Transmission Electron Microscopy of Iron Metal in Almahata Sitta Ureilite

Almahata Sitta (AS) is a polymict breccia mainly composed of variable ureilite lithologies with small amounts of chondritic lithologies [1]. Fe metal is a common accessory phase in ureilites, but our earlier study on Fe metals in one of AS fragments (#44) revealed a unique mineralogy never seen in other ureilites [2,3]. In this abstract we report detailed transmission electron microscopy (TEM) on these metal grains to better understand the thermal history of ureilites. We prepared FIB sections of AS#44 by JEOL JIB-4000 from the PTS that was well characterized by SEM-EBSD in our earlier study [2]. The sections were then observed by STEM (JEOL JEM- 2100F). One of the FIB sections shows a submicron-sized symplectic intergrown texture composed of Fe metal (kamacite), Fe carbide (cohenite), Fe phosphide (schreibersite), and Fe sulfide (troilite). Each phase has an identical SAED pattern in spite of its complex texture, suggesting co-crystallization of all phases. This is probably caused by shock re-melting of pre-existing metal + graphite to form a eutectic-looking texture. The other FIB section is mostly composed of homogeneous Fe metal (93 wt% Fe, 5 wt% Ni, and 2 wt% Si), but BF-STEM images exhibited the presence of elongated lathy grains (approx. 2 microns long) embedded in the interstitial matrix. The SAED patterns from these lath grains could be indexed by alpha-Fe (bcc) while interstitial areas are gamma-Fe (fcc). The elongated alpha-Fe grains show tweed-like structures suggesting martensite transformation. Such a texture can be formed by rapid cooling from high temperature where gamma-Fe was stable. Subsequently alpha-Fe crystallized, but gamma-Fe remained in the interstitial matrix due to quenching from high temperature. This scenario is consistent with very rapid cooling history of ureilites suggested by silicate mineralogy

Transport properties of the layered Rh oxide K_0.49RhO_2

We report measurements and analyses of resistivity, thermopower and Hall coefficient of single-crystalline samples of the layered Rh oxide K_0.49RhO_2. The resistivity is proportional to the square of temperature up to 300 K, and the thermopower is proportional to temperature up to 140 K. The Hall coefficient increases linearly with temperature above 100 K, which is ascribed to the triangular network of Rh in this compound. The different transport properties between Na_xCoO_2 and K_0.49RhO_2 are discussed on the basis of the different band width between Co and Rh evaluated from the magnetotransport.Comment: 3 figures, submitted to PR

Structure of an Al-Fe-Ni Decagonal Quasicrystal Studied by Cs-Corrected STEM

The structure of an Al-Fe-Ni decagonal quasicrystal with two quasiperiodic planes along the periodic axis in an $Al_{72}Ni_{24}Fe_4$ alloy has been examined by spherical aberration (Cs)-corrected scanning transmission electron microscopy with high-angle annular dark-field and annular bright-field techniques. The transition-metal atoms and mixed sites (MSs) of Al and transition-metal atoms are represented as separated bright dots in the observed high-angle annular dark-field scanning transmission electron microscopy images, and consequently the arrangements of transition-metal atoms and mixed sites on the two quasiperiodic planes can be directly determined. The transition-metal atoms are arranged on a pentagonal tiling of an edge-length of 0.76 nm. The close examination of observed annular bright-field- and high-angle annular dark-field scanning transmission electron microscopy images indicates the existence of large decagonal columnar clusters with 3.2 nm diameter, and their arrangement on pentagonal, thin rhombic and squashed hexagonal tiles with an edge-length of 3.2 nm. The arrangements of transition-metal atoms in these three tiles are placed on an ideal pentagonal tiling with an edge-length of 3.2 nm, which is generated by the projection of a five-dimensional hyper-cubic lattice. The vertices are denoted by 5D hyper-cubic indices and then they are projected on the occupation domains in perpendicular space. The arrangement of Al atoms as well as transition-metal atoms and mixed sites in the large decagonal atom cluster with about 3.2 nm diameter is interpreted from the observed high-angle annular dark-field- and annular bright-field scanning transmission electron microscopy ABF-STEM images

Syntheses;Structures and Photophysical Properties of Iron Containing Hydroxyapatite Prepared by a Modified Pseudo-Body Solution

