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A Transmission Electron Microscopy Study of a Refractory Metal Grain from a Calcium-Aluminum-Rich Inclusion in the Leoville CV3 Chondrite

Introduction: Calcium-aluminum-rich inclusions (CAIs) are an important component of chondritic meteorites. They can contain materials that are thermodynamically predicted and isotopically age dated to be among the first-formed solids in our solar system [1-5]. Observed in some CAIs are micron to sub-micron sized inclusions rich in Fe, Ni, and high-Z elements such as Pt, Os, Ir and W, in the form of refractory metal nuggets (RMNs), fremdlinges, and ‘nugget like objects’ (NLOs) [1,6]. Refractory siderophile elements such as Os, Ir and Ru are thermodynamically predicted to condense at temperatures well in excess of the major CAI phases such as melilite, perovskite, spinel and hibonite [2,7-9]. These refractory metal inclusions in CAIs can therefore serve as probes into the thermodynamic landscape of the early solar protoplanetary disk. Here we report on a refractory grain identified in a CAI of the Leoville CV3 chondrite. This work is part of an ongoing effort to gain insight into the thermochemistry of the early solar system through systematic analyses of the structure and chemistry of various components in CAIs [10-13]. Sample and Analytical Techniques: A fluffy type A CAI (Fig. 1A) was identified in a section of the Leoville, CV3 chondrite (Center for Meteorite Studies, Arizona State University collection, #821_C_3) using a JOEL-JXA 8530F electron microprobe at Arizona State University. Backscattered electron (BSE) imaging and energy-dispersive X-ray spectroscopy (EDS) were used to identify refractory metal grains in the CAI using a Thermo Fisher (formerly FEI) Helios NanoLab 660 G3 focused-ion-beam scanning-electron microscope (FIBSEM) located at the Kuiper Materials Imaging and Characterization Facility (KMICF) at the Lunar and Planetary Laboratory, University of Arizona. The FIB is equipped with an EDAX EDS system. We selected one of the larger (micron-sized) refractory metal grains, designated as ‘Spud’ (Fig. 1B) for further analysis. ‘Spud’ was extracted and thinned to electron transparency (<100 nm) using the FIB-SEM located in KMICF, following methods described by [14- 15]. The FIB section was analyzed using a 200 keV Hitachi HF5000 scanning transmission electron microscope (S/TEM) located at KMICF. The HF5000 is equipped with cold-field emission gun, 3rd-order spherical aberration corrector for STEM imaging, and an Oxford Instruments X-Max N 100 TLE energydispersive spectroscopy (EDS) system with dual 100 mm2 windowless silicon-drift detectors (Ω = 2.0 sr). Selected-area electron-diffraction (SAED) patterns were acquired to aid in determination of crystallinity and phase. Results: The mineralogy, texture, and morphology of the CAI are consistent with that of a fluffy type A (FTA) CAI [16]. BSE imaging at high magnifications revealed grains with high contrast, indicative of compositions rich in elements of higher atomic number relative to surrounding material. These high-Z grains have sizes that range from ∼250 nm to 4 µm. EDS analyses confirm that the bright grains are metal-rich inclusions. A minor fraction of the grains are composed of only Fe and Ni, but the majority (∼60%) of the identified inclusions also contained various refractory siderophiles including Os, Ru, Zr, Ir and Mo. EDS analysis on the FIB-SEM of Spud shows that it contains Fe, Ni, Mo and Ru. High-angle annular dark-field (HAAFD) imaging and EDS mapping in the TEM (Fig. 2) show