The Preparation of a Challenging Superconductor Nb3Al by Exploiting Nano Effect.

The Nb3Al superconductor with excellent physical and working properties is one of the most promising materials in high-magnetic-field applications. However, it is difficult to prepare high-quality Nb3Al with a desired superconducting transition temperature (Tc) because of its narrow phase formation area at high temperatures (>1940 °C). This work reports a method to prepare stoichiometric Nb3Al powder samples at a relatively low temperature (1400 °C) by exploiting the nano effect of Nb particles with pretreatment of Nb powder under H2/Ar atmosphere. The obtained Nb3Al samples exhibit high Tc's of ~16.8K. Based on density functional theory (DFT) calculations and statistical mechanics analysis, the crucial role of quantum effect in leading to the success of the preparation method was studied. A new measure of surface energy (MSE) of a model particle is introduced to study its size and face dependence. A rapid convergence of the MSE with respect to the size indicates a quick approach to the solid limit, while the face dependence of MSE reveals a liquid-like behavior. The surface effect and quantum fluctuation of the Nbn clusters explain the success of the preparation method.

MTHFD1 is critical for the negative regulation of retinoic acid receptor signalling in anencephaly.

Neural tube defects are the most severe congenital malformations that result from failure of neural tube closure during early embryonic development, and the underlying molecular mechanisms remain elusive. Retinoic acid, an active derivative of vitamin A, is critical for neural system development, and retinoic acid receptor (RAR) signalling malfunctions have been observed in human neural tube defects. However, retinoic acid-retinoic acid receptor signalling regulation and mechanisms in neural tube defects are not fully understood. The mRNA expression of RARs and retinoid X receptors in the different human neural tube defect phenotypes, including 11 pairs of anencephaly foetuses, 10 pairs of hydrocephalus foetuses and nine pairs of encephalocele foetuses, was investigated by NanoString nCounter technology. Immunoprecipitation-mass spectrometry was performed to screen the potential interacting targets of retinoic acid receptor γ. The interactions between proteins were confirmed by co-immunoprecipitation and immunofluorescence laser confocal microscopy. Luciferase and chromatin immunoprecipitation with quantitative real-time polymerase chain reaction assays were used to clarify the underlying mechanism. Moreover, a neural tube defect animal model, constructed using excess retinoic acid, was used for further analysis with established molecular biology technologies. We report that level of retinoic acid receptor γ (RARγ) mRNA was significantly upregulated in the brain tissues of human foetuses with anencephaly. To further understand the actions of retinoic acid receptor γ in neural tube defects, methylenetetrahydrofolate dehydrogenase 1 was identified as a specific retinoic acid receptor γ target from IP-MS screening. Additionally, methylenetetrahydrofolate dehydrogenase 1 negatively regulated retinoic acid receptor γ transcription factor activity. Furthermore, low expression of methylenetetrahydrofolate dehydrogenase 1 and activation of retinoic acid receptor signalling were further determined in human anencephaly and a retinoic acid-induced neural tube defect mouse model. This study reveals that methylenetetrahydrofolate dehydrogenase 1, the rate-determining enzyme in the one-carbon cycle, might be a specific regulator of retinoic acid receptors; these findings provide new insights into the functional linkage between nuclear folate metabolism and retinoic acid receptor signalling in neural tube defect pathology.

Microscopic Mechanism of the Heat-Induced Blueshift in Phosphors and a Logarithmic Energy Dependence on the Nearest Dopant-Vacancy Distance.

Heat-induced blueshift (HIB) observed in many luminescent materials is a puzzling phenomenon that has remained unexplained for decades. By using the high-throughput first-principles calculations and energy-screening techniques, we generated a number of model structures for five phosphors, RbLi[Li3 SiO4 ]2 :Eu2+ , Na[Li3 SiO4 ]:Eu2+ , K[Li3 SiO4 ]:Eu2+ , Sr[LiAl3 N4 ]:Eu2+ , and Ca[LiAl3 N4 ]:Eu2+ . Our analyses suggest, to a first approximation, a logarithmic energy dependence on the nearest distance between the dopant and the metal-cation vacancy. By identifying the 5 d → 4 f transition energies from the electronic structures calculated for the screened model structures, we show that the vibration of the Eu2+ ion lying in an asymmetric and anharmonic potential well couples with the electronic states, leading to their HIB phenomena.

Dehydrated UiO-66(SH)2 : The Zr-O Cluster and Its Photocatalytic Role Mimicking the Biological Nitrogen Fixation.

