Papers by Wojciech Paszkowicz
Acta Crystallographica Section A, Aug 22, 2011
Thin films built from binary IIIV nitrides or from their solid solutions are basic components of ... more Thin films built from binary IIIV nitrides or from their solid solutions are basic components of modern optoelectronic devices. In particular, indium nitride is a component suitable for high-speed electronics and solar cells. Experimental data on the bulk nitride material behaviour under specific thermal conditions (e.g., on variation of its stability, resistance to oxidation, thermal expansion coefficient with temperature) provide a valuable basic information needed for design and development of such devices. The properties such as decomposition temperature, oxidation rate at elevated temperatures, and thermal expansion are of importance for technologies of growth of low-dimensional structures. The current knowledge of thermal properties [1-3] may require an extension through studies in neutral or oxidizing environment in broad temperature ranges. The aim of the present work is to present selected experimentally determined temperature-dependent properties (thermal decomposition, interaction with air, thermal expansion) and discuss them on the basis of existing literature data. Polycrystalline indium nitride samples differing by grain size were sealed in capillaries filled with air and in argon, and then studied by Xray diffraction in a broad temperature range. High-resolution powder diffractometer at the B2 beamline (DORIS-III, Hasylab, DESY) equipped in a position-sensitive detector ensured collection of data with excellent counting statistics. The X-ray diffraction experiments were conducted at increasing temperatures using a step of 50 K or less; the experiments were stopped when the full decomposition of InN was concluded. High-resolution electron microscopy was used for determination of the microstructure and crystallite size. The structures of InN and minority phases as well as those of the oxidation products were refined using the Rietveld method. The obtained results demonstrate that the decomposition is, as expected, much faster for the sample composed of small grains. The oxide formation, observed when heating indium nitride in air, is attributed to oxidation of the nitride (at moderate temperatures) or to interaction of oxygen with indium vapour (at high temperatures). Thermal expansion was determined from the lattice parameters variation between 22 and 900 K. In this range, the unit-cell volume increases from 61.78 to 62.5 Å 3 , whereas the thermal expansion coefficient, α V , increases from 0 to 19×10-6 K-1. Acknowledgements: The partial support of the European Community in the framework of ITN RAINBOW
Thin films of GdMnO 3 were deposited by RF magnetron sputtering on SrTiO 3 (100) and Si (100) sub... more Thin films of GdMnO 3 were deposited by RF magnetron sputtering on SrTiO 3 (100) and Si (100) substrates. XRD analysis revealed that GdMnO 3 films grown on SrTiO 3 (100) substrates are well textured. The out-of-plane c axis is elongated by 0.75 % - 1.10 % in comparison to bulk lattice value and depends on film thickness. The minimal elongation was found for the GdMnO 3 film deposited on a La 0.7 Sr 0.3 MnO 3 (LSMO) buffer layer. M/H vs. T (5<T<100K) as well as M vs. H (T=5K) dependences obtained for a 100 nm thick sample of GdMnO 3 / SrTiO 3 (100) point to the absence of magnetic anisotropy unlike the case of a GdMnO 3 single crystal. This effect is attributed to the significant structural disorder due to misfit stress. AC resistivity measurements show a measurable influence of the static magnetic field on the impedance parameters, in accordance with the (H, T)-phase diagram of the GdMnO 3 single crystal.
Computer methods in materials science, 2013
Crystals
The structural and luminescence properties of a new material, Ca10.5−xNix(VO4)7, formed by substi... more The structural and luminescence properties of a new material, Ca10.5−xNix(VO4)7, formed by substitution of a fraction of calcium by nickel, are studied as a function of the Ni content (x). The powder X-ray diffraction results for the polycrystals, synthesized using a solid-state reaction method, show that in the studied temperature range (300–1150 K), the structure of the unsubstituted material (space group R3c, whitlockite-β-Ca3(PO4)2 structure type) is conserved up to the solubility limit, x = 0.72(2), determined on the basis of variation of unit cell size with x. The samples of nominal composition exceeding this limit contain a significant amount of the impurity phase.The structural refinements demonstrate that Ni atoms preferentially occupy the M5 site (one of the five independent Ca sites, M1–M5). The unit cell size was equally studied in the range of 300–1150 K, leading to the determination of the thermal expansion coefficients. It was found that with rising Ni content, the ro...
