[en] We present detailed local structure measurements (using the extended x-ray absorption fine structure technique) for the colossal magnetoresistive material La_1-xCa_xMnO₃ (0.21 < x < 0.45) as a function of temperature and magnetic field. The local distortions of the Mn-O bonds are parameterized using σ, the width of the Mn-O pair-distribution function (PDF). After subtracting thermal phonon contributions, we show that the contributions to σ² from polaron and Jahn-Teller (JT) distortions, σ_JT/polaron², are a universal function of the magnetization, independent of how the magnetization is achieved via changes in temperature or magnetic field. However this universal behavior is only observed for B fields (ge) 2 T, likely as a result of domain canting in low B fields. The resulting curve is well described by two straight lines with significantly different slopes. These regimes represent two distinctly differ distortions of the oxygen octahedra about the Mn. For low magnetizations up to ∼65% of the theoretical maximum magnetization, M_T, the slope is low and the distortion removed as the sample becomes magnetized is small - we argue this arises from polarons which have a low distortion around two (or possibly three) Mn sites. At high magnetizations large distortions per Mn site are removed as these sites become magnetized. The data are also analyzed in terms of a two Mn-O peak distribution using experimental standards for Mn-O. The results agree well with recent neutron PDF results but not with some earlier results. We discuss the limitations of assuming a two peak distribution in view of the two distortions needed to describe the Mn-O distortions as a function of T and B for B (ge) 2 T. It is likely that there is a distribution of longer bonds. Finally we show that with increasing B field, the Mn-Mn peak also has a small B-field-induced change - a measure at the unit cell level of magnetostriction but find that there is no observable B-field-induced change in the Mn-La/Ca pair distribution for fields up to ∼10 T.

[en] To develop mechanistic models of contaminant distribution processes, the authors conducted an X-ray absorption fine structure analysis of strontium sorption to hydrous manganese oxide (HMO). Sr K-edge measurements were performed at 298, 220, and 77 K, and at sample loadings from 10^-4 to 10^-2 mol Sr/g HMO. Results from fitting the first shell in the sorbed samples indicate that strontium is surrounded by 10--12 oxygen atoms at an average distance of 2.58 (angstrom). This coordination environment is consistent with the strontium atom remaining hydrated upon sorption to the oxide, where in water hydrated strontium has approximately 9 atoms of oxygen at 2.62 (angstrom). Furthermore, the temperature dependence of the strontium-oxygen bond also suggests physical adsorption due to the large contribution of the dynamic component of the Debye Waller factor. Although second-shell data are consistent with either 3 manganese atoms at 4.12 (angstrom) or 6 strontium atoms at 3.88 (angstrom), both the near-edge and fine structure data for the manganese K-edge indicate that the local coordination environment of the manganese ion remains intact as a function of time or strontium sorption. Furthermore, the local structure of amorphous manganese oxide is highly ordered

[en] Zinc sorption to hydrous manganese oxide (HMO)-coated clay was investigated macroscopically, kinetically, and spectroscopically. Adsorption edges and isotherms revealed that the affinity and capacity of the HMO-coated montmorillonite was greater than that of montmorillonite, and when normalized to the oxide present, the coatings behaved similarly to the discrete Mn oxide. Over two pH conditions, 5 and 6, a linear relationship was observed for the isotherms; further analysis with X-ray absorption spectroscopy (XAS) resulted in one type of sorption configuration as a function of loading and ionic strength at pH 5. However, at a surface loading of 10^-3 mol_Zn g_{HMO-coated clay}^-1 source when the pH increased from 5 to 7, the first shell distance decreased slightly, while the atoms and coordination numbers remained the same; this change may be attributed to an increase in electrostatic interactions. After a contact time of 4 months where an additional 60% of the sites become occupied, the slower sorption process was modeled as intraparticle surface diffusion. Best fit diffusivities ranged from 10^-18 to 10^-17 cm²/s, where a slower process was observed for the coated surface as compared to the discrete oxide. Interestingly, the porosity of the Mn oxide coating appears to be influenced by the substrate during its growth, as its increase and shift to a smaller pore size distribution resulted in a diffusivity between that observed for discrete HMO and montmorillonite

[en] Hydrous Fe and Mn oxides (HFO and HMO) are important sinks for heavy metals and Pb(II) is one of the more prevalent metal contaminants in the environment. In this work, Pb(II) sorption to HFO (Fe₂O₃·nH₂O, n=1-3) and HMO (MnO₂) surfaces has been studied with EXAFS: mononuclear bidentate surface complexes were observed on FeO₆ (MnO₆) octahedra with Pb(single_bond)O distance of 2.25-2.35 Angstroms and Pb(single_bond)Fe(Mn) distances of 3.29-3.36 (3.65-3.76) Angstroms. These surface complexes were invariant of pH 5 and 6, ionic strength 2.8x10^-3 to 1.5x10^-2, loading 2.03x10^-4 to 9.1 x 10^-3 mol Pb/g, and reaction time up to 21 months. EXAFS data at the Fe K-edge revealed that freshly precipitated HFO exhibits short-range order; the sorbed Pb(II) ions do not substitute for Fe but may inhibit crystallization of HFO. Pb(II) sorbed to HFO through a rapid initial uptake (∼77%) followed by a slow intraparticle diffusion step (∼23%) resulting in a surface diffusivity of 2.5x10^-15 cm²/s. Results from this study suggest that mechanistic investigations provide a solid basis for successful adsorption modeling and that inclusion of intraparticle surface diffusion may lead to improved geochemical transport depiction

