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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Local Structure of Zr(OH) 4 and the Effect of Calcination Temperature from X-ray Pair Distribution Function Analysis

Analysis of X-ray pair distribution function data has provided a detailed picture of the local structure of amorphous Zr(OH) 4 and its thermal decomposition into ZrO 2 . In the untreated phase, the Zr atoms tend to be coordinated by six or seven oxygen atoms. The Zr centered polyhedra connect to each other primarily by sharing edges, but also with a significant amount of corner sharing, to form two-dimensional sheets in which the Zr are connected to an average of about five other Zr. This local structure is related to the structure of monoclinic ZrO 2 and can be derived from it by removing certain Zr neighbors to form sheets and reduce the corner to edge sharing ratio. The maximum correlation length in Zr(OH) 4 is about 12 Å. Heating up to 125 °C results in significant water loss but does not alter the network of Zr and bridging O atoms. Additional water loss caused by heating to 250 °C triggers a reorganization into a new type of amorphous phase with a three-dimensional network and a greater number of Zr–Zr neighbors. Further heating to 330 °C causes crystallization into a mixture of tetragonal and monoclinic ZrO 2 , with the minor tetragonal phase having a smaller average domain size. Here, the tetragonal component vanishes by 900 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Moessbauer mineralogy of calcined Murchison meteorite

The three Antarctic meteorites B7904, Y82162, and Y86720 are unusual because they have characteristics in common with both CI and CM groups and because they apparently underwent thermal alteration after hydrous alteration on their parent body. They are also spectrally similar (visible and near-IR) to C, G, B, and F asteroids, which may imply that the surface materials on those asteroids may have undergone thermal alteration. Based on the reflectance spectra of samples of Murchison (CM2 carbonaceous chondrite) that were thermally altered in the laboratory (cryopumped and initial 10(exp -5) atm H2), Hiroi et al. concluded that putative thermal alteration occurred at temperatures of 600 to 1000 C. Similar experiments have been done on Murchison and reported mineralogical changes based on data from transmission electron diffraction microscopy, electron diffraction, and analytical electron microscopy. We report here the Mossbauer mineralogy of the same samples of thermally-altered Murchison. Mossbauer mineralogy gives the molar distribution of Fe among its oxidation states and iron-bearing mineralogies.

Morris, Richard V.↗