Aluminium nitride (AlN) nanopowder was successfully synthesized from aluminium oxide via a single... more Aluminium nitride (AlN) nanopowder was successfully synthesized from aluminium oxide via a single-step reaction with ammonia as a source of nitrogen. The process was carried out in a horizontal mullite tube reactor located in an electric furnace in the ranges of temperature, time and gas ? ow rate of 1050-1350 oC, 1-5 h and 100-150 l/h, respectively. Nanopowders of �� -Al2O3 and �. -Al2O3 were used as starting materials. In? uence of temperature, time, gas ? ow rate and type of aluminium oxide on reaction productivity, powder morphology and product phase composition was investigated. Dynamic light scattering (DLS), Ramman spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and specio c surface area measurements by the BET isotherm technique were used to characterized the obtained materials. Aluminium nitride prepared under optimal reaction conditions, comprising the reaction time of 5 h, the temperature of 1250 oC, and the gas? ow rate of 150 l/h, was found to be nanocrystalline powder with a mean crystallite size of about 60 nm in the synthesis from gamma aluminium oxide and 65 nm in case of alfa aluminium oxide as a starting material. The productivity of nitridation reaction in such conditions was close to 99%, basing on gravimetric measurements. In case of the synthesis from gamma oxide, PXRD analysis indicated the pure hexagonal aluminium nitride structure which was cono rmed by TEM analysis. In case of alfa aluminium oxide nitridation, the obtained aluminium nitride contained the unreacted oxide. Additionally, the investigation of average agglomerate sizes in the water suspension conducted by DLS showed that aluminium nitride obtained from �� -Al2O3 had an average particle size of 282 nm, and in the case of �. -Al2O3 used as a starting material it was 2219 nm..
Journal of Crystal Growth, 1989
GaP/GaP epitaxial layers doped with nitrogen and sulphur, prepared by VPE CVD method, were studie... more GaP/GaP epitaxial layers doped with nitrogen and sulphur, prepared by VPE CVD method, were studied using Bond diffractometry and double crystal spectrometry. A correlation between type and concentration of dopants and temperature dependence of the lattice constants was found. The values of thermal expansion coefficient (TEC) for GaP: S suggest that the defect structure of GaP: S is similar to that of nonstoichiometric GaP crystal at lower temperatures. The presence of the N dopant does not cause any changes of TEC over the whole investigated concentration range while the lattice constant, the misfit factor and the density of misfit dislocations are strongly dependent on the N doping level. The dependence of the misfit factor on the N concentration becomes nonlinear at a high doping level. For GaP: N,S we suggest the existence of a different type of defects than for the case of undoped nonstoichiometric layers. The explanation of the degradation in photoluminescence is discussed on the basis of the results obtained.
Journal of Applied Physics, 2007
A detailed optical characterization of a series of wurtzite-type Cd1-x-yBexZnySe mixed crystals g... more A detailed optical characterization of a series of wurtzite-type Cd1-x-yBexZnySe mixed crystals grown by the modified high pressure Bridgman method has been carried out via photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 15-300 K. Room temperature surface photovoltage spectroscopy has been used as a diagnostic technique for checking the surface condition of the sample. For the
Journal of Alloys and Compounds, 2005
Rietveld refinement was performed for two samples prepared from natural pyrite (a polymorph of ir... more Rietveld refinement was performed for two samples prepared from natural pyrite (a polymorph of iron disulphide or iron persulphide, FeS 2) single crystals originating from Navajun (Spain) and from Ural Mts. (Russia). The present refinements provide the following crystallographic data: a = 5.41784(2) and 5.41819(2)Å, x = 0.3848(1) and 0.3840(1) for the above-mentioned crystals, respectively. They are comparable with those of several crystals from other geological deposits, reported in literature. The obtained diffraction results indicate an off-stoichiometry with a higher sulphur content in the sample from Ural. The last observation is qualitatively supported by the results of X-ray photoelectron spectroscopy. The crystallographic data are discussed on the basis of earlier literature information on composition and structure reported for synthetic bulk and thin-layered pyrite.
Computational Materials Science, 2009
A genetic algorithm aiming for finding the global minimum and multiple deep local minima of a fun... more A genetic algorithm aiming for finding the global minimum and multiple deep local minima of a function exhibiting a complex landscape is studied. A feedback dynamic penalty function is used as a means to direct the algorithm to look for new local minima. The penalty is applied in close vicinity of all minima found before the current search stage. The last one, where the population tends to be trapped in, is treated smoothly. The penalty becomes progressively active there causing that the population progressively transfers outside the trapping area. The method ascertains that, unlike in more classical approaches, after finding the global minimum and a number of local ones on the way, the algorithm continues the exploration and identifies new local minima. Performance tests are described for a task of indexing of a powder diffraction pattern. The presented way of constructing the penalty function is to some extent problem specific, but the applied scheme may be adapted to other global search and optimisation problems, in particular to those requiring identification of multiple deep local minima.