[en] Synchrotron infrared measurements were conducted on a self-doped LaxMnO3-? (x?0.8) film. From these measurements we determined the conductivity and the temperature dependence of the effective number of carriers. While the metal-insulator transition temperature (TMI) and the magnetic ordering temperature (TC) approximately coincide, the onset of the change in the free carrier density occurs at a significantly lower temperature (?45 K below). This suggests that local distortions exist below TMI and TC which trap the eg conduction electrons. These regions with local distortions constitute an insulating phase which persists for temperatures significantly below TMI and TC.

No abstract available

[en] The Bi_1-xCa_xMnO₃ system is known to exhibit charge ordering for a much broader range of x than the La_1-xCa_xMnO₃ system. However, the properties of Bi_1-xCa_xMnO₃ over the entire doping range are not well understood. We have performed magnetization and resistivity measurements as well as x-ray absorption and x-ray diffraction measurements on Bi_1-xCa_xMnO₃ to correlate structural, magnetic, and transport properties. The system is insulating and antiferromagnetic for the entire range of x studied (x≥0.4) except near x∼0.9, where we find a canted spin arrangement with approximately one Bohr magneton per Mn site. Detailed magnetization measurements were performed as a function of field and temperature to explore the net moment on the Mn sites as a function of x and reveal the charge ordering and Neel temperatures. X-ray absorption measurements reveal significant structural distortions of the Mn-O bond distributions with increasing Bi content that correlates directly with increasing charge-ordering temperatures. Moreover, the x-ray diffraction data reveal peak splittings consistent with lower-symmetry cells as Bi content increases. These structural-magnetic correlations point to the importance of Mn-O distortions in stabilizing the charge-ordered state in the manganites

[en] Complementary x-ray emission (XES) and x-ray-absorption (XAS) measurements of La_1-xCa_xMnO₃ as a function of temperature have been performed. For 0.5≤x≤0.8 we find changes in the XAS and XES spectra consistent with a small increase in the average Mn valence accompanying the passage to the low-temperature state. It is suggested that this small Mn-configuration shift could be due to the competition between the energies associated with ferromagnetic (FM) and charge/orbital (CO) correlations. In the region 0.0≤x≤0.4 we observe thermal spectral changes quite different from those found in the x≥0.5 materials. Modest thermal changes in the Mn XAS pre-edge (K-edge), for 0.0≤x≤0.4, were observed to be coupled to the robustness of the FM-metallic ground state. The clearest pre-edge feature variation appears to be related to Mn-e_g-majority-spin state changes. In contrast the thermal XES spectral changes appear similar in character in the antiferromagnetic (AF)-insulating x=0.0 and FM-metallic x=0.3 materials. An enhancement of the effective local moment via a coupling to the internal exchange field, in the magnetically ordered states, is proposed to explain these XES results. The x=1.0 end member (CaMnO₃) was found to exhibit significant temperature dependence of the absorption and emission spectra. These XAS and XES results, for CaMnO₃, are discussed in terms of thermal perturbations on the degree of covalency of this material

[en] Comparison studies of the perovskite compounds system R_1-xCa_xMnO₃ (R=La, Bi) by both Mn K_β-emission spectroscopy and x-ray-absorption near-edge spectroscopy (XANES) are presented. The insensitivity of x-ray-emission measurements to structural distortions coupled with the sensitivity of x-ray-absorption near-edge measurements to changes in both structure and valence enable one to detect the presence of structural distortions, such as local Jahn-Teller (JT) distortions. Theoretical XANES computations for pure cubic perovskite and locally distorted endmembers are used to show the effect of distortions on XANES spectra as well as to comment on the nature of the pre-edge features in the spectra. We show by explicit computations that the near-edge spectra are determined by dipole transitions while pure 1s to 3d electric quadrupole transitions determine a very limited section of the pre-edge region. Simulations of the pre-edge features reveal a direct connection between local distortions and the a1 feature amplitude. The Bi-containing system is found to have significantly higher levels of distortions than the La system. XANES studies of the AMnO₃ (A=La, Pr, and Nd) system reveal a direct relationship between the main line width and the magnitude of the JT distortions

[en] We have performed a detailed study of the long-range and local structure of Nd_0.5Sr_0.5MnO₃ films of varying thicknesses. The detailed nature of the in-plane and out-of-plane d spacings as a function of film thickness was determined. Comparisons between the local structure about Mn and the long-range structure were made. We found that Mn coordination asymmetry exists in thin films and that it is related to orbital ordering. The results yield a confirmation of models of manganite systems, which suggests that biaxial strain enhances electron localization