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Effect of point defects and disorder on structural phase transitions

Description: Since the beginning in 1986, the object of this project has been Structural Phase Transitions (SPT) in real as opposed to ideal materials. The first stage of the study has been centered around the role of Point Defects in SPT`s. Our intent was to use the previous knowledge we had acquired in the study of point defects in non-transforming insulators and apply it to the study of point defects in insulators undergoing phase transitions. In non-transforming insulators, point defects, in low concentrations, marginally affect the bulk properties of the host. It is nevertheless possible by resonance or relaxation methods to study the point defects themselves via their local motion. In transforming solids, however, close to a phase transition, atomic motions become correlated over very large distances; there, even point defects far removed from one another can undergo correlated motions which may strongly affect the transition behavior of the host. Near a structural transition, the elastic properties win be most strongly affected so as to either raise or decrease the transition temperature, prevent the transition from taking place altogether, or simply modify its nature and the microstructure or domain structure of the resulting phase. One of the well known practical examples is calcium-stabilized zirconia in which the high temperature cubic phase is stabilized at room temperature with greatly improved mechanical properties.
Date: June 1, 1997
Creator: Toulouse, J.
Partner: UNT Libraries Government Documents Department

High Temperature Structural Behavior of SrRuO{sub 3}

Description: The unusual metal SrRuO{sub 3} is perhaps the only known 4d transition metal based ferromagnet (Tc = 162K) with a sizable moment. To complement low T polarized neutron diffraction measurements of the magnetization density, high T neutron diffraction measurements are reported here. Two structural phase transitions are observed. Between 10K and 800K SrRuO{sub 3} is orthorhombic and at 800K it appears to be tetragonal until 975K, where it becomes cubic. The temperature variation of the lattice parameters are reported along with a structural description of the tetragonal phase.
Date: July 15, 1997
Creator: Chakoumakos, B.C., Nagler, S.E., Misture, S.T., Christen, H.M.
Partner: UNT Libraries Government Documents Department

Vacancy related defects in La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} thin films

Description: Laser ablated La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} thin films have been studied by Doppler-broadening-detected positron annihilation using a variable-energy positron beam. The oxygen partial pressure during cooling from the growth temperature was altered through the range 760 torr to 10{sup -5} torr to change the oxygen non-stoichiometry of the films. The measured Doppler broadened lineshape parameter S was found to increase with increasing oxygen nonstoichiometry. For films cooled with an oxygen partial pressure of {le} 10{sup -4} Torr positron trapping to monovacancy type defects is inferred. For the film cooled in 10{sup -5} torr oxygen the magnitude of the increase in S, with respect to that measured from the film cooled in 760 Torr oxygen, showed positron trapping to vacancy cluster defects was occurring.
Date: 1997
Creator: Keeble, D. J.; Krishnan, A. & Nielsen, B.
Partner: UNT Libraries Government Documents Department

Tuning ground states and excitations in complex electronic materials

Description: Modern electronic materials are characterized by a great variety of broken-symmetry ground states and excitations. Their control requires understanding and tuning underlying driving forces of spin-charge-lattice coupling, critical to macroscopic properties and applications. We report representative model calculations which demonstrate some of the richness of the phenomena and the challenges for successful microscopic modeling.
Date: September 1, 1996
Creator: Bishop, A.R.
Partner: UNT Libraries Government Documents Department

Ferroelectric-to-Relaxor Crossover and Oxygen Vacancy Hopping in Compositionally-Disordered Perovskites - KtA(1-x)Nb(x)O(3):Ca

Description: It is shown that lattice disorder induced by Nb and Ca substitution has a strong influence on the dielectric and relaxational properties of KTaO{sub 3}. Both substituents are believed to occupy off-center positions at the Ta site, and the difference in valence between the Ca{sup 2+} and Ta{sup 5+} ions leads to the formation of oxygen vacancies (V{sub 0}). Specifically, for a KTa{sub 1{minus}x}Nb{sub x}O{sub 3}:Ca crystal with x = 0.023 and with a 0.055 at.% Ca doping they observe: (1) a ferroelectric transition at atmospheric pressure (1 bar); (2) a large enhancement of the transition temperature by Ca doping; (3) a pressure-induced crossover from ferroelectric-to-relaxor behavior; (4) the impending vanishing of the relaxor phase at high pressure; (5) the reorientation of the Ca-oxygen vacancy (Ca:V{sub 0}) pair defect; and (6) the variation of the energetics and dynamics of this reorientation with pressure. Most of these effects are associated with Nb- and Ca-induced dipolar entities and appear to be general features of soft mode ferroelectrics with random-site polar nanodomains. The ferroelectric-to-relaxor crossover can be understood in terms of a large decrease with pressure in the correlation length among polar nanodomains--a unique property of soft ferroelectric mode systems.
Date: July 26, 1999
Creator: Samara, G.A. & Boatner, L.A.
Partner: UNT Libraries Government Documents Department

Increase of magnetic transition temperatures by reduction of local disorder for perovskite manganites.