that Spud occurs in melilite (Ca1.9Al1.99Si1.06O7). Spud contains a subhedral to anhedral morphology and is compositionally heterogenous (polyphasic, Fig. 2). Local spatial correlation occurs among Fe, Ni, and Pt, and also among Os, Ru, and Mo. SAED patterns show that the Fe-Ni-Pt, Fe-Os-Mo-Ru and Fe-Pt regions are crystalline. Discussion: CAIs can contain various types of inclusions rich in Fe, Ni and refractory siderophiles such as Os, Ru, W and Pt [1]. RMNs are micron-sized, single phase alloy grains and can contain Os, Ir, Ru and Rh [1,7,17]. NLOs are also micron-sized inclusions, but contain two phases, a refractory metal, and an oxide [6]. Fremdlinge are the largest of such inclusions (tens of microns in size) and are complex aggregates of Fe-Ni alloy, silicates, oxides, and sulfides [1,17]. While the size of Spud matches previous descriptions of RMNs and NLOs, Spud is neither a single-phase alloy like RMNs, nor does it contain one metal phase and one oxide like NLOs. Spud does not match the above described categories of refractory metal inclusions. The presence of refractory siderophiles such as Mo, Os, Ru, and Pt suggests a high-temperature origin. Thermodynamic modelling by [7] indicates condensation temperatures of 1917 K, 1693 K, 1613 K and 1415 K for Os, Mo, Ru and Pt respectively. These models also show that following the initial condensation of a refractory metal, alloying of solutes such as Fe, Ni and W, occurs in levels proportional to their partial pressures in the surrounding gas. Such alloying occurs at temperatures above the condensation temperatures of common CAI phases such as melilite (1529 K), perovskite (1441 K), spinel (197 K) and forsterite (1354 K) [2]. The polyphasic nature of Spud could be the result of such high-temperature alloying, possibly shortly after the condensation of Mo and Ru at 1693 K and 1613 K respectively. That Spud occurs as an inclusion is consistent with it having formed prior to and at temperature above that of its host melilite in this FTA CAI, which is qualitatively consistent with such prior thermodynamic modeling. Further, the polyphasic nature of Spud is similar to refractory grains from a FTA CAI in the Northwest Africa (NWA) 8323, CV3 chondrite [11-13]. These data suggest that such refractory metal grains could have been widespread in the inner and early solar protoplanetary disk and represent some of the earliest formed solids to have condensed. Acknowledgments: Research and instrumentation supported by NASA grants #NNX12AL47G, #NNX15AJ22G and #80NSSC19K0509, and NSF grants #1531243 and #0619599. Fig 2. STEM data on ‘Spud’. HAADF Image (Top) False-color EDS Maps (Bottom) References: [1] MacPherson G. J. (2014) T. of Geochem. Vol I: Met. And Cosmochem. Processes, 139-179. [2] Lodders K. (2003) ApJ, 591, 1220-1247. [3] Ebel D. S. (2006) Met. & the Early S. Sys. II., 253- 277. [4] Amelin Y. (2002) Science, 297, 1678-1683. [5] Connelly J.N. et.al. (2012) Science, 338, 651-655. [6] Schwander D. et al. (2015) GCA, 18, 70-87. [7] Palme H. and Wlotzka F. (1976) EPSL, 33, 45-60. [8] Berg T. et al. (2009) ApJ, 702, 172-176. [9] Liffman K. et al. (2021) Icarus, 221, 89-105. [10] Zega T.J. et al. (2021) PSJ, 2, 115. [11] Ramprasad T. et al. (2020) LPSC LI, Abstract #2472. [12] Ramprasad T. et al. (2021) Microscopy & Microanalysis, S1, 2792-2794. [13] Ramprasad T. et al. (2021) 84th MetSoc, Abstract #6123. [14] Zega T.J. et al. (2007) MAPS, 42, 1373-1386. [15] Ramprasad T. et al. (2022) MAPS, in revision. [16] Grossman L. (1975), GCA, 39, 433-454. [17] El Goresy A. et al. (1978) LPSC IX, Abstract#1100

