Chemical disorder reinforces magnetic order in ludwigite (Ni,Mn)(3)BO5 with Mn4+ inclusion

Crystals of ludwigite Ni2.14Mn0.86BO5 were synthesized by flux growth technique and contain Mn3+ and Mn4+. A possible mechanism of the manganese valence states stabilization has been proposed. The structural and magnetic characterization of the synthesized samples has been carried out in detail. The cations composition and Mn valence states of the crystal were determined using X-ray diffraction and EXAFS technique. The comparative analysis was carried out between the studied crystal and Ni2MnBO5 synthesized previously. Magnetic susceptibility measurements were carried out. The magnetic transition in the studied composition occurs at the 100 K temperature that is higher than in Ni2MnBO5 although the studied composition is more disordered. The calculations of the exchange integrals in the frameworks of indirect coupling model revealed strong antiferromagnetic interactions. The several magnetic subsystems existence hypothesis was supposed. The possible magnetic structure was suggested from the energies estimation for different ordering variants

Features of microstructure and magnetic flux dynamics in superconducting Nb-Ti tapes with strong anisotropic pinning

Results of micro-structural, phase and texture analysis of Nb-Ti tapes and results of magnetic flux dynamics fixed by Magneto-Optical Imaging (MOI) method are presented in order to investigate origin and consequences of the anisotropic pinning model proposed, and implemented earlier experimental results including Guided Vortex Motion (GVM) phenomenon

Features of microstructure and magnetic flux dynamics in superconducting Nb-Ti tapes with strong anisotropic pinning

Results of micro-structural, phase and texture analysis of Nb-Ti tapes and results of magnetic flux dynamics fixed by Magneto-Optical Imaging (MOI) method are presented in order to investigate origin and consequences of the anisotropic pinning model proposed, and implemented earlier experimental results including Guided Vortex Motion (GVM) phenomenon

Local atomic and crystal structure rearrangement during the martensitic transformation in Ti50Ni25Cu25Ti_{5}0Ni_{25}Cu_{25} shape memory alloy

The changes of crystal structure and local crystalline environment of Ti, Ni and Cu atoms in Ti50Ni25Cu25 shape memory alloy are investigated using X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) in temperature range of martensite transformation. The analysis of the EXAFS-spectra shows that the bonds involving Ni atoms have the highest degree of disorder and the change in the local environment around Ni atoms is significant for the occurrence of the shape memory effect, while Cu atoms occupy the normal positions in the crystallographic structure and have the lowest displacement amplitude leading to the stabilization of both phases

Manganese Luminescent Centers of Different Valence in Yttrium Aluminum Borate Crystals

We present an extensive study of the luminescence characteristics of Mn impurity ions in a YAl3(BO3)4:Mn crystal, in combination with X-ray fluorescence analysis and determination of the valence state of Mn by XANES (X-ray absorption near-edge structure) spectroscopy. The valences of manganese Mn2+(d5) and Mn3+(d4) were determined by the XANES and high-resolution optical spectroscopy methods shown to be complementary. We observe the R1 and R2 luminescence and absorption lines characteristic of the 2E ↔ 4A2 transitions in d3 ions (such as Mn4+ and Cr3+) and show that they arise due to uncontrolled admixture of Cr3+ ions. A broad luminescent band in the green part of the spectrum is attributed to transitions in Mn2+. Narrow zero-phonon infrared luminescence lines near 1060 nm (9400 cm−1) and 760 nm (13,160 cm−1) are associated with spin-forbidden transitions in Mn3+: 1T2 → 3T1 (between excited triplets) and 1T2 → 5E (to the ground state). Spin-allowed 5T2 → 5E Mn3+ transitions show up as a broad band in the orange region of the spectrum. Using the data of optical spectroscopy and Tanabe–Sugano diagrams we estimated the crystal-field parameter Dq and Racah parameter B for Mn3+ in YAB:Mn as Dq = 1785 cm−1 and B = 800 cm−1. Our work can serve as a basis for further study of YAB:Mn for the purposes of luminescent thermometry, as well as other applications