Description: We report the synthesis of Sr{sub 1-x}Ca{sub x}MnO{sub 3} and La{sub 0.5}Ba{sub 0.5}MnO{sub 3} perovskites over extended cation and oxygen composition ranges and describe the dependence of their phase stability on the tolerance factor t = t(x,T,{delta}) that is a function of composition, temperature, and oxygen content. We show that magnetic transition temperatures depend strongly on the tolerance factor and charge disorder while dependence on the structural disorder is less important. By reducing charge and structural disorder we have significantly increased the Curie and Neel temperatures for perovskite manganites.
Date: July 2, 2002
Creator: Dabrowski, B.; Chmaissem, O.; Mais, J.; Kolesnik, S.; Jorgensen, J. D. & Short, S.
Partner: UNT Libraries Government Documents Department

On the behavior of Bronsted-Evans-Polanyi Relations for Transition Metal Oxides

Description: Versatile Broensted-Evans-Polanyi (BEP) relations are found from density functional theory for a wide range of transition metal oxides including rutiles and perovskites. For oxides, the relation depends on the type of oxide, the active site and the dissociating molecule. The slope of the BEP relation is strongly coupled to the adsorbate geometry in the transition state. If it is final state-like the dissociative chemisorption energy can be considered as a descriptor for the dissociation. If it is initial state-like, on the other hand, the dissociative chemisorption energy is not suitable as descriptor for the dissociation. Dissociation of molecules with strong intramolecular bonds belong to the former and molecules with weak intramolecular bonds to the latter group. We show, for the prototype system La-perovskites, that there is a 'cyclic' behavior in the transition state characteristics upon change of the active transition metal of the oxide.
Date: August 22, 2011
Creator: Vojvodic, Aleksandra
Partner: UNT Libraries Government Documents Department

Charge delocalization and structural response in layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}: Enhanced Jahn-Teller distortion in the metallic regime

Description: Temperature dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the metal-insulator transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.
Date: November 1996
Creator: Mitchell, J. F.; Argyriou, D. N. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Anomalous Hall effect in Gd-doped La{sub 2/3}Ca{sub 1/3}MnO{sub 3}

Description: Ceramic samples of (La{sub 1-x}Gd{sub x}){sub 2/3}Ca{sub 1/3}MnO{sub 3} were prepared and used as targets to grow films onto LaAlO{sub 3} substrates by pulsed laser deposition. Electrical resistance and thermopower, measured vs temperature and applied magnetic fields indicate transport dominated by positive small polarons in the high temperature paramagnetic state. The Hall effect was measured in 0.5 {mu}m thick films of composition x=0 and x=0.25. No evidence for extraordinary hall effect was found in the paramagnetic regime. Instead, the magnitude of the Hall coefficient decreases exponentially with temperature. This behavior and its anomalous negative sign are interpreted to result from face-diagonal hopping of small polarons in the Mn sublattice.
Date: October 1, 1996
Creator: Jaime, M.; Hardner, H.; Salamon, M.B.; Rubinstein, M.; Dorsey, P. & Emin, D.
Partner: UNT Libraries Government Documents Department

OXYGEN TRANSPORT CERAMIC MEMBRANES

Description: This is the third quarterly report on oxygen Transport Ceramic Membranes. In the following, the report describes the progress made by our university partners in Tasks 1 through 6, experimental apparatus that was designed and built for various tasks of this project, thermodynamic calculations, where applicable and work planned for the future. (Task 1) Design, fabricate and evaluate ceramic to metal seals based on graded ceramic powder/metal braze joints. (Task 2) Evaluate the effect of defect configuration on ceramic membrane conductivity and long term chemical and structural stability. (Task 3) Determine materials mechanical properties under conditions of high temperatures and reactive atmospheres. (Task 4) Evaluate phase stability and thermal expansion of candidate perovskite membranes and develop techniques to support these materials on porous metal structures. (Task 5) Assess the microstructure of membrane materials to evaluate the effects of vacancy-impurity association, defect clusters, and vacancy-dopant association on the membrane performance and stability. (Task 6) Measure kinetics of oxygen uptake and transport in ceramic membrane materials under commercially relevant conditions using isotope labeling techniques.
Date: October 1, 2000
Creator: Bandopadhyay, Dr. Sukumar & Nagabhushana, Dr. Nagendra
Partner: UNT Libraries Government Documents Department

