M. Dlouhá

Neutron diffraction study of Pr1 − xCaxMnO3 perovskites

Abstract A thorough powder neutron diffraction study of Pr 1 − x Ca x - Mn 3+ 1 − x Mn 4+ x O 3 (0 x 3+ ions and their orbitals (0 ⩽ x ⪅ 0.3, 0.3 ⪅ x x ⪅ 0.9 and 0.9 ⪅ x ⩽ 1) was revealed. The corresponding prototypes are x = 0, 0.5, 0.75 and 1. The departures from these ideal compositions introduce substancial changes in magnetic interactions and, as a consequence, a variety of magnetic structures (including different noncollinear ones) are observed at low temperatures. The magnetic structures are interpreted by considering the role of superexchange and double exchange.

Neutron diffraction study of crystal and magnetic structures of BaTi0.6Mg0.6Fe10.8O19 and BaTi2Co2Fe8O19

Abstract Two single-crystal samples of the barium hexaferrite with partial substitution of Fe 3+ ions by Ti 4+ + Mg 2+ and Ti 4+ + Co 2+ , respectively, have been investigated at ambient and cryogenic temperatures by the neutron diffraction. A break of the extinction law of group P6 3 /mmc was observed irrespective the temperature. The structure was refined within the trigonal P 3 m 1 symmetry. At the room temperature, the samples display a slightly canted modification of the Gorter-type ferrimagnetism with the easy-axis anisotropy for BaTi 0.6 Mg 0.6 Fe 10.8 O 19 and the easy-cone anisotropy for BaTi 2 Co 2 Fe 8 O 19 . The later type is related to the one-ion contribution of Co 2+ ions in octahedral sites. Conical-spiral arrangements of the block-type are established at low temperature.

Crystal structure of the mixed conductors phases, Li0.5-3xLa0.5+x+yTi1-3yM3yO3 (M = Mn, Cr) with x = 0.133 and y = 0.20

Perovskite-like solid solutions of general formula Li 0.5-3x La 0.5+x+y Ti 1-3y M 3y O 3 (M=Mn, Cr) show three polymorphs; A, beta; and C. The crystal structure of the C polymorphs in manganese- and chromium-lanthanum systems, determined from powder neutron diffraction using Rietveld refinement, are of the ordered perovskite type. The structures of both phases are similar, containing a three dimensional framework of corner-sharing MO 6 (M=Ti or Mn/Cr) octahedra in which the structures are partially collapsed as a result of a cooperative tilting and rotation of octahedra. Orthorhombic unit cell, M=Mn: a=5.5411(11) A, b=7.8120(14) A, c=5.4924(10) A; M=Cr: a=5.5014(9) A, b=7.7735(15) A, c=5.4729(9) A; space group Pnma (no. 62). Ionic conductivity takes place by a hopping mechanism between Li + -occupied and empty A-sites, while electronic conductivity is along octahedra.

Neutronographic Texture Analysis of Zirconium Based Alloys

Neutron diffraction is a very powerful tool in texture analysis of zirconium based alloys used in nuclear technique. Textures of five samples (two rolled sheets and three tubes) were investigated by using basal pole figures, inversion pole figures, and ODF distribution function. The texture measurement was performed at diffractometer KSN2 on the Laboratory of Neutron Diffraction, Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague. Procedures for studying textures with thermal neutrons and procedures for obtaining texture parameters (direct and inverse pole figures, three dimensional orientation distribution function) are also described. Observed data were processed by software packages HEXAL and GSAS. Our results can be summarized as follows: i) All samples of zirconium alloys show the distribution of middle area into two maxima in basal pole figures. This is caused by alloying elements. A characteristic split of the basal pole maxima tilted from the normal direction toward the transverse direction can be observed for all samples, ii) Sheet samples prefer orientation of planes (100) and (110) perpendicular to rolling direction and orientation of planes (002) perpendicular to normal direction, iii) Basal planes of tubes are oriented parallel to tube axis, meanwhile (100) planes are oriented perpendicular to tube axis. Level of resulting texture and maxima position is different for tubes and for sheets. The obtained results are characteristic for zirconium based alloys.

Structure and texture analysis of PVC foils by neutron diffraction

Crystalline order of molded and then bi-axially stretched foils prepared from atactic PVC resin is investigated by means of wide-angle neutron diffraction (WAND). The observed high-resolution WAND patterns of all samples are dominated by a sharp maximum corresponding to the inter-planar distance 0.52 nm. Two weaker maxima are also resolved at 0.62 and 0.78 nm. Intensities of the peaks vary with deformation ratios of the samples and their diffraction position. Average size of the coherently scattering domains is estimated as approximately 4-8 nm. Based on the experimental data, a novel model of crystalline order of atactic PVC is proposed.

