A Mössbauer Study of the Geological Samples from Permian Period

The samples from Permian period in east China were investigated by Mössbauer spectroscopy, X-ray powder diffraction and TGA-DTA methods. According to the different valence states of the iron in samples, the divisions of the Permian may also be separated by such methods at first report. The result shows a new approach for the studies in Permian.     

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite

The origin of dielectric anomalies and magnetodielectric response of La(2)MnCoO(6) has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn(4+)/Co(2+) valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency-temperature region where extrinsic effects dominate the dielectric response.     

Excitation pathways and efficiency of Eu ions in GaN by site-selective spectroscopy

Using combined excitation emission spectroscopy, we performed a comparative study of europium ions in GaN in samples that have been in situ doped during interrupted growth epitaxy (IGE) or conventional molecular beam epitaxy (MBE) as well as samples that were grown using organometallic vapor phase epitaxy (OMVPE) and subsequently ion implanted with Eu ions. Through site-selective resonant excitation, we are able to unambiguously assign all major observed transitions to a combination of different incorporation sites and electron–phonon coupled transitions. We identified at least nine different incorporation sites of Eu ions in GaN and studied how these sites behave under different excitation conditions and how their relative number is modified by different growth and doping conditions. The coupling to phonons has also been studied for a series of Al _x Ga_1−x N samples with x=0…1. We find that a main site most resembling an unperturbed Eu ion on Ga site is always dominant, while the minority sites are changing substantially in relative numbers and can occur in some samples fairly close in emission intensity to the main site. In terms of the excitation pathway after the creation of electron-hole pairs, we found three types of centers: (1) sites that are dominantly excited through shallow defect traps; (2) sites that are excited through a deep defect trap; (3) sites that cannot be excited at all including the majority of the main sites. We interpret this finding to indicate that the ion in this environment is not very efficient in trapping excitation and that the indirect excitation involving other traps depends on the ion/trap distance. Many of the main sites are far away from these traps and cannot be excited through this channel at all. The efficiency of excitation is highest for the deep traps, indicating that it would be desirable to enrich the respective site, as has been done with some success in the IGE grown samples.     

Relation between T c and the defect structure of neodymium cerium cuprite Nd2−x CexCuO4−y

Neutron powder diffraction has been used to study the defect structure of neodymium cerium cuprite Nd_2−x Ce_xCuO_4±y (x=0.15). It has been shown that in addition to oxygen vacancies, O2 sites in superconducting samples may also contain a small quantity of implanted oxygen atoms positioned between copper ions and neodymium/cerium, which control the electrical charge in the Cu-O planes. The oxygen distribution among crystal lattice sites in Nd_2−x Ce_xCuO_4±y (x=0.15) as determined, the average charge of the copper ions was calculated by the method of valence sums, and a correlation was established between the charge of the copper-oxygen plane and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 177–183 (February 1998)     

ZrO_2超微粉的电子顺磁共振谱研究

本文采用共沉淀法合成了不同尺寸的ZrO2超微粉末。对不同粒径的样品进行了X射线衍射和电子顺磁共振谱测量。     

Powder diffraction from a continuous microjet of submicrometer protein crystals

Atomic‐resolution structures from small proteins have recently been determined from high‐quality powder diffraction patterns using a combination of stereochemical restraints and Rietveld refinement [Von Dreele (2007), J. Appl. Cryst. 40 , 133–143; Margiolaki et al. (2007), J. Am. Chem. Soc. 129 , 11865–11871]. While powder diffraction data have been obtained from batch samples of small crystal‐suspensions, which are exposed to X‐rays for long periods of time and undergo significant radiation damage, the proof‐of‐concept that protein powder diffraction data from nanocrystals of a membrane protein can be obtained using a continuous microjet is shown. This flow‐focusing aerojet has been developed to deliver a solution of hydrated protein nanocrystals to an X‐ray beam for diffraction analysis. This method requires neither the crushing of larger polycrystalline samples nor any techniques to avoid radiation damage such as cryocooling. Apparatus to record protein powder diffraction in this manner has been commissioned, and in this paper the first powder diffraction patterns from a membrane protein, photosystem I, with crystallite sizes of less than 500 nm are presented. These preliminary patterns show the lowest‐order reflections, which agree quantitatively with theoretical calculations of the powder profile. The results also serve to test our aerojet injector system, with future application to femtosecond diffraction in free‐electron X‐ray laser schemes, and for serial crystallography using a single‐file beam of aligned hydrated molecules.     

Sound velocities of compressed Fe3C from simultaneous synchrotron X‐ray diffraction and nuclear resonant scattering measurements

The applications of nuclear resonant scattering in laser‐heated diamond anvil cells have provided an important probe for the magnetic and vibrational properties of ⁵⁷Fe‐bearing materials under high pressure and high temperature. Synchrotron X‐ray diffraction is one of the most powerful tools for studying phase stability and equation of state over a wide range of pressure and temperature conditions. Recently an experimental capability has been developed for simultaneous nuclear resonant scattering and X‐ray diffraction measurements using synchrotron radiation. Here the application of this method to determine the sound velocities of compressed Fe₃C is shown. The X‐ray diffraction measurements allow detection of microscale impurities, phase transitions and chemical reactions upon compression or heating. They also provide information on sample pressure, grain size distribution and unit cell volume. By combining the Debye velocity extracted from the nuclear resonant inelastic X‐ray scattering measurements and the structure, density and elasticity data from the X‐ray diffraction measurements simultaneously obtained, more accurate sound velocity data can be derived. Our results on few‐crystal and powder samples indicate strong anisotropy in the sound velocities of Fe₃C under ambient conditions.     

