Differences In Short-range Order Research Articles

Network Connectivity and Crystallization in the Transparent Ferroelectric Nanocomposite LaBGeO5

The LaBGeO5 glass-ceramic composite is a transparent ferroelectric nanocomposite material that has come under attention for its ferroelectric properties and for its straightforward synthesis. The LaBGeO5 crystals-in-glass can be formed through controlled devitrification of glass, as well as through laser irradiation. Recent works have established that both the borate and germanate environments have significant differences in short-range order between the crystalline and glass forms of LaBGeO5. However, there are little available data on the connectivity of the different structural units. We present here a comprehensive nuclear magnetic resonance (NMR) spectroscopy study of the connectivity of crystalline LaBGeO5 and LaBGeO5 glass. Through a combination of 11B, 11B{10B}, 17O, and 139La NMR spectroscopies, we identify specific structural motifs in the glass. In particular, several structures are positively identified in the glass that are not present in the crystal, including BO2O–, BO2O––BO4–, and GeO6. We...

Mineralogical controls on soil black carbon preservation

Black carbon (BC) has long been considered a chemically resistant component of soil organic carbon (SOC). However, there is substantial evidence that the chemistry of most C compounds is less important for long‐term storage than is physical protection (e.g., mineral sorption). We explored BC retention in grasslands that lie along a climate gradient that produces strong differences in short range order (SRO) minerals known to drive landscape‐scale retention of SOC. We measured soil BC content using 13C nuclear magnetic resonance (NMR) spectroscopy, and used radiocarbon dating on a subset of samples to relate BC content to long‐term soil C storage. Black C concentrations in soil ranged from 0.2 to 2.9%, representing 10–30% of SOC and spanning levels found in temperate grasslands around the world. Black C concentrations were significantly correlated with SRO minerals, but the strongest single predictor of BC content was simply SOC. The ratio of BC/OC was fairly insensitive to SRO minerals, suggesting that BC responds similarly to reactive minerals as does OC. Direct links between SOC and BC retention warrant further study. We found no evidence that BC is preferentially retained relative to OC, with soil radiocarbon ages apparently driven primarily by inputs of new OC. These results indicate that physical protection may play a strong role in BC retention, and that BC cycling in soils may be more similar to OC cycling than is generally thought.

Structural Properties of Co and CoFe Electrodes Forming a Magnetic Tunnel Junction

The M1/MgO/M2 trilayer tunnel magnetoresistance systems are studied by means of X-ray diffraction, NMR, and transmission electron microscopy techniques. As M1 and M2 electrodes we used Co, Fe, and CoFe layers. The growth mechanism and structural quality of both electrodes and of the epitaxial MgO barrier forming the magnetic tunnel junctions are experimentally examined. It is shown that the crystallographic coherence of magnetic tunnel junctions across the MgO barrier is significantly disturbed by imperfect crystal structure of magnetic electrodes. The NMR results indicate a difference in short-range order between bottom and top electrodes.

Shear softening and structure in a simulated three-dimensional binary glass

Three-dimensional model binary glasses produced by quenching from a range of liquid temperatures were tested in shear over a range of strain rates using molecular-dynamics techniques. Tests were performed under constant volume and constant pressure constraints. The simulations revealed a systematic change in short-range order as a function of the thermal and strain history of the glass. While subtle signs of differences in short-range order were evident in the pair distribution function, three-body correlations were observed to be markedly more sensitive to the changes in structure. One particular structural parameter, the number of aligned three-atom clusters, was analyzed as a function of the degree of supercooling, the strain and the strain rate. The glasses quenched from the supercooled liquid regime were observed to contain an initially higher number of such clusters, and this number decreased under shear. Those quenched from high-temperature equilibrium liquids contained lower numbers of such clusters and these increased or remained constant under shear. The glasses quenched from the supercooled liquid regime showed higher strength, more marked shear softening, and an increased propensity toward shear localization. The evolution of this structural parameter depended both on its initial value and on the imposed shear rate. These results were observed to hold for simulations performed under both constant density and constant pressure boundary conditions.