Texture Investigations of Zircaloy-4 Tubes by Neutron Diffraction

This article presents the results of texture investigations of Zircaloy-4 tubes. Neutron diffraction and inversion pole figures were used to chracterize the texture of samples. The main interest is given to the analysis of texture changes under different tensile test conditions including tensile force direction, annealing temperature and tensile test temperature. Four different specimens of different orientation to the tube axis were tested.

Texture Analysis of Poly(vinyl Chloride) Foils

The crystallographic textures of the free-standing diamond films which were synthesized by the chemical vapor deposition (CVD) using a CH4/H2/Ar gas mixtures have been investigated. Texture components of the diamond films are analyzed with pole figures and orientation distribution function (ODF). The surface morphology of the diamond films grown at different substrate temperatures as well as different gas concentrations is observed with scanning electron microscopy.(SEM). The experimental results indicate that the intensity of {110} fibre texture becomes much higher with the increase of the substrate temperature. It is considered that the higher multiplicity of <110> crystallographic direction in diamond octahedron leads to the higher appearance possibility of {110} fibre texture and the final texture components of the diamond films depend upon the CVD deposition parameters.

Neutron studies of the structure and dynamics of Rb1 − x(NH4)xI mixed crystals

The structural and dynamical properties of the x-T phase diagram of the system of Rb1 − x(NH4)xI mixed crystals is of great interest, because such solid solutions are almost free of internal stresses due to almost equal ionic radii of ammonium and rubidium. The x-T phase diagram of Rb1 − x(NH4)xI is studied on samples with ammonium concentration ranging from 0.01 to 0.73 over the temperature range from 15 to 300 K by the methods of powder neutron diffraction and inelastic incoherent neutron scattering. The results obtained by powder neutron diffraction show that the α-β phase transition at low temperature is rather extended and occurs at the ammonium concentrations x = 0.50 and 0.66. The region of orientational state glass is determined by inelastic incoherent neutron scattering at the concentrations x = 0.29 and 0.40 at the temperature T = 20 K.

New Data Collection Subsystem for Powder Neutron Diffractometer with Position Sensitive Detectors

Project INDECS (Integrated Neutron Diffraction Experiment Control System) is a newly developed software system created for the purpose of data acquisition from, and controlling of the upgraded version of the KSN-2 powder neutron diffractometer equipped with Position Sensitive Detectors. The KSN-2 neutron diffractometer (belonging to LND, DSSE, FNSPE, CTU in Prague, CZ, placed at the NRI, Řež near Prague, CZ) was recently upgraded with the possibility to contain up to three parallel linear PSDs which can be mounted on the detector arm instead of the original single counter detector. That was the motivation for the project INDECS to be started. For the actual data acquisition and initial data analysis of the raw sampled signals within the project INDECS a special modular structure called the PSD Acquisition Path (or PSDAP) was designed. Raw sampled signals from either end of the PSD are taken as the main input to the subsystem. The signals are then split into multiple events, analyzed for the position of the obtained events on the PSD, and a histogram of the diffraction events is created as the main output of this subsystem. Moreover, the PSDAP is capable of storing the raw signals either before event splitting or after, so that later the whole process of acquisition can be replayed in software, perhaps with different settings of the processing parameters or perhaps even algorithms. It can also be switched into a mode where under special conditions a correction curve specific for the given PSD can be constructed and later in normal operation it can be used to do the corrections of detected event positions to enhance the results on that particular PSD. This whole subsystem then acts as a single modular command unit (External Execution Module or EEM) within the entire system of project INDECS.

Structural, magnetic, and transport properties of the single-layered perovskites La{sub 2-x}Sr{sub x}CoO{sub 4} (x=1.0-1.4)

The layered cobaltites La{sub 2-x}Sr{sub x}CoO{sub 4} have been complexly studied over the range of mixed Co{sup 3+/4+} valency and their properties discussed in comparison with cubic perovskites La{sub 1-x}Sr{sub x}CoO{sub 3}. The sample x=1.0 exhibits a paramagnetic ground state based on the intermediate spin state of Co{sup 3+} and undergoes a broad resistivity transition at 400-900 K, associated with diffusive changes of magnetic susceptibility and volume expansion. The Co{sup 4+} doping in samples with x>1 induces gradually ferromagnetism and leads to a dramatic decrease of resistivity and thermopower, though the metallicity is not achieved. The neutron diffraction on the limit composition x=1.4 suggests that the long-range ordered ferromagnetic regions coexist with paramagnetic ones, and their amount changes little with an application of high pressures up to 4.3 GPa.