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1. Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk

   https://doi.org/10.2110/jsr.2020.173  

   Rinderknecht, Chanse J.; Hasiuk, Franek J.; Oborny, Stephan C. (August 2021, Journal of Sedimentary Research)

   null (Ed.)

   ABSTRACT Diagenetic low-magnesium calcite (LMC) microcrystals constitute the framework that hosts most micropores in limestone reservoirs and therefore create the storage capacity for hydrocarbons, water, and anthropogenic CO2. Limestones dominated by LMC microcrystals are also commonly used for paleoclimate reconstructions and chemostratigraphic correlations. LMC microcrystals are well known to exhibit a range of textures (e.g., granular, fitted, clustered), but there exists uncertainty with regard to how these textures form. One hypothesis is that during crystal growth, Mg is incorporated into diagenetic overgrowths (cement), where the chemical zonation and microtexture may reflect diagenetic processes. To evaluate small-scale geochemical zonation in LMC microcrystals, this study uses scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to measure the Mg/Ca ratio across the interiors of LMC granular microcrystals from a Late Cretaceous marine chalk from the Tor Fm. (Norwegian North Sea). Mg/Ca zonation was identified in all LMC microcrystals with a diameter > 5 μm. Generally, the cores of large crystals have lower Mg/Ca (≈ 5.9 mmol/mol) and the rims have elevated Mg/Ca (≈ 13 mmol/mol). Smaller microcrystals (< 5 μm) show no resolvable zonation, but do exhibit a wide range in Mg/Ca content from 2.9 to 32.2 mmol/mol. Measured Mg/Ca values are arbitrarily divided into three populations: low Mg (average ≈ 5.9 mmol/mol), intermediate Mg (average ≈ 13.3 mmol/mol), and high Mg (average ≈ 20 mmol/mol). The observed zonation and Mg enrichment within LMC microcrystals is interpreted to reflect depositional as well as multiple diagenetic signals, such as constructive precipitation through recrystallization and pore-filling cementation. Although chalk from the Tor Fm. is dominated by granular euhedral LMC microcrystals, using SEM-EDS to find Mg/Ca heterogeneity in other LMC microcrystal textures may provide insight into the diagenetic processes that create textural variations in micropore-dominated limestones. The Mg data also more broadly suggest that there is useful, measurable diagenetic information in material that is otherwise considered homogeneous. Distinguishing between possible primary compositions and secondary cementation has implications for studies that rely on the primary chemistry of fine-grained carbonate deposits (e.g., micrite), such as paleoclimatology, Mg paleothermometry, and chemostratigraphy. 

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2. First-Principles Investigation of the Early-Stage Precipitations in Mg-Sn and Mg-Zn Alloys

   Cheng, D.; Wang, K.; Zhou, BC. (February 2022, Magnesium Technology)

   Maier, P.; Barela, S.; Miller, V.M.; Neelameggham, N.R. (Ed.)

   Mg-Sn and Mg-Zn alloys exhibit a strong age-hardening effect and have become promising bases for high-strength and low-cost Mg alloys. However, the atomic structures and phase stabilities of various precipitates and intermetallic compounds during the heat treatment in these systems remain unclear. Here we use a combined approach of first-principles calculations and cluster expansion (CE) to investigate the atomic structures and thermodynamic stabilities of the experimentally reported precipitates as well as orderings on the FCC and HCP lattices in Mg-Sn and Mg-Zn alloys. From the low energy structures searched by CE, potential Guinier–Preston (GP) zones are identified from preferred HCP orderings. The slow convergence for CE of HCP Mg-Zn, compared with that of Mg-Sn system, is attributed to the long-ranged interactions resulting from the larger lattice mismatch. This study could help design better age-hardened Mg alloys. 

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3. Mg-related charge transitions in Mg-doped Ga2O3

   https://doi.org/10.1117/12.2582040  

   Bhandari, Suman; Zvanut, Mary E. (March 2021, SPIE 11687, Oxide-based Materials and Devices XII)

   Teherani, Ferechteh H.; Look, David C.; Rogers, David J. (Ed.)

   Gallium oxide (Ga2O3), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga2O3 crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga2O3. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies. 

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4. Optical transitions of neutral Mg in Mg-doped β -Ga2O3

   https://doi.org/10.1063/5.0081925  

   Bhandari, Suman; Lyons, John_L; Wickramaratne, Darshana; Zvanut, M_E (February 2022, APL Materials)

   Gallium oxide when doped with Mg becomes semi-insulating and can be useful for power electronic devices. The present work investigates optical transitions of neutral Mg (MgGa0) using photoinduced electron paramagnetic resonance spectroscopy, a variation of the traditional optical absorption. Steady-state and time-dependent measurements are carried out at 130 K by illuminating the samples with photon energies from 0.7 to 4.4 eV. Interpretation of the data using a model that incorporates electron–phonon coupling yields a defect transition level that is consistent with the MgGa−/0 level obtained from hybrid density functional theory calculations. We conclude that the neutral to negative transition of MgGa that we observe involves an electron transition from the valence band to the defect, and the MgGa−/0 level is located 1.2 eV above the valence band maximum, with a relaxation energy of 1.3 eV. 

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5. Surface and interfacial energies of Mg17Al12-Mg system

   Wang, F.X. (April 2018, TMS 2018)