Local crystal structure of TiNiCu shape memory alloys

The features of long-range crystal structure and local environment of Ni and Cu atoms in shape memory alloy Ti50Ni25Cu25 are investigated using X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) in the temperature range of direct and reverse martensitic transformations. According to XRD study, alloy has the B2 type structure in austenitic phase and B19 type structure (space group Pmmb) in martensitic phase. EXAFS demonstrates the difference between Cu-Ti and Ni-Ti bond lengths ~ 0.09 Å not typical for common TiNi lattice. The static disordering is more significant in the local environment of Ni atoms than in local environment of Cu atoms

Energy-transfer luminescence of a zinc oxide/ytterbium oxide nanocomposite

© The Royal Society of Chemistry 2012. A newly structured nanocomposite material based on nanocrystalline ZnO/Yb 2 O 3 has been prepared by thermal decomposition of Yb-doped zinc carbonate hydroxide. Transmission electron microscopy has revealed that the prepared nanopowder consists of ZnO nanocrystals of about 50 to 100 nm size decorated by attached smaller Yb 2 O 3 nanocrystals of about 10 to 15 nm size. X-Ray absorption spectroscopy, in particular XANES and EXAFS, indicate the charge of Yb ions equals to +3 and their coordination is oxygen octahedral with the Yb-O and Yb-Yb interatomic distances the same as in bulk Yb 2 O 3 . Photoluminescence spectroscopy unambiguously proves an efficient excitation energy transfer from the ZnO nanocrystals to the Yb 3+ ions. The energy transfer from the ZnO nanocrystals (absorption range from 250 to 400 nm) to the Yb 3+ ions (emission range from 950 to 1100 nm) has been explained by a model, which considers the quantum cutting effect. The prepared nanocomposite is promising for application as a down-conversion layer for enhanced solar cells.status: publishe

Key Roles of Size and Crystallinity of Nanosized Iron Hydr(oxides) Stabilized by Humic Substances in Iron Bioavailability to Plants

Availability of Fe in soil to plants is closely related to the presence of humic substances (HS). Still, the systematic data on applicability of iron-based nanomaterials stabilized with HS as a source for plant nutrition are missing. The goal of our study was to establish a connection between properties of iron-based materials stabilized by HS and their bioavailability to plants. We have prepared two samples of leonardite HS-stabilized iron-based materials with substantially different properties using the reported protocols and studied their physical chemical state in relation to iron uptake and other biological effects. We used Mössbauer spectroscopy, XRD, SAXS, and TEM to conclude on iron speciation, size, and crystallinity. One material (Fe-HA) consisted of polynuclear iron­(III) (hydr)­oxide complexes, so-called ferric polymers, distributed in HS matrix. These complexes are composed of predominantly amorphous small-size components (<5 nm) with inclusions of larger crystalline particles (the mean size of (11 ± 4) nm). The other material was composed of well-crystalline feroxyhyte (δ’-FeOOH) NPs with mean transverse sizes of (35 ± 20) nm stabilized by small amounts of HS. Bioavailability studies were conducted on wheat plants under conditions of iron deficiency. The uptake studies have shown that small and amorphous ferric polymers were readily translocated into the leaves on the level of Fe-EDTA, whereas relatively large and crystalline feroxyhyte NPs were mostly sorbed on the roots. The obtained data are consistent with the size exclusion limits of cell wall pores (5–20 nm). Both samples demonstrated distinct beneficial effects with respect to photosynthetic activity and lipid biosynthesis. The obtained results might be of use for production of iron-based nanomaterials stabilized by HS with the tailored iron availability to plants. They can be applied as the only source for iron nutrition as well as in combination with the other elements, for example, for industrial production of “nanofortified” macrofertilizers (NPK)