T. Ramprasad↗
DOE OSTI2022

Computational determination of a primary diffusion mode in γ U-10Mo under irradiation

Low enriched uranium (< 20 % 235 U)-molybdenum (U-Mo) monolithic fuel is the primary candidate for high performance research and test reactors, and is in the process of being qualified to replace highly enriched uranium (≥ 20 % 23 5U) fuel. As part of the qualification process, it is critical to understand and predict the behavior of fission gas bubbles under irradiation, which affects fuel swelling and fuel failure. Mechanistic fuel models are being developed that can both reproduce the existing experimental data for fuel swelling, and be further applied to irradiation conditions beyond the experimental scope. Diffusion of species under irradiation conditions is an important parameter in the mechanistic fuel models; however, no temperature-relevant experimental diffusion data exists. In the present work, radiation-enhanced diffusion coefficients of U, Mo, and Xe in λU-10wt.%Mo were calculated in the temperature range between 300 K and 1400 K via rate-theory models and molecular dynamics simulations with an embedded-atom method interatomic potential for the U-Mo-Xe system. Accordingly, total diffusion coefficients under relevant irradiation conditions are determined using previously obtained intrinsic thermal diffusion and radiation-driven diffusion coefficients, as well as the newly calculated radiation-enhanced diffusion coefficients presented herein. Radiation-enhanced diffusion of U and Mo was dominant in the intermediate temperature range, whereas radiation-enhanced diffusion of Xe did not significantly contribute to total diffusion of Xe at the relevant fission rate densities. Radiation-enhanced diffusion of Xe became faster than both intrinsic thermal diffusion and radiation-driven diffusion at a fission rate density of 5 x 10 22 fissions/m 3 /s, which is higher than the typical fission rate density range in research reactors. The temperature regime that radiation-enhanced diffusion of each element dominated was dependent on the fission rate density. Finally, the total diffusion coefficients of U, Mo, and Xe, updated in this work, will be utilized as parameters in the mechanistic fuel models to help predict the behavior of fission gas bubbles under irradiation more accurately.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Materials Data on ZrMoP by Materials Project

ZrMoP crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. Zr is bonded in a 11-coordinate geometry to six equivalent Mo and five P atoms. There are two shorter (2.96 Å) and four longer (3.01 Å) Zr–Mo bond lengths. There are four shorter (2.73 Å) and one longer (2.82 Å) Zr–P bond lengths. Mo is bonded in a 10-coordinate geometry to six equivalent Zr and four P atoms. There are two shorter (2.46 Å) and two longer (2.65 Å) Mo–P bond lengths. There are two inequivalent P sites. In the first P site, P is bonded in a 9-coordinate geometry to three equivalent Zr and six equivalent Mo atoms. In the second P site, P is bonded in a 9-coordinate geometry to six equivalent Zr and three equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mo3Ir by Materials Project

Mo3Ir crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mo is bonded in a 6-coordinate geometry to two equivalent Mo and four equivalent Ir atoms. Both Mo–Mo bond lengths are 2.50 Å. All Mo–Ir bond lengths are 2.80 Å. Ir is bonded to twelve equivalent Mo atoms to form a mixture of edge and face-sharing IrMo12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on BeMo3 by Materials Project

BeMo3 crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Be is bonded to twelve equivalent Mo atoms to form a mixture of face and edge-sharing BeMo12 cuboctahedra. All Be–Mo bond lengths are 2.71 Å. Mo is bonded in a 6-coordinate geometry to four equivalent Be and two equivalent Mo atoms. Both Mo–Mo bond lengths are 2.43 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MoIr by Materials Project

IrMo is beta-prime cadmium gold structured and crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. Mo is bonded to four equivalent Mo and eight equivalent Ir atoms to form MoMo4Ir8 cuboctahedra that share corners with eight equivalent IrMo8Ir4 cuboctahedra, corners with ten equivalent MoMo4Ir8 cuboctahedra, edges with six equivalent MoMo4Ir8 cuboctahedra, edges with twelve equivalent IrMo8Ir4 cuboctahedra, faces with eight equivalent IrMo8Ir4 cuboctahedra, and faces with twelve equivalent MoMo4Ir8 cuboctahedra. All Mo–Mo bond lengths are 2.78 Å. There are a spread of Mo–Ir bond distances ranging from 2.73–2.81 Å. Ir is bonded to eight equivalent Mo and four equivalent Ir atoms to form IrMo8Ir4 cuboctahedra that share corners with eight equivalent MoMo4Ir8 cuboctahedra, corners with ten equivalent IrMo8Ir4 cuboctahedra, edges with six equivalent IrMo8Ir4 cuboctahedra, edges with twelve equivalent MoMo4Ir8 cuboctahedra, faces with eight equivalent MoMo4Ir8 cuboctahedra, and faces with twelve equivalent IrMo8Ir4 cuboctahedra. There are two shorter (2.78 Å) and two longer (2.80 Å) Ir–Ir bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mo3Pt by Materials Project

