811Crossref PubMed Google Scholar The former with an electrophilic oxygen atom can proceed via the water nucleophilic-attack AB pathway to form the O–O bond, whereas the latter favors the oxygen RC pathway for O–O bond formation. The two different pathways are believed to be concurrent in the Mn-based complex during water oxidation in nature. It is necessary to point out that the radical coupling process on metal-oxide-based electrocatalysts in water oxidation is usually very complicated because the lattice oxygen atoms can easily participate in the coupling. In the HO–Co(III)–(μ-O)2–Co(IV)–OH intermediate proposed by the authors, the HO–Co(III)– unit should be too stable to proceed in the RC process with the –Co(IV)–OH unit with only a partial radical character. The radical coupling might be more likely to involve the bridged oxygen atoms, as generally proposed in the OEC in nature.7Najafpour M.M. Renger G. Hołyńska M. Moghaddam A.N. Aro E.-M. Carpentier R. Nishihara H. Eaton-Rye J.J. Shen J.-R. Allakhverdiev S.I. Manganese compounds as water-oxidizing catalysts: from the natural water-oxidizing complex to nanosized manganese oxide structures.Chem. Rev. 2016; 116: 2886-2936Crossref PubMed Scopus (461) Google Scholar Overall, the observed mechanism switch from Wang and co-workers is thought provoking not only for the Co-based heterogeneous electrocatalytic OER but also for the biological water-oxidation process. Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalystsLang et al.ChemApril 14, 2021In BriefThe O–O bond formation is a key elementary step of the water-oxidation reaction, of which the dependence on the applied potential remains ambiguous. Using water-in-salt electrolyte, we systematically tuned the water activity and probed the mechanism as a function of applied potentials. We found that the mechanism is sensitive to the applied potential. The O–O bond forms via an intramolecular oxygen coupling mechanism at low potentials, whereas it proceeds through a water nucleophilic attack mechanism at high potentials. Full-Text PDF Open Archive

Effect of Transition Metal Impurities on the Adsorption of Flotation Reagents on Pyrite Surface

Pyrite is one of the most common sulfide minerals on earth, which is usually grown with chalcopyrite and other sulfide minerals. In flotation practice, pyrite from different mines or different sections in a same mine may have different flotation behaviors. This is because the pyrite crystal contains lattice defect, including ionic vacancy and lattice impurity. The present work investigates the effects of three typical 3d transition metal (Co, Ni, and Fe) on the adsorption of CH3OCS2−, OH−, and CN− on pyrite surface by density functional theory (DFT) calculations. Calculated results show that Co impurity reduces the adsorption energies of CH3OCS2− and CN− on the surface of pyrite, while Ni and Cu impurities significantly increase the adsorption energies of CH3OCS2− and CN− on the surface of pyrite. Co impurity has small influence on the adsorption of OH−, but Ni and Cu impurities significantly increase the adsorption energies. Calculations on Mulliken charge population of Co2+, Ni2+, and Cu2+ indicate that the Mulliken charges of Co2+, Ni2+, and Cu2+ after OH− adsorption are more than those after CN− adsorption. This suggests that the ionic property of the M-OH- coordination bond is greater than that of M-CN- coordination bond, while covalent bonding of M-CN- coordination bond is greater than that of M-OH- coordination bond. The adsorption results are discussed based on coordination field theory (CFT).

Effect of transition metal impurities on the strength of grain boundaries in vanadium

Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

Degradation of a Refractory Organic Contaminant by Photocatalytic Systems

In this research, the photocatalytic degradation of benzothiophene in TiO 2 aqueous suspension has been studied. TiO 2 photocatalysts are prepared by a sol-gel method. The dominant anatase-structure on TiO 2 particles is observed after calcining the TiO 2 gel at 500℃ for 1hr. Photocatalysts with various transition metals (Nd, Pd and Pt) loading are tested to evaluate the effect of transition metal impurities on photodegradation. The photocatalytic degradation in most cases follows first-order kinetics. The maximum photodegradation efficiency is obtained with TiO 2 dosage of 0.4g/L. The photodegradation efficiency with Pt-TiO 2 is higher than pure TiO 2 powder. The optimal content value of Pt is 0.5wt%. Also we investigate the applicability of H 2 O 2 to increase the efficiency of the TiO 2 photocatalytic degradation of benzothiophene. The optimal concentration of 2 O 2 is 0.05M. The effect of pH is investigated; we obtain the maximum photodegradation efficiency at pH 9. Hydroxy- benzothiophenes and dihydroxybenzothiophenes are identified as reaction intermediates. It is proposed that benzothiophene is oxidized by OH radical to sequentially form hydroxyl-benzothiophenes, dihydroxybenzothiophenes, and benzothiophenedione.

Electronic properties and reactivity of simulated Fe(3+) and Cr(3+) substituted α-Al(2)O(3) (0001) surface.

