Crystal Engineering最新文献

The origin of the correlation between the microscopic and macroscopic material properties that are formed as a result of large-scale fluctuations in the process of evolution of the material to a new state was investigated. Each value of the state parameter corresponds to a certain set of large-scale fluctuations and microscopic states. It was established that the correlations between the dependences of the microscopic and macroscopic properties of the materials are determined by the interatomic interactions and the spatial arrangement of atoms.

Recently, the crystal structure of synthetic and natural Cu-O compounds was analyzed in the unified classification scheme. Structural units of cuprates as salts of hypothetical copper acids were distinguished and assumed to be responsible for characteristic physical properties of these Cu-O compounds. In this work the optical and magnetic properties were studied in a series of synthetic and natural cuprates. The fine structure in the optical spectra of (M₂Cu₂O₃)_m(CuO₂)_n, CuGeO₃, linarite PbCu(OH)₂(SO₄) and malachite Cu₂(OH)₂CO₃ and other Cu-O compounds was correlated to the electronic excitations in particular structural units typical of cuprates. The temperature dependence of magnetic susceptibility was investigated on a series of Cu oxysalt minerals. The dimerization of magnetic moments was considered as a property, which was determined by the type of cuprate structural units. The observed relationship between the crystal structure and physical properties is useful in search of new magnetic materials.

The family (SrO)(SrNbO_2-xN)_n is the first example of Ruddlesden-Popper strontium niobium oxynitrides and provides layered structures to obtain mixed valence niobium compounds with a variety of oxidation states and transport properties. We have prepared members with n=1 (Sr₂NbO₃N) and n=2 (Sr₃Nb₂O₅N₂) by solid state reaction of Nb₂O₅ and SrCO₃ at 900–1050 °C under ammonia flow. These compounds crystallize in the I4/mmm space group and show a range of anionic stoichiometry that together with the N/O ratio determines the formal oxidation state for niobium and therefore the resultant physical properties. Samples of the n=1 member show mixed valence Nb⁵⁺/Nb⁴⁺ and are paramagnetic and very resistive at room temperature. This behavior may be understood by considering electron localization due to N/O disorder.

Perovskite manganites are known as functional materials showing colossal magnetoresistance and are used as magnetic sensors. We report on the synthesis and characterization of La_0.67Ca_0.33MnO₃, La_0.67Sr_0.33MnO₃, and La_0.67Ba_0.33MnO₃ polycrystalline bulk materials. Detailed measurements of the magnetization as function of temperature and magnetic field for these samples were carried out. Significant entropy changes near the Curie temperatures are obtained from the magnetization data. The specific heat changes of these samples near their phase transition temperatures are derived from magnetic measurements. Our results and the relevant data from various references are summarized. Furthermore the magnetocaloric effects and potential applications in magnetic cooling of perovskite manganite materials are evaluated.

The structure of Ag₂Cu₂O₄ synthesized by electrochemical oxidation of its precursor Ag₂Cu₂O₃ is studied under a new perspective. Irradiation of the oxide with X-ray beam transforms the electronic structure of the phase by redistributing the internal charge within the material. The refined structure shows several possibilities all with metals in the same arrangement, but with oxygen atoms distributed in a rather disordered way. This is typical of ionic conductors.

Single crystals of Bi₁₂TiO₂₀, and Bi₄₀Ga₂O₆₃ (titanium, and gallium sillenite) were grown with use of Top Seeded Solution Growth (TSSG) technique. Mixed sillenites described by the formula Bi₁₂Ti_1–xM_xO₂₀ (M=V, Ga, Pb), some of them doped additionally with Cu were also obtained. Photoconductivity under permanent, chopped, and pulse illumination and photochromism of obtained crystals were investigated and discussed. Some characteristic common features were found for photoconductivity obtained under conditions of chopped and constant illumination. Comparing the best known sillenite photoconductors we managed to obtain mixed titanium sillenites having photoconductivity four to five orders of magnitude higher.

The recent discovery of metallic-like superconductivity in bulk MgB₂ material [1] opened a new class of possible applications in current transport, magnetic field devices and electronic devices. Fabrication of thin films in situ is essential in order to explore the device applications of this material, and in spite of a few reports [2], [3], [4], this still remains a challenge. The main reasons for this are the high vapour pressure of Mg even at low temperatures, and the high susceptibility of Mg to oxidation. A number of c-axis oriented MgB₂ films were grown on Al₂O₃-R cut, and the stoichiometry of the films was measured by ICP as a function of deposition pressure and laser fluence. It was found that the relative concentration of Mg on the substrate varies considerably with pressure and laser fluence. A relative concentration of around 53 at% Mg, which is necessary in order to form stoichiometric MgB₂ phase, can be obtain only in a narrow range of deposition pressures and laser fluences. The films obtain under these conditions were characterised by X-ray diffraction and magnetic moment in applied fields up to 9T.

Polycrystalline KClO₃-, K₂CO₃-, Rb₂CO₃-doped and undoped YBa₂Cu₃O_7−x (YBCO) ceramic high-T_c superconductors were examined using electron backscatter diffraction (EBSD) technique. We present orientation maps and pole figures of the samples. It is shown that the pure YBCO samples and the ones with Rb₂CO₃ and K₂CO₃ additives do not exhibit a very pronounced texture (i.e. preferred orientation). On the polycrystalline KClO₃-doped YBCO samples we find two major grain orientations (0 0 1) and (1 0 0) normal to the sample surface while the pure YBCO sample does not exhibit any orientation maxima. This observation is ascribed to the effect of a liquid phase due to the KClO₃-additive during the preparation process.

The crystal structure of zirconium tungstate exhibits large isotropic negative thermal expansion. As a result, zirconium tungstate might find use in applications which require tailored thermal expansion properties. In this work, the sintering ability of commercially available ZrW₂O₈ and the reactive sintering of an oxide mixture was studied. The microstructure obtained when sintering oxide mixtures on porous alumina based refractory fibre-boards is characterised by large pores and clear grain boundaries, arising from a liquid phase sintering mechanism due to chemical interaction of the oxide mixture with the fibre-board. Although this interaction is avoided when using commercially available zirconium tungstate its sintering ability is very poor. When reactive sintering of oxide mixtures was performed in sealed quartz tubes in air, dense zirconium tungstate was obtained. The reactivity of the oxide mixtures using quartz tubes as support, is dependent on the homogeneity and particle size of the precursor oxide mixture.

LaMO₃-type perovskites are promising ceramic materials used as electrode materials in fuel cells or as the catalytic active component in motorcar exhaust catalysts. In this work we present a systematic study on the morphology of LaMO₃-perovskites containing late 3d-transition metals prepared by thermal decomposition of the corresponding citrate precursors at the lowest possible temperature. The calcination temperature was determined by thermogravimetric analysis and XRD. The specific surface areas as well as the particle morphologies of the perovskite powders are investigated by SEM and compared with Brunauer-Emett-Teller (BET) surface area measurements. The effect of the solvent, which was used for the precursor synthesis as well as the citrate stochiometry on the nanoparticle morphology will be taken into evaluation.