This work reports the dehydrated Zr-based MOF UiO-66(SH)2 as a visible-light-driven photocatalyst to mimic the biological N2 fixation process. The 15 N2 and other control experiments demonstrated that the new photocatalyst is highly efficient in converting N2 to ammonia. In-situ TGA, XPS, and EXAFS as well as first-principles simulations were used to demonstrate the role of the thermal treatment and the changes of the local structures around Zr due to the dehydration. It was shown that the dehydration opened a gate for the entry of N2 molecules into the [Zr6 O6 ] cluster where the strong N≡N bond was broken stepwise by µ-N-Zr type interactions driven by the photoelectrons aided by the protonation. This mechanism was discussed in comparison with the Lowe-Thorneley mechanism proposed for the MoFe nitrogenase, and with emphasis on the [Zr6 O6 ] cluster effect and the leading role of photoelectrons over the protonation. The results shed new light on understanding the catalytic mechanism of biological N2 fixation and open a new way to fix N2 under mild conditions.

Harnessing Electrostatic Interactions for Enhanced Conductivity in Metal-Organic Frameworks.

The poor electrical conductivity of metal-organic frameworks (MOFs) has been a stumbling block for its applications in many important fields. Therefore, exploring a simple and effective strategy to regulate the conductivity of MOFs is highly desired. Herein, anionic guest molecules are incorporated inside the pores of a cationic MOF (PFC-8), which increases its conductivity by five orders of magnitude while maintaining the original porosity. In contrast, the same operation in an isoreticular neutral framework (PFC-9) does not bring such a significant change. Theoretical studies reveal that the guest molecules, stabilized inside pores through electrostatic interaction, play the role of electron donors as do in semiconductors, bringing in an analogous n-type semiconductor mechanism for electron conduction. Therefore, we demonstrate that harnessing electrostatic interaction provides a new way to regulate the conductivity of MOFs without necessarily altering the original porous structure. This strategy would greatly broaden MOFs' application potential in electronic and optoelectronic technologies.

Structure and Origin of the Second-Harmonic Generation Response of Nonlinear Optical Material Sr2Be2B2O7.

Sr2Be2B2O7 (SBBO) has long been considered as one of the most promising deep-ultraviolet nonlinear optical materials, but its crystal structure described by space group P6̅c2 in previous studies has remained questionable. On the basis of first-principles calculations coupled with the high-throughput crystal structure prediction method, we found three energetically favorable structures for SBBO with space groups Cm, Pm, and P6̅. These structures and a superstructure of space group Pm-S derived from the Cm structure were refined by the Rietveld method using the available powder X-ray diffraction data. These analyses show that the Pm-S structure is the best one, but its parent Cm structure is almost equally good and has the advantage of having higher symmetry. Via atom response theory analysis, we resolved the cause for the second-harmonic generation (SHG) responses of SBBO at the atomic and orbital level to elucidate the importance of local inversion symmetry in reducing the SHG response.

Knowledge and intake of folic acid to prevent neural tube defects among pregnant women in urban China: a cross-sectional study.

BACKGROUND: The prevalence of neural tube defects (NTDs) in China declined during 2000-2017 with periconceptional folic acid (FA) supplementation, which is effective in reducing the risk of birth defects. We aimed to assess the knowledge and actual use of FA among Chinese pregnant women and to explore factors associated with FA use before pregnancy. METHODS: All data were collected in face-to-face interviews during health visits among pregnant women. We collected information about knowledge and use of FA supplements and demographic, socioeconomic, and health status. One maternity and childcare hospital was chosen in each of four cities: Beijing, Huaibei, Kunming, and Haikou. In total, 435 pregnant women were randomly recruited for interviews conducted from June to December 2016. RESULTS: A total of 428 pregnant women were included in this survey. Of these, 82.0% (351/428) knew that FA can prevent NTDs, and 75.9% (325/428) knew the correct time to take FA. Overall, 65.9% (282/428) of women knew both that FA can prevent NTDs and the recommended time to take FA before pregnancy. Approximately 95.1% (407/428) of women reported having ever taken FA, only 46.3% (198/428) had begun to take FA supplementation before conception, and 64.5% (109/169) of women from rural areas failed to take FA before pregnancy. Women living in northern China (odds ratio [OR] = 1.81, 95% confidence interval [CI], 1.18-2.77), those with unplanned pregnancy (OR = 1.99, 95% CI 1.30-3.04), and highly educated women (OR = 2.37, 95% CI 1.45-3.88) were more likely to know about FA. Women who were homemakers (OR = 1.94, 95% CI 1.21-3.11) and had unplanned pregnancy (OR = 6.18, 95% CI 4.01-9.53) were less likely to begin taking FA before pregnancy. CONCLUSIONS: Our survey showed that most pregnant women knew about FA. Although preconception intake of FA can help to reduce NTDs, improving the rate of FA intake before pregnancy is needed in urban areas of China, especially among homemakers and women from rural areas or with unplanned pregnancy. Campaigns are needed to increase awareness about FA and FA use before pregnancy among rural women, homemakers, and those with unplanned pregnancy and lower education levels.