Metal oxide based minerals naturally contain transition metal impurities isomorphically substituted into the structure that can alter the structural and electronic properties as well as the reactivity of these metal oxides. Natural α-Al(2)O(3) (corundum) can contain up to 9.17% (w/w) Fe(2)O(3) and 1.81% (w/w) of Cr(2)O(3.) Here we report on changes in the structural and electronic properties of undoped and doped α-Al(2)O(3) (0001) surfaces using periodic density functional theory (DFT) methods with spin unrestricted B3LYP functional and a local atomic basis set. Both structural and electronic properties are altered upon doping. Implications for doping effects on photochemical processes are discussed.As metal oxides are major components of the environment, including atmospheric mineral aerosol, DFT was also used to study the effect of transition metal impurities on gas/surface interactions of a model acidic atmospheric gas molecule, carbon monoxide (CO). The theoretical results indicated that the presence of Fe(3+) and Cr(3+) impurities substituted on the outer layer of natural corundum surfaces reduces the propensity toward CO adsorption relative to the undoped surface. However, CO-surface interactions resemble that of bulk α-Al(2)O(3) when the impurity is substituted below the first surface layer. The presence and location of the mineral dopant was found to significantly alter the structural and electronic properties and gas/surface interactions studied here.

Effects of transition metal impurities in alpha alumina: a theoretical study

Ab-initio calculations based on density functional theory have been employed to study the electronic and some optical properties of Y and Nb doped α-Al2O3 with corundum structure. The Nd presence elevates the static dielectric constant of the pure alumina, but changes significantly its band offset. Doping with Y preserves the band gap value and increases slightly the dielectric constant of the pure alumina, thus showing a potential to be used in semiconductor industry.

The Effects of Composition and Microstructure on the Thermal Conductivity of Liquid-Phase-Sintered W-Cu

Liquid-phase sintering of high-purity, submicron, co-reduced W-15Cu powders at temperatures of 1463 to 1623 K (1190 to 1350 °C) produces W grain sizes ranging from 0.6 to 1.2 μm while maintaining less than 2 pct porosity. Measured thermal conductivities of 185 to 221 W/(m·K) are related to the grain size and contiguity, which ranged from 0.51 to 0.62. The effects of composition and microstructure on thermal conductivity are further investigated with a model based on a computational cell that allows adjustment of the grain shape to produce selected matrix volume fractions and contiguities. The model considers porosity, the effects of transition metal impurities on the thermal conductivities of the W and Cu phases, and the role of an interfacial resistance between W grains. The effects of grain size and contiguity on thermal conductivity are shown for thermal boundary conductances ranging from 0 to 1.7 × 1010 W/(m2·K). Comparison of the model predictions with those of prior models, the experimental results, and previously reported thermal conductivities shows that impurities are highly detrimental to the thermal conductivity, but the thermal boundary conductance is a significant factor for high-purity W-Cu.

Degradation of chlorophenol by photocatalysts with various transition metals

In this research, the photocatalytic degradation of 4-chlorophenol (4-CP) in TiO2 aqueous suspension was studied. TiO2 photocatalysts were prepared by sol-gel method. The dominant anatase-structure on TiO2 particles was observed after calcining the TiO2 gel at 500 °C for 1 hr. Photocatalysts with various transition metals (Fe, Cu, Nd, Pd and Pt) loading were tested to evaluate the effect of transition metal impurities on photodegradation. The photocatalytic degradation in most cases follows first-order kinetics. The maximum photodegradation efficiency was obtained with TiO2 dosage of 0.4 g/L, retention time of 1 min and air flow rate of 2,500 cc/min. The photodegradation efficiency with Pt-TiO2 or Pd-TiO2 is higher than pure TiO2 powder. The optimal content value of Pt and Pd is 2 wt%. However, the photodegradation efficiency with Fe(1.0 wt%)-TiO2 and Cu(1.0 wt%)-TiO2 is lower than pure TiO2 powder.

Anisotropic S-wave superconductors containing transition metal impurities: High TK Kondo impurities

The effects of transition metal impurities whose magnetic behavior is that of high T K Kondo impurities and of anisotropy on the T c's of conventional superconductors are studied within the framework of the strong coupling Eliashberg formalism. The Matsuura-Ichinose-Nagaoka theory is used to describe the high T K Kondo impurities. It is shown that the strong coupling and anisotropy combine to change the magnitudes of the initial decrease in T c due to the impurities and of the critical concentration at which the impurities completely suppress superconductivity in different ways. The value of the former is lowered while the value of the latter is raised.