It is known that bones and teeth are mostly composed of hydroxyapatite (HAp) and iron is present in them as a trace element. In order to search for helpful information for understanding the behavior of trace iron element in bones and teeth, very small amounts of iron containing HAp(FeHAp) were synthesized from a modified pseudo-body solution at low temperature. The effects of iron on the structural and photo-absorption properties of FeHAp were characterized by XRD, the Rietveld structural refinement, TEM and UV-vis photo-absorption spectroscopy. The results suggested that Fe(III) ions with high spin could get into the apatite structure and FeHAp with a pure apatitic phase could be obtained. At the same time, iron could modify the photo-absorption property of FeHAp. Compared with the pure HAp material, the prepared iron containing materials showed obviously photo-absorption property in the UV light region

F-AlCoPdGe Alloy with Three Types of Pseudo-Mackay Clusters

The structure of an F-AlCoPdGe alloy was determined by single-crystal X-ray diffraction analysis: space group Pa3̅ (No. 205), Pearson symbol cP1128-142.4, a = 24.4338(4) Å; R1 = 0.0526 for the observed 5106 reflections with $F_0$ > 4.0σ$(F_0)$. The F-AlCoPdGe alloy exhibited three types of pseudo-Mackay clusters (pMCs). The first shell of the Co(13)-pMC at 24d indicated a coordination polyhedron typically found in Al-Co binary approximants; the second shell was a combination of an M icosahedron and an Al icosidodecahedron, where M is a mixed site of Pd and/or Co. The first shell of the Co(14)-pMC at 4b consisted of 20 partially occupied Al positions arranged in a dodecahedral fashion; its second shell was also composed of an M icosahedron and an Al icosidodecahedron. The first shell of the Al(18)-pMC at 4a consisted of a combination of an M cube and an Al octahedron, resulting in the rhombic dodecahedral arrangement; its outer shell structure was similar to those of the other pMCs. The structure of the F-AlCoPdGe phase comprised an arrangement of these three types of pMCs together with interstitially placed Al icosahedra around the Pd/Al(1) and Pd/Al(2) sites

Studies on formation and structures of ultrafine Cu precipitates in Fe-Cu model alloys for reactor pressure vessel steels using positron quantum dot confinement in the precipitates by their positron affinity. JAERI's nuclear research promotion program, H11-034 (Contract research)

Positron annihilation experiments on Fe-Cu model dilute alloys of nuclear reactor pressure vessel (RPV) steels have been performed after neutron irradiation in JMTR. Nanovoids whose inner surfaces were covered by Cu atoms were clearly observed. The nanovoids transformed to ultrafine Cu precipitates by dissociating their vacancies after annealing at around 400degC. The nanovoids and the ultrafine Cu precipitates are strongly suggested to be responsible for irradiation-induced embrittlement of RPV steels. Effects of Ni, Mn and P addition on the nanovoid and Cu precipitate formations were also studied. The nanovoid formation was enhanced by Ni and P, but suppressed by Mn. The Cu precipitates after annealing around 400degC were almost free from these doping elements and hence were pure Cu in the chemical composition. Furthermore the Fermi surface of the 'embedded' Cu precipitates with a body centered cubic crystal structure was obtained from two dimensional angular correlation of annihilation radiation (2D-ACAR) in a Fe-Cu single crystal and was agreed well with that from a band structure calculation. Theoretical calculation of positron confinement in Fe-Cu model alloys showed that a positron quantum dot state induced by positron affinity is attained for the embedded precipitates larger than 1 nm. A new position sensitive detector with a function of one dimensional angular correlation of annihilation radiation (1D-ACAR) has been developed that enables high resolution experiments over wide ranges of momentum distribution

Zr60Al15(Ni,Cu)(25) noncrystalline alloys created by referring to ionic arrangements of a garnet structure with molecular dynamics simulations based on a plastic crystal model

International audienceThe Zr60Al15(Ni,Cu)(25) noncrystalline alloys with their initial crystalline states from the ionic arrangements of a Fe3Al2Si3O12 garnet structure were created with molecular dynamics simulations based on a plastic crystal model (MD-PCM). The analyses with pair-distribution function revealed that the randomly-rotated octahedral clusters around the Cu sites or tetrahedral clusters around the Ni sites and subsequent annealing with MD-PCM make the Zr60Al15(Ni,Cu)(25) crystalline alloys to vitrify. The interference function indicated that the Zr60Al15Ni25 noncrystalline alloy created through rotating operation for the octahedral clusters gives the best fit with the experimental data in an as-quenched state. Crystallographic analyses indicated that the prototype of the garnet structure, Weaire-Phelan (A15) structure, gives inhomogeneous Wigner-Seitz cell for the solute elements and an optimized heat of mixing for atomic pairs in the Zr60Al15(Ni,Cu)(25) noncrystalline alloys. These crystallographic features due to Weaire-Phelan structure are a reason for the Zr60Al15(Ni,Cu)(25) alloys to have high glass-forming ability