The controlled growth of perovskite thin films: Opportunities, challenges, and synthesis

Description: The broad spectrum of electronic and optical properties exhibited by perovskites offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired. Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the monolayer-level; its use for the integration of perovskites with similar nanoscale customization appears promising. Composition control and oxidation are often significant challenges to the growth of perovskites by MBE, but we show that these can be met through the use of purified ozone as an oxidant and real-time atomic absorption composition control. The opportunities, challenges, and synthesis of oxide heterostructures by reactive MBE are described, with examples taken from the growth of oxide superconductors and oxide ferroelectrics.
Date: October 1, 1997
Creator: Schlom, D.G.; Theis, C.D. & Hawley, M.E.
Partner: UNT Libraries Government Documents Department

Spin excitations in [La{sub 1-x}Ca{sub x}MnO{sub 3}] in the mixed-phase region.

Description: The magnetic excitations and the ferromagnetic order parameter have been studied by neutron scattering in a series of the manganese-based CMR perovskites [La{sub 1-x}Ca{sub x}MnO{sub 3}] (x=0.46, 0.48, 0.50) near the metallic ferromagnetic to insulating antiferromagnetic phase. Well-defined ferromagnetic spin waves were detected for the x=0.46 and x=0.48 compositions. From the measurements of the order parameter, only the x=0.48 sample showed conclusive evidence of a coexistence of an antiferromagnetic phase with the ferromagnetic phase. For this composition, hysteresis was observed in the spin wave intensity but not in the spin stiffness parameter. This effect indicates that the ferromagnetic exchange is not perturbed by the antiferromagnetic ordering. No measurable ferromagnetic magnetization was found in the x=0.50 sample; thus no spin waves could be detected. The results indicate that the onset of the antiferromagnetism upon hole doping for the series occurs in a narrow region of x below the x=0.50 phase boundary.
Date: December 18, 2001
Creator: Stumpe, L.; Kirby, B.; Kaiser, H.; Rhyne, J. J. & Mitchell, J. F.
Partner: UNT Libraries Government Documents Department

Short range spin correlations in the CMR material La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The (La{sub 1{minus}} x Sr{sub x}){sub 3}Mn{sub 2}O{sub 7} compounds are layered materials that exhibit higher magneto-resistance than the corresponding 3D manganite perovskites. Quasi-elastic neutron scattering on a polycrystalline sample of La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} shows that the spin fluctuation spectrum of the these layered CMR materials is qualitatively similar to those found in the perovskite manganites (La,Ca)MnO{sub 3}; their concentration, lifetime, and coherence length increase as T decreases to T{sub c}. Unlike the perovskites we find a lower spin-diffusion constant above T{sub c} of {approximately}5 meV {angstrom}{sup 2}.
Date: March 23, 1998
Creator: Kelley, T. M.
Partner: UNT Libraries Government Documents Department

SYNTHESIS AND CHARACTERIZATION OF PROTON CONDUCTORS

Description: The morphological and electrical properties of yttrium (Y) and indium (In) doped barium cerate perovskites of the form BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} (with x=0-0.3) prepared by a modified Pechini method were investigated as potential high temperature proton conductors with improved chemical stability. The sinterability increased with the increase of In-doping, and the perovskite phase was found in the BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} solid solutions over the range 0 {le} x {le} 0.3. The conductivities decreased (from x to x, insert quantitative values) while the tolerance to wet CO{sub 2} improved for BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} samples with an increase of In-doping.
Date: February 18, 2010
Creator: Brinkman, K.
Partner: UNT Libraries Government Documents Department

Two-Dimensional Anisotropy in a Layered Metallic Antiferromagnet RE{sub 1-x}Sr{sub x}MnO{sub 3} with x approx.1/2