Site-specific electronic structure of bacterial surface protein layers

We applied resonant photoemission and X-ray absorption spectroscopy for a detailed characterization of the valence electronic structure of the regular two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602. Using this approach, we detected valence electron emission from specific chemical sites. In particular, it was found that electrons from the π clouds of aromatic systems make large contributions to the highest occupied molecular orbitals.     

Understanding the instrumental profile of synchrotron radiation X‐ray powder diffraction beamlines

A Monte Carlo algorithm has been developed to calculate the instrumental profile function of a powder diffraction synchrotron beamline. Realistic models of all optical elements are implemented in a ray‐tracing software. The proposed approach and the emerging paradigm have been investigated and verified for several existing X‐ray powder diffraction beamlines. The results, which can be extended to further facilities, show a new and general way of assessing the contribution of instrumental broadening to synchrotron radiation data, based on ab initio simulations.     

X‐ray fluorescence determination of the manganese valence state and speciation in manganese ores

This work concerns determination of the manganese valence state and speciation by wavelength‐dispersive X‐ray fluorescence analysis. The authors investigated the effect of the manganese valence state and speciation on the intensity of some К‐series lines of the X‐ray emission spectrum for the samples of manganese compounds. The intensities of MnKβ₅ line and MnKβ′ satellite are least influenced by speciation, and they may be used for evaluating the manganese valence state for the samples containing low iron. The intensities of MnKβ″ and MnKβ^x satellites may be employed for assessing the manganese speciation. The results of X‐ray fluorescence determination of the manganese valence state and speciation in the manganese ores of the South Ural deposits agree with the X‐ray diffraction data. The X‐ray fluorescence method is definitely advantageous, because it does not require a complicated process of sample preparation and allows to receive fast information on the manganese valence state and speciation with the purpose to assess the quality of manganese ores. Copyright © 2015 John Wiley & Sons, Ltd.     

Structures of superconducting Ba2YCu3O7-ϖ and semiconducting Ba2YCu3O6 between 25°C and 750°C

The structure and copper valence states of the 100K superconductor, Ba₂YCu₃O₇ have recently been determined by neutron powder diffraction between 5K and 300K¹, and at room temperature². This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed^3,4, is well ordered and stochiometric, and contains one Cu⁺⁺⁺ and two Cu⁺⁺ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO₄ squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO₄ squares form infinite chains on the second Cu-site. In this paper we show that when Ba₂YCu₃O₇ is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal⁵, with the P4/mmm Ba₂YCu₃O₆ structure obtained by X-ray diffraction at room temperature⁶. This material is well ordered, stochiometric and stable when cooled under vacuum. The valence of Cu2 in the a-b sheets remains close to 2, while that of Cu1 in the b-axis chains falls to near 1. The valence of copper in these chains can perhaps fluctuate, since the vibration of O4 out of the chain, which we observed even at 5K¹, implies a reduction of the valence charge on this copper site. We have observed similar large amplitude oxygen vibration at low temperature in La₂CuO₄.     

Materials Research with X-ray Micro-beams at the Advanced Photon Source

X-ray scattering has had a rich and prominent place in the history of materials research. For samples that are large single crystals, X-ray diffraction provides very detailed, high-quality information about the atomic structure of almost every kind. However, most engineering materials are not a large single crystal, but mixtures of materials or a poly-crystalline material. Still, X-ray diffraction using X-ray beams significantly larger than the structures being investigated has produced much excellent information. Examples of this are powder diffraction, texture analysis, strain analysis, etc. While these types of analyses have been very useful in providing a microscopic understanding, they do not provide the microscopic information that we really want. What we really want to have is this useful X-ray diffraction information for every volume element in our sample; not just the diffraction information about small volume elements, but all of the volume elements, exactly where they are in our sample, and how each one diffracts. This detailed spatial and structural information is what is needed for a full understanding of the source of the properties that we find in engineering materials.     

Properties of three-dimensional composites based on opal matrices and magnetic nanoparticles

Three-dimensional nanocomposites consisting of an opal matrix and a metal have been prepared by the interaction of salts and oxides of different elements (Ni, Co, Fe, etc.) embedded in an opal matrix with isopropanol in the range of supercritical state parameters of the alcohol. According to X-ray powder diffraction analysis and transmission electron microscopy data, the composites consist of an X-ray amorphous opal matrix with pores filled by nanoparticles of Co (or CoO _x ), metallic Ni, or Fe₃O₄ with a magnetite structure of various morphology. The sizes of the nanoparticles do not exceed the diameter of the pores in the opal matrix. A complex investigation of the nanocomposites has been performed using the electron magnetic resonance and vibrating magnetometry methods. All the studied samples at room temperature exhibit a ferromagnetic behavior. The coercive force of the samples lies in the range from 150 Oe for iron-containing nanocomposites to 565 Oe for cobalt-containing nanocomposites.     