PtMo3 crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mo is bonded in a 6-coordinate geometry to two equivalent Mo and four equivalent Pt atoms. Both Mo–Mo bond lengths are 2.51 Å. All Mo–Pt bond lengths are 2.81 Å. Pt is bonded to twelve equivalent Mo atoms to form a mixture of edge and face-sharing PtMo12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MoRh by Materials Project

MoRh is beta-prime cadmium gold structured and crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. Mo is bonded to four equivalent Mo and eight equivalent Rh atoms to form MoMo4Rh8 cuboctahedra that share corners with eight equivalent RhMo8Rh4 cuboctahedra, corners with ten equivalent MoMo4Rh8 cuboctahedra, edges with six equivalent MoMo4Rh8 cuboctahedra, edges with twelve equivalent RhMo8Rh4 cuboctahedra, faces with eight equivalent RhMo8Rh4 cuboctahedra, and faces with twelve equivalent MoMo4Rh8 cuboctahedra. There are two shorter (2.76 Å) and two longer (2.77 Å) Mo–Mo bond lengths. There are a spread of Mo–Rh bond distances ranging from 2.72–2.80 Å. Rh is bonded to eight equivalent Mo and four equivalent Rh atoms to form RhMo8Rh4 cuboctahedra that share corners with eight equivalent MoMo4Rh8 cuboctahedra, corners with ten equivalent RhMo8Rh4 cuboctahedra, edges with six equivalent RhMo8Rh4 cuboctahedra, edges with twelve equivalent MoMo4Rh8 cuboctahedra, faces with eight equivalent MoMo4Rh8 cuboctahedra, and faces with twelve equivalent RhMo8Rh4 cuboctahedra. There are two shorter (2.76 Å) and two longer (2.79 Å) Rh–Rh bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiMo3 by Materials Project

Mo3Si crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mo is bonded in a 6-coordinate geometry to two equivalent Mo and four equivalent Si atoms. Both Mo–Mo bond lengths are 2.46 Å. All Mo–Si bond lengths are 2.75 Å. Si is bonded to twelve equivalent Mo atoms to form a mixture of edge and face-sharing SiMo12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mn3Mo3C by Materials Project

Mo3Mn3C crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mo is bonded in a 8-coordinate geometry to six Mn and two equivalent C atoms. There are a spread of Mo–Mn bond distances ranging from 2.72–2.79 Å. Both Mo–C bond lengths are 2.13 Å. There are two inequivalent Mn sites. In the first Mn site, Mn is bonded to six equivalent Mo and six equivalent Mn atoms to form MnMn6Mo6 cuboctahedra that share edges with six equivalent CMo6 octahedra and faces with six equivalent MnMn6Mo6 cuboctahedra. All Mn–Mn bond lengths are 2.38 Å. In the second Mn site, Mn is bonded in a 12-coordinate geometry to six equivalent Mo and six Mn atoms. All Mn–Mn bond lengths are 2.46 Å. C is bonded to six equivalent Mo atoms to form CMo6 octahedra that share corners with six equivalent CMo6 octahedra and edges with six equivalent MnMn6Mo6 cuboctahedra. The corner-sharing octahedral tilt angles are 45°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on GaMo4CS8 by Materials Project

Mo4GaCS8 crystallizes in the cubic F-43m space group. The structure is three-dimensional. Mo+4.25+ is bonded in a 3-coordinate geometry to one C4- and six S2- atoms. The Mo–C bond length is 2.70 Å. There are three shorter (2.36 Å) and three longer (2.72 Å) Mo–S bond lengths. Ga3+ is bonded to four equivalent S2- atoms to form GaS4 tetrahedra that share corners with four equivalent CMo4S4 tetrahedra. All Ga–S bond lengths are 2.53 Å. C4- is bonded to four equivalent Mo+4.25+ and four equivalent S2- atoms to form distorted CMo4S4 tetrahedra that share corners with four equivalent GaS4 tetrahedra. All C–S bond lengths are 1.88 Å. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Mo+4.25+ atoms. In the second S2- site, S2- is bonded in a 2-coordinate geometry to three equivalent Mo+4.25+, one Ga3+, and one C4- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Tl(TeMo)3 by Materials Project

