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The role of selected f ions in the suppression of high-Tc superconductivity.

Description: The initial observations of superconductivity at temperatures above 77 K in copper-oxide based materials was surprising from a variety of different perspectives. Among the unexpected findings were reports of superconductivity for the series RBa{sub 2}Cu{sub 3}O{sub 7} where R is a rare earth (Y, Nd-Tm), which may carry a large, local magnetic moment. Superconductivity was subsequently demonstrated for all 4f analogs in this series except Ce, Pr, and Tb. In addition to the RBa{sub 2}Cu{sub 3}O{sub 7} series, there are several other CuO based series of superconductors that are formed by substituting R ions. The most studied of these are listed in Table 1, together with the f ions that form isostructural compounds and their superconducting critical temperatures (T{sub c}). The presence of an R ion with a large magnetic moment does not significantly influence the superconductivity. In contrast, even the presence of small concentrations of magnetic impurity ions in a conventional superconductor inhibits superconductivity by interfering with the formation of Cooper pairs. Most R ions substitute into an isostructural series with no observable effect on the superconducting properties of the material. As can be seen from Table 1, there are notable exceptions to this observation. In particular, the rare-earth ions Ce, Pr, and Tb, together with the 5f-actinide ions Am and Cm, sometimes behave anomalously. These ions either do not form an isostructural phase, or if the phase forms it may not be superconducting. The fact that an f-ion can suppress superconductivity is clearly demonstrated in the isostructural series Y{sub 1{minus}x}Pr{sub x}Ba{sub 2}Cu{sub 3}O{sub 7} (0 < x < 1){sup 3}, in which the critical temperature decreases with increasing x, such that for concentrations of Pr greater than x {approx} 0.5, the samples no longer superconduct.
Date: September 25, 1998
Creator: Soderholm, L.
Partner: UNT Libraries Government Documents Department

Short range Tb{sup +3} spin correlations far above the two dimensional Neel temperature in Pb{sub 2}Sr{sub 2}TbCu{sub 3}O{sub 8}

Description: Whereas the Tb{sup +3} moments undergo antiferromagnetic ordering at T{sub N}=5.5 K, our susceptibility and inelastic neutron scattering experiments indicate that significant magnetic Tb-Tb short range correlations persist to temperatures T{ge}100 K. Magnetic correlations at such high temperatures relative to T{sub N} are very unusual and they may shed new light on the relation between superconductivity and rare earth magnetism in these systems.
Date: March 1, 1996
Creator: Staub, U.; soderholm, L. & Skanthakumar, S.
Partner: UNT Libraries Government Documents Department

Magnetic Properties of Dy in Pb

Description: Superconductivity can be induced at high temperatures in Pb{sub 2}Sr{sub 2}RCu{sub 3}O{sub 8} (R - rare earth) by partially doping Ca{sup 2+} for R{sup 3+}. In order to understand the interplay between magnetism and superconductivity, the magnetic properties of the parent compounds, Pb{sub 2}Sr{sub 2}RCu{sub 3}O{sub 8}, have been studied. The work presented here includes magnetic susceptibility and specific heat measurements on R=Dy and extends the previous studies on R=Ce, Pr, Tb, Ho and Er. Specific heat experiments suggest that the Dy ions order antiferromagnetically with an ordering temperature of 1.3K. The magnetic susceptibility data are in good agreement with the susceptibility calculated using crystal field parameters that are extrapolated from previous modeling of the R=Er and Ho analogs of this series.
Date: July 10, 1999
Creator: Skanthakumar, S.; Soderholm, L. & Movshovich, R.
Partner: UNT Libraries Government Documents Department

Magnetic properties of Dy in Pb{sub 2}Sr{sub 2}DyCu{sub 3}O{sub 8}.

Description: Superconductivity can be induced at high temperatures in Pb{sub 2}Sr{sub 2}RCu{sub 3}O{sub 8} (R - rare earth) by partially doping Ca{sup 2+} for R{sup 3+}. In order to understand the interplay between magnetism and superconductivity, the magnetic properties of the parent compounds, Pb{sub 2}Sr{sub 2}RCu{sub 3}O{sub 8}, have been studied. The work presented here includes magnetic susceptibility and specific heat measurements on R = Dy and extends our previous studies on R = Ce, Pr, Tb, Ho and Er. Specific heat experiments suggest that the Dy ions order antiferromagnetically with an ordering temperature of 1.3K. The magnetic susceptibility data are in good agreement with the susceptibility calculated using crystal field parameters that are extrapolated from previous modeling of the R = Er and Ho analogs of this series.
Date: July 2, 1999
Creator: Movshovich, R.; Skanthakumar, S. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Implications of the unusual redox behavior exhibited by the teteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-}

