Sten G. Eriksson

Neutron Diffraction Studies of Pb(ZrxTi1-x)O3 Ceramics

Neutron diffraction studies of lead zirconate titanate Pb(ZrxT1-x)O-3 (PZT) powders, 0.20 less than or equal to x less than or equal to 0.54, were carried out at 10 K and 297 K, The phase transitio ...

Neutron total scattering study of the delta and beta phases of Bi2O3

The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable alpha and metastable beta phases at ambient temperature, which is characteristic of the cations 6s2 lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3.

Fabrication and characterization of highly pure and homogeneous YBa2Cu3O7 superconductors from sol‐gel derived powders

The high Tc superconducting oxide YBa2Cu3O7−d has been fabricated by a simple colloidal sol‐gel precursor technique based on citrate complexes uniformly dispersed into ethyleneglycol and water. The present method permits easier fabrication of highly pure and homogeneous superconducting materials when compared with other more elaborate preparation procedures based on solution techniques as it eliminates many steps such as centrifugation, filtration, and pH control. The purity of the final product was estimated to be >99.5% based on the combined results of x‐ray diffraction and Raman scattering analyses. The composition of the sample was determined to be Y1.00Ba1.98Cu3.05O6.85 by inductively coupled plasma spectroscopy and idiometry. The sol‐gel process produced homogeneous materials with particle sizes smaller than ∼3 μm. The superconducting transition determined with resistivity measurements was shown to be sharp with a transition width narrower than 1 K. The magnetic susceptibility measurements exhibited...

A neutron powder diffraction study of the ferroelectric relaxor Pb(Fe1/2Ta1/2)O3

The complex perovskite lead iron niobate, Pb(Fe1/2Ta1/2)O3 (PFT), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. A straightforward unit cell of symmetry R3m was obtained for the 300 K structure, with the same symmetry, and a similar unit cell also obtained at the low temperature. In both cases, the iron and titanium ions were found to be disordered over the perovskite B-sites. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic arrangement. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFT are discussed by comparison with the related system of Pb(Fe1/2Nb1/2)O3.

Structural Properties, Mössbauer Spectra, and Magnetism of Perovskite-Type Oxides SrFe1–xTixO3–y

Perovskite-type phases SrFe1–xTixO3–y with 0.1 ≤ x ≤ 0.7 have been prepared from the oxides, and, in order to reach high oxygen contents and FeIV fractions, annealed at oxygen pressures of 60 MPa. The materials were characterised by powder x-ray and neutron diffraction, 57Fe Mossbauer spectroscopy, and magnetic susceptibility measurements. All samples of the series crystallise in a cubic perovskite structure and reveal considerable oxygen deficiency. The Mossbauer parameters suggest that for x = 0.1, where the FeIV fraction is about 90%, the itinerant electronic state of SrFeO3 is essentially retained. In materials with larger x increasing amounts of TiIV and FeIII ions lead to a stronger localisation of the σ* (Fe 3 d – O 2 p) electrons. There is no evidence for a charge disproportionation of FeIV in any of the materials. Magnetic susceptibility measurements show a divergence of zero-field cooled and field-cooled data below a temperature Tm and deviations from Curie-Weiss behaviour above Tm. The data are indicative of spin-glass behaviour due to disorder and competing exchange interactions. Strukturelle Eigenschaften, Mosbauer-Spektren und Magnetismus von Perowskit-artigen Oxiden SrFe1–xTixO3–y Perowskit-artige Phasen SrFe1–xTixO3–y mit 0.1 ≤ x ≤ 0.7 wurden aus den Oxiden hergestellt und, um hohe Sauerstoffgehalte und FeIV-Anteile zu erreichen, unter Sauerstoffdrucken von 60 MPa erhitzt. Die Praparate wurden mit Rontgen- und Neutronenpulverdiffraktometrie, 57Fe-Mosbauer-Spektroskopie und magnetischen Suszeptibilitatsmessungen charakterisiert. Alle Proben der Serie kristallisieren in der kubischen Perowskitstruktur und weisen Sauerstoffdefizite auf. Die Mosbauer-Parameter deuten darauf hin, das fur x = 0.1, wo der FeIV-Gehalt ca. 90% betragt, der itinerante Elektronenzustand von SrFeO3 im wesentlichen erhalten bleibt. In Praparaten mit groserem x fuhren wachsende Konzentrationen an TiIV- und FeIII-Ionen zu einer starkeren Lokalisierung der σ*(Fe 3 d – O 2 p) Elektronen. Fur keine der untersuchten Proben gibt es Hinweise auf eine Ladungsdisproportionierung von FeIV. Unterhalb einer Temperatur Tm werden unterschiedliche Suszeptibilitatskurven beobachtet, je nachdem, ob ohne magnetisches Feld oder im magnetischen Feld abgekuhlt wurde. Oberhalb von Tm treten Abweichungen vom Curie-Weiss-Verhalten auf. Diese Befunde deuten auf Spin-Glas-Verhalten hin, das durch Unordnung und konkurrierende Austauschwechselwirkungen bedingt ist.