Phase changes in Fe72−x Al28Cr x (x = 0,2,4,6) alloys due to mechanical strain

Fe_72−x Al₂₈Cr _x (x = 0,2,4,6) are made by arc melting a mixture of constituent elements in stoichiometric proportion, in argon atmosphere. The ingots so obtained are filed to make powder samples thereby giving them substantial mechanical deformation. It is observed that as-powdered samples show hyperfine field distribution typical of α-phase, where the atoms are randomly distributed on the available sites. Annealing at 900°C for 60 h leads to preferential occupation of lattice sites by the atoms and this results in better defined groups of hyperfine magnetic field (HMF) which can be associated with specific configuration in the neighbourhood of probe iron atoms. The average HMF is found to decrease sharply with increasing Cr concentration even though the net chromium concentration remains low (≤6 at%). The results show that cold working on samples is very important in changing the atomic ordering and must be taken into account if properties of equilibrium phases are probed.     

Resonant diffraction in stishovite near the <Emphasis Type="Italic">K</Emphasis> absorption edge of silicon

The X-ray resonant diffraction in a stishovite crystal near the K absorption edge of silicon (E _K = 1839 eV) is studied theoretically. For such a long wavelength, the only possible Bragg reflection is the 100 reflection, which is forbidden by the space group of the crystal. It can be excited solely due to anisotropy of the X-ray scattering amplitude. The crystal symmetry is used to determine the polarization and azimuthal dependence of the reflection intensity. Since this reflection is single, it can be detected upon diffraction from a powder, which substantially widens the possibilities of investigations. The numerical calculations of the energy dependences of the forbidden reflection intensity and the absorption coefficient demonstrate that the dipole-quadrupole, quadrupole-quadrupole, and dipole-octupole contributions to the resonant diffraction and absorption are small and that the dipole-dipole contribution is the most important one.     

Spin liquid in an almost ferromagnetic Kondo lattice

A theory of stabilization of a spin liquid in a Kondo lattice at temperatures close to the temperature of antiferromagnetic instability has been developed. Kondo exchange scattering of conduction electrons leads to emergence of a state of the spin liquid of the resonating valence bonds (RVB) type at T>T _K. Owing to this stabilization, low-energy processes of Kondo scattering with energies below T _K are frozen so that the “singlet” state of the Kondo lattice is not realized; instead a strongly correlated spin liquid with developed antiferromagnetic fluctuations occurs. A new version of the Feynman diagram technique has been developed to describe interaction between spin fluctuations and resonant valence bonds in a self-consistent manner. Emergence of a strongly anisotropic RVB spin liquid is discussed. Zh. éksp. Teor. Fiz. 112, 729–759 (August 1997)     

Novel results on structural investigations of natural minerals of clathrate hydrates

High-resolution powder X-ray diffraction measurements were carried out on gas hydrates in the angle-dispersive mode using a synchrotron radiation source. In contrast to the structural studies of laboratory-grown gas hydrates, this study has been performed on naturally grown clathrate hydrates obtained from the sea floor at different geographic locations. While the hydrate samples of the Cascadia Margin exhibit a preponderance of structure I, those from the Gulf of Mexico consist of mixed structures, namely structure II, structure I and structure H. Ice in structure Ih is inherently present in all the clathrate hydrate samples. PACS 61.10.Nz; 61.50.Ah; 61.66.Fn; 91.50.-r     

Mössbauer and X-ray diffraction studies of mechanically alloyed Fe-Al

Powder samples of Fe₂₅Al₇₅ were prepared by the mechanical alloying method. Mössbauer effect, X-ray diffraction and DSC measurements indicate that Fe and Al crystalline powder transform into Fe-Al amorphous powder with increasing milling time. The X-ray diffraction patterns of the milled Fe₂₅Al₇₅ do not clearly show a sign of the existence of the intermetallic phases or Fe-Al solid solution. However, Mössbauer measurements reveal two sites with hyperfine magnetic fields 30.2 and 26.0 T. These sites form locally during the milling process and then they disappear.     

Possible metallization of the Mott insulators TiOCl and TiOBr: Effects of doping and external pressure

We report the results of doping- and pressure-dependent experimental investigations on the low-dimensional Mott-Hubbard insulators TiOCl and TiOBr. As observed by photoelectron spectroscopy, doping with Na and K shifts the valence band to higher binding energies associated with a jump of the chemical potential. With increasing doping, a broad hump develops close to the Fermi energy and the energy gap “softens”. The application of pressure induces the appearance of spectral weight close to the Fermi energy, as probed by infrared transmittance and reflectance measurements. The pressure-induced changes in the electronic properties coincide with a structural phase transition according to powder X-ray diffraction studies under pressure. The experimental findings are compared to theoretical predictions.