(MoTe)3Tl crystallizes in the hexagonal P6_3/m space group. The structure is one-dimensional and consists of two thallium molecules and one MoTe ribbon oriented in the (0, 0, 1) direction. In the MoTe ribbon, Mo is bonded in a 10-coordinate geometry to six equivalent Mo and four equivalent Te atoms. There are two shorter (2.67 Å) and four longer (2.78 Å) Mo–Mo bond lengths. There are two shorter (2.84 Å) and two longer (2.89 Å) Mo–Te bond lengths. Te is bonded in a 4-coordinate geometry to four equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZrMo2 by Materials Project

Mo2Zr is Cubic Laves structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Zr is bonded in a 12-coordinate geometry to twelve equivalent Mo atoms. All Zr–Mo bond lengths are 3.16 Å. Mo is bonded to six equivalent Zr and six equivalent Mo atoms to form a mixture of corner, edge, and face-sharing MoZr6Mo6 cuboctahedra. All Mo–Mo bond lengths are 2.70 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Co3Mo3N by Materials Project

Co3Mo3N crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mo is bonded in a 2-coordinate geometry to six Co and two equivalent N atoms. There are a spread of Mo–Co bond distances ranging from 2.67–2.73 Å. Both Mo–N bond lengths are 2.13 Å. There are two inequivalent Co sites. In the first Co site, Co is bonded to six equivalent Mo and six equivalent Co atoms to form CoCo6Mo6 cuboctahedra that share edges with six equivalent NMo6 octahedra and faces with six equivalent CoCo6Mo6 cuboctahedra. All Co–Co bond lengths are 2.37 Å. In the second Co site, Co is bonded in a 12-coordinate geometry to six equivalent Mo and six Co atoms. All Co–Co bond lengths are 2.51 Å. N is bonded to six equivalent Mo atoms to form NMo6 octahedra that share corners with six equivalent NMo6 octahedra and edges with six equivalent CoCo6Mo6 cuboctahedra. The corner-sharing octahedral tilt angles are 47°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on HfMo2 by Materials Project

HfMo2 is Cubic Laves structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Hf is bonded in a 12-coordinate geometry to twelve equivalent Mo atoms. All Hf–Mo bond lengths are 3.14 Å. Mo is bonded to six equivalent Hf and six equivalent Mo atoms to form a mixture of edge, corner, and face-sharing MoHf6Mo6 cuboctahedra. All Mo–Mo bond lengths are 2.68 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on GaMo3 by Materials Project

Mo3Ga crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mo is bonded in a 6-coordinate geometry to two equivalent Mo and four equivalent Ga atoms. Both Mo–Mo bond lengths are 2.49 Å. All Mo–Ga bond lengths are 2.78 Å. Ga is bonded to twelve equivalent Mo atoms to form a mixture of face and edge-sharing GaMo12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on AlMo3 by Materials Project

AlMo3 crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mo is bonded in a 6-coordinate geometry to two equivalent Mo and four equivalent Al atoms. Both Mo–Mo bond lengths are 2.49 Å. All Mo–Al bond lengths are 2.78 Å. Al is bonded to twelve equivalent Mo atoms to form a mixture of edge and face-sharing AlMo12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ni6Mo6C by Materials Project

Ni6Mo6C crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mo is bonded in a distorted single-bond geometry to six Ni and one C atom. There are a spread of Mo–Ni bond distances ranging from 2.59–2.73 Å. The Mo–C bond length is 2.16 Å. There are two inequivalent Ni sites. In the first Ni site, Ni is bonded in a 12-coordinate geometry to six equivalent Mo and six Ni atoms. There are three shorter (2.36 Å) and three longer (2.58 Å) Ni–Ni bond lengths. In the second Ni site, Ni is bonded to six equivalent Mo and six equivalent Ni atoms to form NiNi6Mo6 cuboctahedra that share corners with six equivalent CMo6 octahedra and faces with six equivalent NiNi6Mo6 cuboctahedra. The corner-sharing octahedral tilt angles are 45°. C is bonded to six equivalent Mo atoms to form CMo6 octahedra that share corners with twelve equivalent NiNi6Mo6 cuboctahedra.

36 MATERIALS SCIENCE↗