Spectroscopically Determined Collagen Pyr/deH-DHLNL Cross-Link Ratio and Crystallinity Indices Differ Markedly in Recombinant Congenic Mice with Divergent Calculated Bone Tissue Strength

Whole bone strength can be partitioned into structural and material components. In three-point bending tests of 6-month-old female humeri from the HcB/Dem recombinant congenic series, strains HcB/8 and HcB/23 differed markedly in calculated failure stress but not ash percentage. Fourier transform infrared spectroscopic imaging was used to determine whether differences in the ratio of pyridinoline (pyr; nonreducible) to dehydrodihydroxynorleucine (de-DHLNL; reducible) collagen cross-links (XLR), mineral crystallinity, or spatial ordering could account for the strains' differing biomechanical performance. HcB/8 had significantly higher XLR and significantly higher crystallinity than HcB/23. XLR and crystallinity were highly and similarly correlated in both strains. There were no significant differences between the strains' one-dimensional spatial correlation functions, suggesting no difference in short-range order between them. The strong correlation between XLR and crystallinity reflects the interdependence of the protein and mineral elements of bone. The data illustrate the importance of material properties in addition to mineral quantity to bone tissue strength.

Mössbauer study of the microstructure of electrodeposited Co92-xWxFe8 films

Co92-xWxFe8 polycrystalline and amorphous films were produced by electrolysis and examined by means of Mossbauer spectroscopy and other structural techniques. Results suggest that there are no important differences in short range order between polycrystalline and amorphous films. In the crystalline and amorphous samples, there appear to exist three different local environments for the resonant 57Fe atoms: one magnetic and two non-magnetic. These results can be interpreted by the existence of two different fcc-Co short range orders: one magnetic and the other paramagnetic. Some part of the magnetic phase evolves towards a new non-magnetic phase when the W content in the alloy increases, giving rise to a new additional paramagnetic peak in the Mossbauer spectrum.

Molecular dynamics simulations of the bulk phases of 4-cyano-4′-n-pentyloxybiphenyl

Abstract Extensive molecular dynamics (MD) simulations of the crystalline, the isotropic, and the nemadc phase of 4-cyano-4′-npentyloxybiphenyl (50CB) are reported. Basing on the AMBER and the OPLS force field parameters for 5OCB were modified in order to perform the simulations within a model scenario, where the dipole moment and the rotational barriers are in agreement with experimental data. Up to now this is the first investigation of the problem, whether long-range orientational equilibrium can be achieved on the time scale of MD simulations with a full-detail molecular model. The simulations of the liquid-crystalline phase were started from configurations with different initial order parameters. Each order parameter remains stable even over the longest MD run of more than 1.6 ns. Collective motions of molecules and slow self-diffusion are discussed in the context of very slow equilibration of long-range order. In contrast, equilibrium of short-range orientational and positional order is reached. Antiparallel dimers are formed due to strong interactions between two cyano groups. Furthermore, a high probability density for cyano groups adjacent to the phenyl rings of the biphenyl moiety is observed. Although both effects are more pronounced in the liquid-crystalline phase, the differences in short-range order between the liquid and the liquid-crystalline phases are small.

Magnetism in amorphous metals and alloys

Abstract In amorphous transition metal-metalloid alloys, nearest neighbor short range order about metalloid atoms, and to a lesser extent about transition metal atoms, is similar, at least in an average sense, to that in the corresponding crystalline intermetallic compounds. In amorphous transition metal-metalloid alloys which are strongly ferromagnetic, magnetic moments and Curie temperatures are only weakly affected by the absence of crystalline long range order or by dispersions in near neighbor configurations. Magnetic moments and ordering temperatures of amorphous rare earth metal-transition metal alloys RE Fe 2 and RE Co 2 differ significantly from those of the corresponding Laves phase crystalline RE TM 2 compounds, and X-ray and neutron scattering studies of several of the amorphous alloys indicate significant differences in nearest neighbor short range order between the Laves phase crystalline compounds and the amorphous alloys. These differences in short range order may be largely responsible for the observed changes in magnetic properties.

Magnetostriction of ferromagnetic metallic glasses

Linear saturation magnetostrictions (295 K) of Fe 80t- x Co x B 20, 0⩽ x⩽80, and Fe 80− x Ni x B 20, 0⩽ x⩽60 (at. %) glasses are reported. The saturation magnetostrictions of the Fe-Ni-base glasses vary as the sqaure of their respective saturation magnetizations. The behavior in the Fe-Co-base glasses is strikingly different. The compositional trends suggest a difference in origin of magnetostrictions between Fe-Ni-base and Co-containing glasses. When magnetostrictions for these glasses are compared with those for Fe-Co and Fe-Ni polycrystalline alloys, the most significant discrepancies appear in the Fe-rich compositions. This is probably related to the difference in short range order between the crystalline and non-crystalline Fe-rich alloys: α-Fe is 8-fold coordinated whereas all of the metallic glasses studied show ∼ 12-fold metal coordination.