High-quality ceramics of YBa2Cu4O8 from citrate sol-gel precursors sintered at one atmosphere oxygen pressure

Abstract Single phase ceramic material of the superconducting phase YBa2Cu4O8 has been synthesized at 780–790°C under one atmosphere oxygen pressure with a sol-gel powder precursor technique. The precursor was derived from citrate metal ion complexes uniformly dispersed in a solvent mixture of ethyleneglycol and water. The main advantages of the present method are, apart from the low oxygen pressure needed, the elimination of centrifugation, ageing and fine control of pH usually required for other solution techniques. X-ray diffraction analysis show that the material is virtually of single-phase character with only minor traces of CuO (∼1%). AC-susceptibility measurements indicate a high-quality sample with an onset of the superconducting transition at Tc=82.6 K. The superconducting transition is much sharper, ΔTc(10–90%)=4.5K, than for ceramic samples of previously reported synthesis procedures.

Polymerized complex synthesis of a pure 93 K Y2Ba4Cu7O15−d superconductor without the need of high oxygen pressure and additive catalysts

High‐purity ceramic material of the superconducting phase Y2Ba4Cu7O14.82 (247) has been synthesized at 870 °C by the polymerized complex method using neither high oxygen pressure nor additive catalysts. The method is based on the formation of a polymer‐metal complex precursor which is prepared through polyesterification between metal citrate complexes and ethylene glycol. Apart from the fact that high oxygen pressure is unnecessary, the present preparation technique offers more convenient and easier fabrication of highly pure 247 material compared with other wet chemical routes since it eliminates many steps such as centrifugation, filtration, aging, and pH control. X‐ray diffraction and Raman scattering analyses show that the material is single‐phase without any indication of secondary phases. Zero‐resistance has been achieved at 88.0 K with a transition width narrower than 4 K. Complex ac magnetic susceptibility measurements confirm the presence of a single bulk superconducting 247 phase with Tc(onset)=...

Electrochemically deposited polycrystalline n-CdSe thin films studied with laterally resolved photoelectrochemistry and SEM

Semiconducting thin films of n-CdSe were electrochemically deposited on Ti substrates with the potential difference as the driving force. By using a photomasking technique different film thicknesses could be obtained on the same substrate. The film electrodes were characterized with photoelectrochemical (PEC) imaging, optical microscopy and scanning electron microscopy (SEM) /energy-dispersive X-ray analysis. PEC imaging is done with a fibre-optical laser scan microscope that allows a laterally resolved mapping of the photoelectrochemical response. The results show that substrate preparation is crucial for the photoelectrochemical performance of the film. At scratches in the substrate the film shows a lower photoresponse. Very thin films give low photoresponse. These films also show a different morphology in the SEM pictures. The films are not homogeneous and have a less ideal composition when they are too thin. We have found PEC imaging a good complement to other methods for characterization of semiconducting thin films.

THE HUME - ROTHERY COMPOUND MN8GA27.4ZN13.6 : SEPARATED ZN13-CLUSTERS INTERSPERSED IN A PRIMITIVE CUBIC HOST LATTICE

Separated Zn13 cluster entities unexpectedly occur in the solid-state structure of Mn8 Ga27.4 Zn13.6 (the central building block is shown). They correspond to centered cuboctahedra, that is, small volumes of face-centered cubic metal. The intriguing segregation of Ga and Zn atoms in Mn8 Ga27.4 Zn13.6 was verfied by Rietveld refinement of neutron powder diffraction data.

Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: A play in the octahedra to tetrahedra ratio in oxygen-deficient perovskites

Three different perovskite-related phases were isolated in the SrGa(1-x)Sc(x)O(2.5) system: Sr(2)GaScO(5), Sr(10)Ga(6)Sc(4)O(25), and SrGa(0.75)Sc(0.25)O(2.5). Sr(2)GaScO(5) (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å] with complete ordering of Sc(3+) and Ga(3+) over octahedral and tetrahedral positions, respectively. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. I4(1)/a, a = 17.517(1) Å, c = 32.830(3) Å]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) can be described as a stacking of eight perovskite layers along the c axis ...[-(Sc/Ga)O(1.6)-SrO(0.8)-(Sc/Ga)O(1.8)-SrO(0.8)-](2).... Similar to Sr(2)GaScO(5), this structure features a complete ordering of the Sc(3+) and Ga(3+) cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr(10)Ga(6)Sc(4)O(25) is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc(3+) by Ga(3+) leads to the formation of the cubic perovskite phase SrGa(0.75)Sc(0.25)O(2.5) (x = 0.25) with a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa(0.75)Sc(0.25)O(2.5)·xH(2)O hydrate, which exhibits a proton conductivity of ∼2.0 × 10(-6) S/cm at 673 K.