Journal of Magnetism and Magnetic Materials, 1999
ABSTRACT The magnetic properties of zinc-blende Hg1−xMnxS, where x=0.024 and 0.066 were presented... more ABSTRACT The magnetic properties of zinc-blende Hg1−xMnxS, where x=0.024 and 0.066 were presented. The measurements were done by the SQUID technique. The magnetic susceptibility was measured in the temperature range from 1.8 to 180 K. The magnetisation was recorded in magnetic fields up to 50 kOe at liquid helium temperatures. The experimental data were analyzed simultaneously in extended nearest neighbour par approximation leading to the Mn–Mn nearest-neighbour exchange constant JNN=−7.7±1.1 K and its radial decay for further neighbours proportional to: ∝R−5. The standard analysis of high-temperature susceptibility gave the value of effective exchange constant Jeff=−11.8±1.2 K. The obtained data were compared with those published earlier for Hg1−xMnxS. Our data confirmed the chemical trend expected within the whole family of mercury chalcogenides.
Acta Crystallographica Section A Foundations and Advances, 2017
Nanocrystals are produced by many different methods, such as hyfrothermal route, sputtering, epit... more Nanocrystals are produced by many different methods, such as hyfrothermal route, sputtering, epitaxy and many others. One of most original is the microbial route serving for fabrication of metallic and oxide nanoparticles. The obtained nanoparticles can adopt very simple and very complex forms. The variety of the reported shapes is quite large. The nanocrystal shape can be controlled by choice of the synthesis technique and its parameters as shown, e.g., in refs. [1,2]. Some technologies produce the simplest shapes of cubes and other simple polyhedra, spheres, discs and rods. Many others lead to various nanocrystalline forms of high geometrical, morphological and compositional complexity. These forms are typically named after shape of objects belonging to the macroworld (cylinders, nails, necklaces, ribbons, boxes, capsules, belts, pyramids, snowflowers), space objects (stars), plants (flowers, dendrites, forests), and animals (hedgehogs, multipods, corals, drones). Unusual forms include, for example, disordered pyramidal pits ('negative pyramids') on a ZnSe film surface and SiO2 hedgehogs built from a spherical gallium-based droplet and covered by short quasi-symmetrical coneshaped needles, characterized by a composition gradient. Nanocrystals can be porous or hollow, single phase or polyphase, may have uniform or non-uniform composition. Many forms of crystals growing in the nanoscale do not resemble those growing in the macroscale. The smallest ones belong to the subnanoworld, whereas the upper size limit is not strictly defined (the frequently adopted limit is 1 micrometer in at least one dimension). Nanocrystals are organized or selforganized in various ways: as separate crystals, nano-objects deposited on films or as inclusions within bulk crystals, as more or less ordered objects. Some of them belong to the category of fractals, some others are organized hierarchically. Nanocrystals can form arrays, superlattices, complexes, nanoclusters, bunches, nanoinclusions, core-shell structures etc. Frequently, individual nanocrystals can be treated as building blocks of these complex structures. The organization of such small elements is usually termed as nanoarchitecture. The shape of a nanocrystal is an important factor determining the physicochemical properties of the material. A large free surface is one of known shape-dependent features enabling application of specific materials as catalysts. Specific nanoarchitectures have been designed and reported, being characterized by low density leading to possible construction of ultralight materials. Almost twenty simple shapes have been distinguished for metallic nanocrystals [3]. The anticipated extension of nanocrystal shape classification and analysis is expected to be helpful in further work on design of nanomaterials and in understanding of their properties. [1] S.
Acta Physica Polonica A, 2011
Introduction The Ln1-xSrxCoO3-δ solid solution oxides crystallise in perovskite-type ABO3 structu... more Introduction The Ln1-xSrxCoO3-δ solid solution oxides crystallise in perovskite-type ABO3 structures with a lanthanide ion (e.g. La) or an alkaline earth ion (e.g. Sr) at the A site and with a cobalt ion at the B site. These oxides are subject of many studies because of their unique physical properties. It was recently shown that the presence of oxygen vacancies changes the valence state of Co ion. The oxygen content may thus influence the structural, electrical and magnetic properties of these materials. A distinctive feature that results from the oxygen deficiency is so-called mixed conductivity. This phenomenon is based on a combination of high ionic conductivity and high electronic conductivity [1]. It opens possibilities for practical applications of these materials as high temperature ceramic membranes [2] and as cathode materials for solid oxide fuels [3-5]. Ln1-xSrxCoO3-δ undergoes phase transitions which are dependent on the kind and substitution amount of the lanthanide io...
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Papers by Wojciech Paszkowicz