Localversusaverage structure: a study of neighborite (NaMgF3) utilizing the pair distribution function method for structure determination

2007 ◽  
Vol 40 (3) ◽  
pp. 441-448 ◽  
Author(s):  
C. David Martin ◽  
Peter J. Chupas ◽  
Karena W. Chapman ◽  
John B. Parise

The temperature-dependent local structure (< 2 nm) of neighborite (NaMgF3) is probed through least-squares refinement of structure models fit to the pair distribution function [G(r)] derived from the total high-energy X-ray scattering of sample powders. In contrast to previous temperature-dependent structure models obtained through Rietveld refinement and statistical modelling of powder diffraction data, it is found that the average Mg—F bond length, corresponding to a ∼2 Å peak in theG(r), increases between 323 and 1123 K. At each temperature, asymmetry in this peak is consistent with an orthorhombic (Pbnm) perovskite local structure, allowing three unique Mg—F values and deformation of MgF6octahedra. Defined by the three orthogonal Mg—Mg distances, the pseudo-cubic unit cell of local structure models becomes metrically tetragonal and cubic at temperatures greater than ∼623 and 1038 K, respectively. A discontinuity in the temperature dependence of the fluorine atomic displacement parameters at 1038 K suggests thermal activation of new vibrational modes in NaMgF3at high temperature, consistent with transverse vibration of the bridging fluorine atoms (Mg—F—Mg).

Minerals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 272
Author(s):  
Seungyeol Lee ◽  
Huifang Xu ◽  
Hongwu Xu ◽  
Joerg Neuefeind

The crystal structure of moganite from the Mogán formation on Gran Canaria has been re-investigated using high-resolution synchrotron X-ray diffraction (XRD) and X-ray/neutron pair distribution function (PDF) analyses. Our study for the first time reports the anisotropic atomic displacement parameters (ADPs) of a natural moganite. Rietveld analysis of synchrotron XRD data determined the crystal structure of moganite with the space group I2/a. The refined unit-cell parameters are a = 8.7363(8), b = 4.8688(5), c = 10.7203(9) Å, and β = 90.212(4)°. The ADPs of Si and O in moganite were obtained from X-ray and neutron PDF analyses. The shapes and orientations of the anisotropic ellipsoids determined from X-ray and neutron measurements are similar. The anisotropic ellipsoids for O extend along planes perpendicular to the Si-Si axis of corner-sharing SiO4 tetrahedra, suggesting precession-like movement. Neutron PDF result confirms the occurrence of OH over some of the tetrahedral sites. We postulate that moganite nanomineral is stable with respect to quartz in hypersaline water. The ADPs of moganite show a similar trend as those of quartz determined by single-crystal XRD. In short, the combined methods can provide high-quality structural parameters of moganite nanomineral, including its ADPs and extra OH position at the surface. This approach can be used as an alternative means for solving the structures of crystals that are not large enough for single-crystal XRD measurements, such as fine-grained and nanocrystalline minerals formed in various geological environments.


IUCrJ ◽  
2015 ◽  
Vol 2 (5) ◽  
pp. 481-489 ◽  
Author(s):  
Kirsten M. Ø. Jensen ◽  
Anders B. Blichfeld ◽  
Sage R. Bauers ◽  
Suzannah R. Wood ◽  
Eric Dooryhée ◽  
...  

By means of normal-incidence, high-flux and high-energy X-rays, total scattering data for pair distribution function (PDF) analysis have been obtained from thin films (tf), suitable for local structure analysis. By using amorphous substrates as support for the films, the standard Rapid Acquisition PDF setup can be applied and the scattering signal from the film can be isolated from the total scattering data through subtraction of an independently measured background signal. No angular corrections to the data are needed, as would be the case for grazing incidence measurements. The `tfPDF' method is illustrated through studies of as-deposited (i.e.amorphous) and crystalline FeSb3films, where the local structure analysis gives insight into the stabilization of the metastable skutterudite FeSb3phase. The films were prepared by depositing ultra-thin alternating layers of Fe and Sb, which interdiffuse and after annealing crystallize to form the FeSb3structure. The tfPDF data show that the amorphous precursor phase consists of corner-sharing FeSb6octahedra with motifs highly resembling the local structure in crystalline FeSb3. Analysis of the amorphous structure allows the prediction of whether the final crystalline product will form the FeSb3phase with or without excess Sb present. The study thus illustrates how analysis of the local structure in amorphous precursor films can help to understand crystallization processes of metastable phases and opens for a range of new local structure studies of thin films.


Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 124 ◽  
Author(s):  
Seungyeol Lee ◽  
Huifang Xu

Determination of the atomic-scale structures of certain fine-grained minerals using single-crystal X-ray diffraction (XRD) has been challenging because they commonly occur as submicron and nanocrystals in the geological environment. Synchrotron powder diffraction and scattering techniques are useful complementary methods for studying this type of minerals. In this review, we discussed three example studies investigated by combined methods of synchrotron radiation XRD and pair distribution function (PDF) techniques: (1) low-temperature cristobalite; (2) kaolinite; and (3) vernadite. Powder XRD is useful to determine the average structure including unit-cell parameters, fractional atomic coordinates, occupancies and isotropic atomic displacement parameters. X-ray/Neutron PDF methods are sensitive to study the local structure with anisotropic atomic displacement parameters (ADP). The results and case studies suggest that the crystal structure and high-quality ADP values can be obtained using the combined methods. The method can be useful to characterize crystals and minerals that are not suitable for single-crystal XRD.


1989 ◽  
Vol 156 ◽  
Author(s):  
B. H. Toby ◽  
W. Dmowskia ◽  
T. Egami ◽  
J. D. Jorgensen ◽  
M. A. Subramanian ◽  
...  

ABSTRACTAtomic pair distribution function analysis has demonstrated displacements of Tl and O atoms within the Tl-O layers with shortrange ordering of lower symmetry than the crystallographic lattice. Two models have been proposed for these displacements. Rietveld analysis neither confirms nor contradicts these models. Temperature-dependent PDF measurements show changes in local structure correlating with temperature.


Author(s):  
Seungyeol Lee ◽  
Huifang Xu

Determination of the crystal structures of low-temperature tridymite and cristobalite using single-crystal XRD has been challenging because they generally occur as metastable fine-grained crystals in the geological environment. Therefore, using powder diffraction and scattering techniques is critical to study the low-temperature crystals. Synchrotron powder X-ray diffraction (XRD), pair distribution function (PDF) and transmission electron microscopy were used to investigate the structure of orthorhombic tridymite with C2221 symmetry and tetragonal cristobalite with P41212 symmetry, including their anisotropic atomic displacement parameters (ADPs). Rietveld refinement was used to determine the unit-cell parameters, fractional coordinates and isotropic atomic displacement parameters (U iso) of the tridymite and cristobalite. The PDF method was used to determine ADPs for each atom. The results suggest that the crystal structure with high quality ADP values can be obtained using the combined methods of XRD and PDF analyses. The method can be used for characterizing crystals that are not suitable for single-crystal XRD.


2020 ◽  
Author(s):  
Adam Sapnik ◽  
Duncan Johnstone ◽  
Sean M. Collins ◽  
Giorgio Divitini ◽  
Alice Bumstead ◽  
...  

<p>Defect engineering is a powerful tool that can be used to tailor the properties of metal–organic frameworks (MOFs). Here, we incorporate defects through ball milling to systematically vary the porosity of the giant pore MOF, MIL-100 (Fe). We show that milling leads to the breaking of metal–linker bonds, generating more coordinatively unsaturated metal sites, and ultimately causes amorphisation. Pair distribution function analysis shows the hierarchical local structure is partially</p><p>retained, even in the amorphised material. We find that the solvent toluene stabilises the MIL-100 (Fe) framework against collapse and leads to a substantial rentention of porosity over the non-stabilised material.</p>


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