Description: We report on the novel electrochemical behavior of the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup -12}, with emphasis on the Eu valence in both the oxidized and reduced forms. In aqueous electrolytes, the P-W-O framework is electroactive and can reversibly accept up to 10 electrons before the potential becomes reducing enough for H{sub 2} evolution. Eu L{sub 3}-edge x-ray absorption near edge structure (XANES) results demonstrate that Eu is also electroactive under the same conditions. Implications of this work are discussed re long-standing questions about intermediate- valent materials.
Date: December 1996
Creator: Antonio, M. R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Synthesis and properties of lanthanide-exchanged Preyssler`s heteropolyanions

Description: Na{sup +} in the Preyssler heteropolytungstate anion [NaP{sub 5}W{sub 30}O{sub 110}]{sup 14{minus}} can be exchanged for a trivalent lanthanide ion. The potential significance of this new class of lanthanide heteropolyanions relates to their applications in catalysis science. This view follows from the fact that Keggin heteropolyanions and their free acids are used as heterogeneous solid catalysts and homogeneous solution catalysts. The authors describe synthetic conditions that lead to the incorporation of Ce{sup 3+} and Pr{sup 3+} within the Preyssler anion, and the coprecipitation of Ce{sup 3+} and the Preyssler anion. Initial studies indicate that the latter, coprecipitated, material deserves study for bifunctional catalytic activity.
Date: December 1994
Creator: Antonio, M. R.; Malinsky, J. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Oxidation states of the ``unusual`` rare earths (R=Ce, Pr and Tb) in double layer high-T{sub c} superconductors

Description: X-ray absorption and magnetization experiments are used to describe the oxidation states of the rare earths R= Ce, Pr, and Tb in the double layer high-Tc superconducting series RBa{sub 2}Cu{sub 3}O{sub 7} and Pb{sub 2}Sr{sub 2}R{sub 1-x}Ca{sub x}Cu{sub 3}O{sub 8}. We obtained the same oxidation states for the rare earths in these two classes of compounds, namely, R=Ce tetravalent and R=Pr and Tb trivalent.
Date: July 1, 1996
Creator: Staub, U.; Soderholm, L.; Skanthakumar, S. & Antonio, M.R.
Partner: UNT Libraries Government Documents Department

Nanoscale encapsulation: the structure of cations in hydrophobic microporous aluminosilicates

Description: Hydrophobic microporous aluminosilicates, created by organic surface modification of inherently hydrophilic materials such as zeolites and clays, are currently being investigated as storage media for hazardous cations. Use of organic monolayers to modify the surface of an aluminosilicate after introducing an ion into the zeolite/clay reduces the interaction of water with the material. Resulting systems are about 20 times more resistant to leaching of stored ion. XAS spectra from the encapsulated ion demonstrate that byproducts from the organic modifier can complex with the stored cation. This complexation can result in a decreased affinity of the cation for the aluminosilicate matrix. Changing the organic modifier eliminates this problem. XAS spectra also indicate that the reactivity and speciation of the encapsulated ion may change upon application of the hydrophobic layer.
Date: October 1, 1996
Creator: Wasserman, S.R.; Yuchs, S.E.; Giaquinta, D.; Soderholm, L. & Song, Kang
Partner: UNT Libraries Government Documents Department

Implications of the unusual redox behavior exhibited by the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12{minus}}

Description: Materials containing more than one electroactive species may exhibit a variety of unusual properties, including mixed or intermediate valence, and their related phenomena. Intermediate valence arises when localized orbitals, notably valence f-states, interact in a bonding fashion with delocalized, molecular orbitals in the same material. Electrons are viewed to be rapidly hopping between these two very different kinds of orbitals, and it is their relative residence times that are interpreted as a non-integral valence. Whereas there has been considerable efforts expended to understand this unusual behavior, these efforts have not proven successful, largely because the problem is many-bodied and is usually studied on infinite band states, often in chemically complex systems. The authors report here on the novel electrochemical behavior of the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12}, and examine the valence of Eu in both the oxidized and reduced form of the anion, as determined from Eu L{sub 3}-edge X-ray absorption near edge structure (XANES). The results demonstrate that, with careful electrochemical control, Eu in this ion may become intermediate valent. Such a result would provide an entirely new and unique approach to longstanding questions related to the phenomenon of intermediate valence.
Date: August 1, 1997
Creator: Antonio, M.R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Solution spectroelectrochemical cell for in situ X-ray absorption fine structure