Effect of Transition Metal Impurities on the Photoluminescence of Deformed Si Crystal

Effect of Transition Metal Impurities on the Photoluminescence of Deformed Si Crystal

Electrical transport of YBa 2(Cu 1−xCo x) 3O 7+δ: A Kondo type behaviour

New data on resistivity and thermopower for YBa 2(Cu 1−xCo x) 3O 7+δ (0≤x≤0.12) over an extended temperature range 20–600K has been used to study systematically the effect of transition metal impurities in the YBa 2Cu 3O 7(Y123) system. In order to distinguish between possible scattering mechanisms the data has been fitted to theoretical results based on Mott and Kondo models. Although resistivity can be fitted reasonably well to both models the thermopower clearly favour a Kondo model. The results suggest that the antiferromagnetic spin correlations between conduction electron and localised moments associated with 3d ions play a prominent role in this system.

The effect of transition metal impurities on the superconducting transition temperature of the compound Lu2Fe3Si5

The superconducting transition temperature of the compounds Lu 2 (Fe 1− x T x ) 3 Si 5 (where T = Cr , Mn , Co , Ni , Cu and Ru ) are reported. The fitting of the T c data to Kaiser's theory is presented. Among these transition metals, Mn and Cu behave like magnetic impurities while all others behave like non-magnetic impurities. The covalent bonding between the Fe and its Si neigbors is believed to be related directly to the non-magnetic behavior of these transition metals in the compounds Lu 2 (Fe 1− x T x ) 3 Si 5 .

Impurity and thermodynamic effects on superconducting properties of Y1Ba2Cu3O7-x

The effect of transition-metal impurities on the superconducting properties of Y1Ba2Cu3O7-x was studied and the transition temperature was found to be sensitive to the orthorhombic-tetragonal phase transition, which in turn depends on a number of elemental properties including the oxidation state. Thermodynamic calculations show that dopants with high oxidation states have the same effect as oxygen partial pressure in stabilising the superconducting orthorhombic phase. This effect is consistent with experimental results.

Effects of transition-metal impurities on the crystallographic and magnetic properties of iron sulfide

Though the 2.5% transition-metal impurities have no appreciable influence on T N of FeS, V 2+(3d 3) impurities increase the spin-rotation temperature T M by 80 K while Co 2+(3d 7) impurities decrease T M by 200 K. With V 2+ impurities the crystallographic transition takes place in a narrow temperature region of 15 K while with Co 2+ impurities there is a wide temperature range of 300 K.

The effect of transition metal impurities on the low-field hall coefficient of aluminum

The change of the low-field Hall coefficient RoH of polycrystalline aluminum due to the addition of 3d transitional impurities Ti, Cr, Mn, Fe, Ni and Cu is measured at 4.2 K. The results show that RoH depends systematically on the valence of the impurities, and has the lowest value for chromium. An analysis of the results is given using a three group model of the Fermi surface of Al and the Friedel-Anderson model for a localized virtual bound state.

‘‘Defect state model’’ and effect of transition metal impurities on metal-free phthalocyanine: Electrical and photoconductive properties

A mechanistic study of the effect of transition metal impurities on trap density, electrical and photoelectrical properties of metal-free phthalocyanine was carried out. The experimental results showed that electrical and photoconductive properties of H2Pc were a function of trap density (nt) and concentration of metallic impurities (particularly iron). The results were analyzed in terms of several theoretical models. Both the hydrogenic-type shallow impurity state model and conduction due to bridging of host molecules via iron impurity atoms were incompatible with the results. An excellent description of the observed electrical and photoconductive behavior of H2Pc as a function of trap density and impurity can be given using a defect state model, analogous to deep state in inorganic system. This model correctly predicts the dependence of specific conductivity, σ, of H2Pc on the inverse of trap density (1/nt) at low concentration and on 1/nt2 at high trap concentration. Similarly, the model offers an explanation of the change in the dependence of conductivity under illumination from 1/Ne to 1/Ne1/2 as a function of increased trap density.

Effect of Transition Metal Impurities on Superconductivity

The existing experimental results on the effect of transition metal impurities on superconductivity in several alloy systems were compared with theories based on two different models, the Anderson model and the s-d model. It was found that the theory of Muller-Hartmann and Zittartz (s-d model) reproduces the experimental results in Kondo alloys with \(T_{\text{K}}/T_{\text{co}}{\lesssim}1\) fairly well if T K of the theory is replaced by T K /10, where T K is the Kondo temperature of alloy and T co is the critical temperature of host metal. It was also found that the thermodynamic quantities at T =0 of Kondo alloys fit the theory of Shiba (Anderson model) remarkably well. From the observed variation of the position of the localized excited states with in the gap with T K / T co , it was inferred that the transition from the pair breaking behavior to the nonmagnetic BCS behavior takes place rather abruptly at about T K / T co ≈80.

Effect of Transition-Metal Impurities in Superconductors: An Extension of Yoshimori's Theory

Effect of Transition-Metal Impurities in Superconductors: An Extension of Yoshimori's Theory