Description: Some RE{sub 1-x}Sr{sub x}MnO{sub 3} (RE=Nd,Pr) systems with x {approximately} 1/2 exhibit an A-type anti-ferromagnetic (AFM) structure instead of the expectged CE-type magnetic structure. This can be understood as being a consequence of the increase of the band width (W). These A-type magnetic systems also show salient features in their crystal structures. We proposed a two dimensional anisotropy of the transport as well as the magnetic properties for the A-type AFM structure and we show that we have, indeed, observed anisotropic spin wave dispersion relations by inelastic neutron measurements.
Date: July 1, 1997
Creator: Kawano, H.; Yoshizawa, H.; Fernandez-Baca, J.A.; Tomioka, Y. & Kuwahara, H.
Partner: UNT Libraries Government Documents Department

Zero-field {mu}{sup +}SR study of the collossal magnetoresistance material La{sub 0.67}Ca{sub 0.33}MnO{sub 3}

Description: Zero-field {mu}{sup +}SR and resistivity experiments on La{sub 0. 67}Ca{sub 0.33}MnO{sub 3} powder shows that the ferromagnetic transition temperature (Tc = 274 K) and resistivity peak temperature coincide to within 1K, about 10 K higher that T{sub c} determined from the bulk magnetization. The sublattice magnetization v{sub mu}(T) is well described for T {<=} T{sub c} by (1-T/T{sub c}){sup beta}, where {beta} = 0.345 {+-} 0.015. Unusual relaxational dynamics suggest a wide distribution of Mn-ion correlation times. These results are discussed in terms of the effects of polarons on the spin and charge dynamics.
Date: April 1, 1996
Creator: Heffner, R.H.; Le, L.P. & Hundley, M.F.
Partner: UNT Libraries Government Documents Department

Crystal chemistry and electronic properties of the n=2 Ruddlesden-Popper manganates: unconventional CMR materials

Description: The crystallography and electronic properties of the Ln[sub 2- x]Sr[sub 1+x]Mn[sub 2]O[sub 7] manganese oxides adopting the n=2 Ruddlesden-Popper (RP) structure are discussed, focusing on the structural phase diagrams and electronic properties in the vicinity of the Mn +3.5 oxidation state and in particular the ease of synthesis of single phases of these materials.
Date: December 31, 1996
Creator: Battle, P.D.; Green, M.A.; Millburn, J.E.; Spring, L.E.; Vente, J.F.; Blundell, S.J. et al.
Partner: UNT Libraries Government Documents Department

Structure versus properties in High-temperature superconductors

Description: This is the final report of a one-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). High-temperature superconductors (HTS) have crystal structure that make them a distorted subset of Perovskite materials. The complexity of the superconductors arising from the large number of elements that they possess gives us all sorts of imperfect regions in our materials, and these regions tend to show Perovskite-like properties. This project sought to expand our fundamental understanding of high-temperature superconductors by combining experimental and theoretical studies of this class of novel materials. The research objectives were to interrelate structural and superconducting properties of HTS materials to further understand parameters influencing their behavior in important power and electronic applications.
Date: October 1, 1996
Creator: Smith, J.L.; Chou, F; Foltyn, S.; Hammel, P.C.; Hawley, M.; Johnson, D. et al.
Partner: UNT Libraries Government Documents Department

Temperature-induced phase transitions during ion-beam irradiation of the perovskite-structure oxides

Description: Several perovskite-structure oxide compounds, including CaTiO{sub 3}, SrTiO{sub 3}, BaTiO{sub 3}, KNbO{sub 3}, and KTaO{sub 3} were irradiated by 800 keV Kr{sup +} ions in order to investigate and compare their response to heavy-ion irradiation. The critical amorphization temperature T{sub c}, above which amorphization temperature T{sub c}, above which amorphization could not be induced, was found to increase in the order SrTiO{sub 3} {yields} CaTiO{sub 3} {yields} BaTiO{sub 3} {yields} KNbO{sub 3} {yields} KTaO{sub 3}. No single physical parameter explains the observed sequence, although T{sub c} correlates well with the melting temperatures. The well-known temperature-driven phase transformations in these materials did not have a significant effect on the dose required for amorphization. Domain boundaries were observed in the pristine samples; however, after only a low dose, the boundaries became poorly defined and, with increasing dose, eventually disappeared. Dislocation loops were observed to aggregate at the domain boundaries.
Date: December 1997
Creator: Meldrum, A.; Boatner, L. A. & Ewing, R. C.
Partner: UNT Libraries Government Documents Department

Heteroepitaxy of perovskite ferroelectrics on silicon: a path to silicon-integrated ferroelectrics