Description: A purpose-built spectroelectrochemical cell for in situ fluorescence XAFS (X-ray Absorption Fine Structure) measurements of bulk solution species during constant-potential electrolysis is described. The cell performance was demonstrated by the collection of europium L{sub 3}-edge XANES (X-ray Absorption Near Edge Structure) throughout the course of electrolysis of an aqueous solution of EuCl{sub 3}{center_dot}6H{sub 2}O in 1 M H{sub 2}SO{sub 4}. The europium L{sub 3}-edge resonances reported here for the Eu{sup III} and Eu{sup II} ions demonstrate that their 2p{sub 3/2} {yields} 5d electronic transition probabilities are not the same.
Date: June 12, 1995
Creator: Antonio, M.R.; Soderholm, L. & Song, I.
Partner: UNT Libraries Government Documents Department

Redox behavior of europium in the Preyssler heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-}

Description: In aqueous, mineral-acid electrolytes, the cyclic voltammetry of the europium- exchanged Preyssler heteropolyanion, [Eu{sup III}P{sub 5}W{sub 30}O{sub 110}]{sup 12-}, is unique among all the other trivalent-lanthanide-exchanged anions, [Ln{sup III}P{sub 5}W{sub 30}O{sub 110}]{sup 12-} for Ln {triple_bond} Ce-Lu. All [LnP{sub 5}W{sub 30}O{sub 110}]{sup 12-}, including Eu, form heteropoly blues upon reduction. In order to obtain insights about this issue, we conducted in situ Eu L{sub 3}-edge XANES (X-ray absorption near edge structure) spectroelectrochemical experiments on an aqueous solution of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-} (5.5 mM) in a supporting electrolyte of 1 M H{sub 2}SO{sub 4} at two extreme potentials. The results demonstrate that the Eu{sup III} ion in the colorless Preyssler anion solution at open circuit potential (+0.21 V vs Ag/AgCl) is electroactive and is reduced to Eu{sup II} in the resulting dark blue solution from constant-potential bulk electrolysis at -0.55 V vs Ag/AgCl. This unusual redox behavior of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-} may be of technological importance in the area of oxidation catalysis.
Date: August 1, 1997
Creator: Antonio, M.R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Complexants for actinide element coordination and immobilization

Description: The goal of this project is to develop inorganic metal oxide clusters known as polyoxoanions (POAs) as complexants for the immobilization of actinide (An) ions from high-level waste (HLW). A diverse array of rugged isopolyoxoanions, [M{sub x}O{sub y}]{sup z{minus}}, and heteropolyoxoanions, [X{sub a}M{sub b}O{sub c}]{sup d{minus}}, comprised of M = V, Mo, W and X = Si, P polyhedra will be investigated for their ability to incarcerate An ions. The research combines two objectives--An-POA coordination and An-POA containment. The first involves the synthesis, isolation, and characterization of POAs that can selectively bind An ions to form stable An-POA complexes in alkaline and acidic solutions. The second involves investigations of the thermochemistry of the An-POA complexes under vitrification conditions germane to the formation of proposed HLW forms, such as borosilicate glass. The approach is envisioned to provide two levels of An encapsulation for maximum stability and durability as well as the potential to incorporate higher levels of An ions (particularly Pu) in waste forms than now possible. Such versatility bodes well for the potential application of POAs as An complexants in technology of significance to the environmental management of HLW. This report summarizes work performed since the commencement of the project on 1 October 1998. As an overview of the research progress-to-date (15 June 1999), the strategic point is that selective binding of An ions by POAs depends upon the An valence. Actinide reduction-oxidation (redox) chemistry is a pivotal property in the proposed, rational use of POAs as coordinating ligands for An ions. Because POAs are oxidants and the transuranium elements are multivalent, the combined redox chemistry is rich and, oftentimes, confusing. The evidence of contrasting and conflicting valence behavior makes it difficult, if not impossible, to make broad generalizations about the collective properties of An-POA systems without appropriate experimentation. To ...
Date: March 14, 2000
Creator: Antonio, M.R.; Soderholm, L.; Williams, C.W. & Francesconi, L.C.
Partner: UNT Libraries Government Documents Department

The structure of actinide ions exchanged into native and modified zeolites and clays

