Structural prediction of Fe-Mg-O compounds at Super-Earth's pressures

Terrestrial exoplanets are of great interest for being simultaneously similar to and different from Earth. Their compositions are likely comparable to those of solar-terrestrial objects, but their internal pressures and temperatures can vary significantly with their masses/sizes. The most abundant non-volatile elements are O, Mg, Si, Fe, Al, and Ca, and there has been much recent progress in understanding the nature of magnesium silicates up to and beyond ~3 TPa. However, a critical element, Fe, has yet to be systematically included in materials discovery studies of potential terrestrial planet-forming phases at ultra-high pressures. Here, using the adaptive genetic algorithm (AGA) crystal structure prediction method, we predict several unreported stable crystalline phases in the binary Fe-Mg and ternary Fe-Mg-O systems up to pressures of 3 TPa. The analysis of the local packing motifs of the low-enthalpy Fe-Mg-O phases reveals that the Fe-Mg-O system favors a BCC motif under ultra-high pressures regardless of chemical composition. Besides, oxygen enrichment is conducive to lowering the enthalpies of the Fe-Mg-O phases. Our results extend the current knowledge of structural information of the Fe-Mg-O system to exoplanet pressures

Preparations of Meiotic Pachytene Chromosomes and Extended DNA Fibers from Cotton Suitable for Fluorescence In Situ Hybridization

Fluorescence in situ hybridization (FISH) has become one of the most important techniques applied in plant molecular cytogenetics. However, the application of this technique in cotton has lagged behind because of difficulties in chromosome preparation. The focus of this article was FISH performed not only on cotton pachytene chromosomes, but also on cotton extended DNA fibers. The cotton pollen mother cells (PMCs) instead of buds or anthers were directly digested in enzyme to completely breakdown the cell wall. Before the routine acetic acid treatment, PMCs were incubated in acetic acid and enzyme mixture to remove the cytoplasm and clear the background. The method of ice-cold Carnoy's solution spreading chromosome was adopted instead of nitrogen removed method to avoid chromosomes losing and fully stretch chromosome. With the above-improved steps, the high-quality well-differentiated pachytene chromosomes with clear background were obtained. FISH results demonstrated that a mature protocol of cotton pachytene chromosomes preparation was presented. Intact and no debris cotton nuclei were obtained by chopping from etiolation cotyledons instead of the conventional liquid nitrogen grinding method. After incubating the nuclei with nucleus lysis buffer on slide, the parallel and clear background DNA fibers were acquired along the slide. This method overcomes the twist, accumulation and fracture of DNA fibers compared with other methods. The entire process of DNA fibers preparation requires only 30 min, in contrast, it takes 3 h with routine nitrogen grinding method. The poisonous mercaptoethanol in nucleus lysis buffer is replaced by nonpoisonous dithiothreitol. PVP40 in nucleus isolation buffer is used to prevent oxidation. The probability of success in isolating nuclei for DNA fiber preparation is almost 100% tested with this method in cotton. So a rapid, safe, and efficient method for the preparation of cotton extended DNA fibers suitable for FISH was established

Superconductivity in the Li-B-C system at 100 GPa

Layer Li-B-C compounds have been shown to have feasible superconductivity. Using the adaptive genetic algorithm, we predict the structures of the Li-B-C system at 100 GPa. We identify several low-enthalpy metallic phases with stoichiometries of LiB2C, LiB3C, Li2BC2, Li3B2C3, Li3BC, and Li5BC. Using a fast screening method of electron-phonon interaction, we find that LiB3C is a promising candidate for superconductivity. The consecutive calculations using the full Brillouin zone confirm the existence of the strong electron-phonon coupling (EPC) in this system. The anharmonic B-C phonon modes near the zone center provide the major contribution to the EPC. The EPC constant is 1.40, and the estimated critical temperature is 22 K. In this paper, we indicate that superconductivity can also happen without a layered structural motif in the Li-B-C system. We also demonstrate an effective strategy for crystal structure prediction of superconducting materials.This article is published as Zheng, Feng, Yang Sun, Renhai Wang, Yimei Fang, Feng Zhang, Shunqing Wu, Cai-Zhuang Wang, Vladimir Antropov, and Kai-Ming Ho. "Superconductivity in the Li-BC system at 100 GPa." Physical Review B 107, no. 1 (2023): 014508. DOI: 10.1103/PhysRevB.107.014508 Copyright 2023 American Physical Society. Posted with permission. DOE Contract Number(s): AC02-07CH11358; 11874307; 2021A1515110328; 2022A1515012174; DMR-213266

Stabilizing the crystal structures of NaFePO4 with Li substitutions

Due to the high cost and insufficient resources of lithium, alternative sodium-ion batteries have been widely investigated for large-scale applications. NaFePO4 has the highest theoretical capacity of 154 mA h g−1 among the iron-based phosphates, which makes it an attractive cathode material for Na-ion batteries. Experimentally, LiFePO4 has been highly successful as a cathode material in Li-ion batteries because its olivine crystal structure provides a stable framework during battery cycling. In NaFePO4, maricite replaces olivine as the most stable phase. However, the maricite phase is experimentally found to be electrochemically inactive under normal battery operating voltages (0–4.5 V). We found that partial substitutions of Na with Li stabilize the olivine structure and may be a way to improve the performance of NaFePO4 cathodes. Using the previously developed structural LiFePO4 database, we examined the low-energy crystal structures in the system when we replace Li with Na. The known maricite and olivine NaFePO4 phases are reconfirmed and an unreported phase with energy between them is identified by our calculations. Besides, the Li-doped olivine type compound LixNa1−xFePO4 with mixed alkali ions retains better energetic stability compared with the other two types of structures of the same composition, as long as the proportion of Li exceeds 0.25. The thermodynamic stability of o-type LixNa1−xFePO4 can be further improved at finite temperatures. The primary limitation of the calculations is that we mainly focus on the zero-temperature condition; however, the relative stability of the structures may vary depending on the ambient temperature

Stabilizing the crystal structures of NaFePO4 with Li substitutions

Due to the high cost and insufficient resources of lithium, alternative sodium-ion batteries have been widely investigated for large-scale applications. NaFePO4 has the highest theoretical capacity of 154 mA h g−1 among the iron-based phosphates, which makes it an attractive cathode material for Na-ion batteries. Experimentally, LiFePO4 has been highly successful as a cathode material in Li-ion batteries because its olivine crystal structure provides a stable framework during battery cycling. In NaFePO4, maricite replaces olivine as the most stable phase. However, the maricite phase is experimentally found to be electrochemically inactive under normal battery operating voltages (0–4.5 V). We found that partial substitutions of Na with Li stabilize the olivine structure and may be a way to improve the performance of NaFePO4 cathodes. Using the previously developed structural LiFePO4 database, we examined the low-energy crystal structures in the system when we replace Li with Na. The known maricite and olivine NaFePO4 phases are reconfirmed and an unreported phase with energy between them is identified by our calculations. Besides, the Li-doped olivine type compound LixNa1−xFePO4 with mixed alkali ions retains better energetic stability compared with the other two types of structures of the same composition, as long as the proportion of Li exceeds 0.25. The thermodynamic stability of o-type LixNa1−xFePO4 can be further improved at finite temperatures. The primary limitation of the calculations is that we mainly focus on the zero-temperature condition; however, the relative stability of the structures may vary depending on the ambient temperature.</p