S. Vratislav

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.

Structure and magnetic properties of Pr1−xSrxMnO3 perovskites

Abstract A comprehensive account of structural features in mixed Mn3+, Mn4+ perovskites, Pr1−xAxMnO3 (A = Ca, Sr, Ba) is given. The newly investigated Sr system was studied at various temperatures by X-ray and neutron diffraction, magnetic, and conductivity measurements. A low-temperature transition from the activated type to the degenerate type of conductivity was observed in the Sr system for x = 0.3–0.4. It occurs 20–60 K below the ferromagnetic Curie point.

Electric transport and magnetic properties of perovskites LaMn1−xCoxO3 up to 900 K

The structural, magnetic and transport properties of the LaMn1−xCoxO3 system were investigated over a wide temperature range, 10–900 K. Two structural types, depending on x, were detected—orthorhombic Pbnm () and rhombohedral (x>0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn1−xCoxO3 system, the respective substituents are unambiguously characterized as Co2+ () and Mn4+ (). On the Mn-rich side, up to x<0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co2+ valence towards the prevailing Co3+, while for the Co-rich samples (), the Co3+ states tend to disproportionate at high temperatures to Co2++Co4+, which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below K and coagulate below K.

Distribution of cations in nanosize and bulk Co–Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.

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.