Description: X-ray absorption spectroscopy (XAS) has been used to investigate the structure and valence of thorium (Th{sup 4+}) and uranyl (UO{sub 2}{sup 2+}) cations exchanged into two classes of microporous aluminosilicate minerals: zeolites and smectite clays. XAS is also employed to examine the fate of the exchanged cations after modification of the mineral surface using self-assembled organic films and/or exposure to hydrothermal conditions. These treatments serve as models for the forces that ultimately determine the chemical fate of the actinide cations in the environment. The speciation of the cations depends on the pore size of the aluminosilicate, which is fixed for the zeolites and variable for the smectites.
Date: February 16, 2000
Creator: Wasserman, S. R.; Soderholm, L. & Giaquinta, D. M.
Partner: UNT Libraries Government Documents Department

The influence of desulfovibrio desulfuricans on neptunium chemistry.

Description: The role of biotic Np(V) reduction is studied in light of its potential role in the environmental immobilization of this hazardous radionuclide. The speciation of Np in Desulfovibrio desulfuricans cultures is compared with Np speciation in the spent medium and in the uninoculated medium. Precipitates formed in all three samples. Optical spectroscopy and X-ray absorption near edge structure (XANES) were used to determine that Np(V) is almost quantitatively reduced in all three samples and that the precipitate is an amorphous Np(IV) species. These results demonstrate that the reduction of Np is independent of Desulfovibrio desulfuricans. The underlying chemistry associated with these results is discussed.
Date: December 20, 1999
Creator: Soderholm, L.; Williams, C.; Antonio, M. R.; Tischler, M. L. & Markos, M.
Partner: UNT Libraries Government Documents Department

Complexants fsro Actinide Element Coordination & Immobilization

Description: The goal of this project is to develop inorganic metal oxide clusters known as polyoxoanions (POAs) as complexants for the immobilization of actinide (An) ions from high-level waste (HLW). A diverse array of rugged isopolyoxoanions, [MxOy]z- , and heteropolyoxoanions, [XaMbOc]d-, comprised of M = V, Nb, Mo, W and X = Si, P polyhedra are under investigation for their ability to incarcerate An ions. The research combines two objectives-An-POA coordination and An-POA containment. The first involves the synthesis, isolation, and characterization of POAs that can selectively bind An ions to form stable An-POA complexes in alkaline and acidic solutions. The second involves investigations of the thermochemistry of the An-POA complexes under vitrification conditions germane to the formation of proposed HLW forms, such as borosilicate glass. The approach is envisioned to provide two levels of An encapsulation for maximum stability and durability as well as the potential to incorporate higher levels of An ions (particularly Pu) in waste forms than now possible. Such versatility bodes well for prospective applications of POAs as An complexants in technology of significance to the environmental management of HLW.
Date: June 1, 2000
Creator: Antonio, Mark R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Actinide speciation in glass leach-layers: An EXAFS study

Description: Uranium L{sub 3} X-ray absorption data were obtained from two borosilicate glasses, which are considered as models for radioactive wasteforms, both before and after leaching. Surface sensitivity to uranium speciation was attained by a novel application of simultaneous fluorescence and electron-yield detection. Changes in speciation are clearly discernible, from U(VI) in the bulk to (UO{sub 2}){sup 2+}-uranyl in the leach layer. The leach-layer uranium concentration variations with leaching times are also determined from the data.
Date: December 1996
Creator: Biwer, B. M.; Soderholm, L.; Greegor, R. B. & Lytle, F. W.
Partner: UNT Libraries Government Documents Department

The effects of surface modification on the speciation of metal ions intercalated into aluminosilicates

Description: Microporous aluminosilicates, including clay minerals and zeolites, are ion-exchange materials. In their most common forms, they have the ability to incorporate cationic species within their matrices. Because of this property, microporous aluminosilicates have been proposed as storage media for hazardous waste. In this paper the authors use X-ray absorption spectroscopy (XAS) to examine the structure of cations held within smectite clay minerals and to determine how modification of the surface of the clay using an organic monolayer affects the coordination of the stored cation. The effects of hydrothermal and thermal processing on the coordination of the ions contained within these systems are also investigated. The presence of the monolayer changes the surface of the clay from hydrophilic to hydrophobic. It inhibits the interlayer ions from exchanging freely into environmental water and reduces the leach rate of cations out of the clay by approximately a factor of 20. Significant changes are observed when these coated samples are treated under hydrothermal and thermal conditions. Reductions of uranium (VI), in the form of uranyl, and cupric ions occur. In addition, the uranium aggregates, forming small particles that appear similar to UO{sub 2}. Comparable conglomeration occurs with lead cations and with the reduced copper species.
Date: December 31, 1996
Creator: Wasserman, S.R.; Giaquinta, D.M.; Yuchs, S.E. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Speciation of uranium in surface-modified, hydrothermally treated, (UO{sub 2}){sup 2+}-exchanged smectite clays