Description: We have studied the thin-film heteroepitaxy between simple perovskite oxides and Si as it is accomplished with molecular beam epitaxy growth. Interface chemistry and electrostatics play critical roles in determining whether a commensurate interface develops. Submonolayer silicides are the required precursors for the transition to commensurate epitaxial oxide structures in these systems. This approach is illustrated with BaTiO{sub 3}; high crystalline perfection is achieved and resistivities as high as 10{sup 13} ohm-cm can be developed for BaTiO{sub 3} MOS capacitors on (001) Si. The epitaxy is (100)BaTiO{sub 3}//(001)Si with [0001]BaTiO{sub 3} aligned in plane with the polar axis along the [110] of Si. This epitaxy is dominated by a strong propensity for Ba to order along the [110] of Si during the submonolayer silicide formation. This commensurate heteroepitaxial growth leads to a truly monolithic crystal and attendant properties. 9 figs, 23 refs.
Date: October 1, 1996
Creator: McKee, R.A.; Walker, F.J. & Chisholm, M.F.
Partner: UNT Libraries Government Documents Department

Growth of epitaxial (Sr,Ba){sub n+1}Ru{sub n}O{sub 3n+1} films

Description: We have grown epitaxial (Sr,Ba) (n+1)Ru(n)O(3n+1) films, n = 1, 2, and infinity, by pulsed laser deposition (PLD) and controlled their orientation by choosing appropriate substrates. The growth conditions yielding phase pure films have been mapped out. Resistivity versus temperature measurements show that both a and c axis films of Sr2RuO4 are metallic, but not superconducting. The latter is probably due to the presence of low-level impurities that are difficult to avoid given the target preparation process involved in growing these films by PLD.
Date: December 1, 1997
Creator: Schlom, D.G.; Knapp, S.B. & Wozniak, S.
Partner: UNT Libraries Government Documents Department

Local and average crystal structure and displacements of La{sup 11}B{sub 6} and EuB{sub 6} as a function of temperature

Description: Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction (NPD) data and the local structure from La L{sub III}-edge x-ray-absorption fine-structure (XAFS) are presented for a La{sup 11}B{sub 6} sample as a function of temperature ({approx}10-320 K). These data are compared to XAFS results on a EuB{sub 6} sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB{sub 6} indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB{sub 6} place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.
Date: January 30, 2001
Creator: Booth, C.H.; Sarrao, J.L.; Hundley, M.F.; Cornelius, A.L.; Kwei, G.H.; Bianchi, A. et al.
Partner: UNT Libraries Government Documents Department

Structural disorder and transport in ternary oxides with the pyrochlore structure. Final report

Description: This research program has focused on the structure-electrical property relations in families of pyrochlore compounds which exhibit, on the one hand, controlled levels of structural disorder and on the other, controlled levels of ionic and electronic conductivities. Models have been developed to evaluate the often complex defect chemistry of these systems. Much progress has been made in extracting key thermodynamic and kinetic data. From a technological standpoint, novel solid electrolytes and compatible mixed conducting electrodes have been identified and the concept of the single phase monolithic fuel cell design has been demonstrated and patented. Related work on lanthanum gallate-based perovskites has shown even more promising results for use of such materials in the monolithic fuel cell structures. Recent work on the Bi{sub 3}Zn{sub 2}Sb{sub 3}O{sub 14} Pyrochlore, a phase found at grain boundaries in varistors, was also completed. This material was found to be a mixed ionic-electronic conductor with interesting implications for grain boundary equilibration kinetics in SnO-base varistor materials. Three of the most recent projects are summarized in this paper. The results of work on the perovskites are reported in recent publications.
Date: June 17, 2001
Creator: Tuller, Harry L.
Partner: UNT Libraries Government Documents Department

Diffusion-controlled creep in mixed-conducting oxides

Description: Steady-state creep rate of the mixed conducting oxides La{sub 1-x}Sr{sub x}MnO{sub 3} (x=0.1, 0.15, 0.25) and La{sub 0.7}Ca{sub 0.3}MnO{sub 3} has been investigated between 1150 and 1300 C. Creep parameters and TEM indicate that deformation is controlled by lattice diffusion of one of the cations. Dependence of creep rate on Sr concentration, combined with a point-defect model, confirms this hypothesis; however the oxygen partial pressure dependence of creep (from 10{sup -1} to 2x10{sup 4} Pa) cannot be accounted for within the framework of a simple point-defect model.
Date: June 1, 1996
Creator: Routbort, J.L.; Goretta, K.C.; Cook, R.E.; Wolfenstine, J.; Armstrong, T.R.; Clauss, C. et al.
Partner: UNT Libraries Government Documents Department