Description: A successful solution to the problem of disposal and permanent storage of water soluble radioactive species must address two issues: exclusion of the radionuclides from the environment and the prevention of leaching from the storage media into the environment. Immobilization of radionuclides in clay minerals has been studied. In addition to the use of clays as potential waste forms, information about the interactions of radionuclides with clays and how such interactions affect their speciations is crucial for successful modeling of actinide-migration. X-ray absorption spectroscopy (XAS) is used to determine the uranium speciation in exchanged and surface-modified clays. The XAS data from uranyl-loaded bentonite clay are compared with those obtained after the particle surfaces have been coated with alkylsilanes. These silane films, which render the surface of the clay hydrophobic, are added in order to minimize the ability of external water to exchange with the water in the clay interlayer, thereby decreasing the release rate of the exchanged-uranium species. Mild hydrothermal conditions are used in an effort to mimic potential geologic conditions that may occur during long-term radioactive waste storage. The XAS spectra indicate that the uranyl monomer species remain unchanged in most samples, except in those samples that were both coated with an alkylsilane and hydrothermally treated. When the clay was coated with an organic film, formed by the acidic deposition of octadecyltrimethoxysilane, hydrothermal treatment results in the formation of aggregated uranium species in which the uranium is reduced from U{sup VI} to U{sup IV}.
Date: August 1, 1997
Creator: Giaquinta, D.M.; Soderholm, L.; Yuchs, S.E. & Wasserman, S.R.
Partner: UNT Libraries Government Documents Department

Structural relationships in high temperature superconductors

Description: The recent discovery of two types of metallic copper oxide compounds which are superconducting to above 90/sup 0/K has renewed interest in the search for new high temperature superconducting materials. It is significant that both classes of compounds, La/sub 2-x/Sr/sub x/CuO/sub 4-y/ and YBa/sub 2/Cu/sub 3/O/sub 7-delta/ are intimately related to the extensively studied perovskite family. Both compounds contain highly oxidized, covalently bonded Cu-O sublattices, however, they differ in geometry. In this paper we discuss the relationship of these features to the superconducting properties. 30 refs., 6 figs.
Date: September 1, 1987
Creator: Schuller, I.K.; Segre, C.U.; Hinks, D.G.; Jorgensen, J.D.; Soderholm, L.; Beno, M. et al.
Partner: UNT Libraries Government Documents Department

Coordination and valence of europium in [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(W{sub 5}O{sub 18}){sub 2}]{sup 9{minus}}

Description: Europium L{sub 3}-edge X-ray absorption fine structure (XAFS) spectroelectrochemistry was used to determine the valence of europium in [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(w{sub 5}O{sub 18}){sub 2}]{sup 9{minus}}. Dilute solutions of these anions in aqueous supporting electrolytes were examined at ambient temperature and at extreme potentials. In situ XANES (X-ray absorption near edge structure) data revealed that Eu is trivalent in both [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(W{sub 5}O{sub 18}){sub 2}]{sup 9{minus}} at rest potential. Furthermore, it was not reduced to Eu{sup 2+} by constant-potential bulk electrolysis at significantly reducing potentials under the electrochemical conditions used herein. These results stand in obvious contrast to the redox behavior of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12{minus}}, in which Eu{sup 3+} is reduced to Eu{sup 2+} under similar electrochemical conditions.
Date: September 1, 1997
Creator: Antonio, M.R.; Soderholm, L.; Jennings, G.; Francesconi, L.C.; Dankova, M. & Bartis, J.
Partner: UNT Libraries Government Documents Department

The possible occurrence of multi-phase behavior in the high T sub c superconductor YBa sub 2 Cu sub 3 O sub x

Description: We report here the results obtained for powder diffraction studies on four YBa{sub 2}Cu{sub 3}O{sub x} samples in which a model with two orthorhombic phases was used to obtain an improved agreement. The implications of this structural model on the systematic variation of {Tc} with oxygen content will be discussed. 28 refs., 3 figs., 2 tabs.
Date: January 1, 1989
Creator: Beno, M.A.; Soderholm, L.; Capone, D.W. II; Hinks, D.G.; Jorgensen, J.D.; Grace, J.D. et al.
Partner: UNT Libraries Government Documents Department