Engineering the Bandgap and Surface Structure of CsPbCl3 Nanocrystals to Achieve Efficient Ultraviolet Luminescence.

Herein, we report the design of novel ultraviolet luminescent CsPbCl3 nanocrystals (NCs) with the emission peak at 381 nm through doping of cadmium ions. Subsequently, a surface passivation strategy with CdCl2 is adopted to improve their photoluminescence quantum yield (PLQY) with the maximum value of 60.5 %, which is 67 times higher than that of the pristine counterparts. The PLQY of the surface passivated NCs remains over 50 % after one week while the pristine NCs show negligible emission. By virtue of density functional theory calculations, we reveal that the higher PLQY and better stability after surface passivation may result from the significant elimination of surface chloride vacancy (VCl ) defects. These findings provide fundamental insights into the optical manipulation of metal ion-doped CsPbCl3 NCs.

The partition principles for atomic-scale structures and their physical properties: application to the nonlinear optical crystal material KBe2BO3F2.

In implementing the materials genome approach to search for new materials with interesting properties or functions, it is necessary to find the correct functional motif. To this end, it is common to partition an extended structure into various building units and then partition its properties to find the appropriate functional motif. We have developed the general principles for partitioning a structure and its properties in terms of a set of atoms and bonds by analyzing the differential cross-sections of neutron and X-ray scattering phenomena and proposed the procedures with which to partition an extended structure and its properties. We demonstrate how these procedures work by analyzing the nonlinear optical crystal KBe2BO3F2. Our partitioning analysis of KBe2BO3F2 leads to the conclusion that the second harmonic generation response of KBe2BO3F2 is dominated by the ionically bonded metal-centered groups.

Physical Properties of Molecules and Condensed Materials Governed by Onsite Repulsion, Spin-Orbit Coupling and Polarizability of Their Constituent Atoms.

The onsite repulsion, spin-orbit coupling and polarizability of elements and their ions play important roles in controlling the physical properties of molecules and condensed materials. In celebration of the 150th birthday of the periodic table this year, we briefly review how these parameters affect the physical properties and are interrelated.

Key Factors Controlling the Large Second Harmonic Generation in Nonlinear Optical Materials.

The second harmonic generation (SHG) responses of nonisostructural nonlinear optical (NLO) compounds, ß-BaB2O4, LiB3O5, CsB3O5, CsLiB6O10, KBe2BO3F2, and LiCs2PO4, were examined by density functional theory (DFT) calculations, and the contributions of their individual cations and anions were determined by performing atomic response theory analyses. In all of these compounds, the contribution of all metal cations lies in the range of ∼9.3 to ∼29.7% of the total SHG responses, and that of all anions lies between ∼57.4 and ∼72.3%. However, in terms of individual atom contribution, a large metal cation can contribute more than does an anion to the total SHG response. Our work shows that the SHG contribution of an individual anion (e.g., O2-) is weakened when it forms covalent bonds with its surrounding cations (e.g., O-B). The contribution of an individual cation is further affected by the homogeneity of its surrounding anion distribution and also by how strongly the polarizabilities of its surrounding anions are weakened by covalent bonding with other cations. The SHG response increases with αsum/(NEg), an important parameter useful in searching for new NLO materials, where αsum is the sum of the polarizabilities of all of the ions in the primitive unit cell, N is the total number of atoms per primitive cell, and Eg is the band gap.

Second harmonic generation responses of KH2PO4: importance of K and breaking down of Kleinman symmetry.

The second harmonic generation (SHG) responses of the paraelectric and ferroelectric phases of KH2PO4 (KDP) were calculated by first-principles density functional theory (DFT) calculations, and the individual atom contributions to the SHG responses were analyzed by the atom response theory (ART). We show that the occurrence of static polarization does not enhance the SHG responses of the ferroelectric KDP, and that the Kleinman symmetry is reasonably well obeyed for the paraelectric phase, but not for the ferroelectric phase despite that the latter has a larger bandgap. This is caused most likely by the fact that the ferroelectric phase has lower-symmetry local structures than does the paraelectric phase. The contribution to the SHG response of an individual K+ ion is comparable to that of an individual O2- ion. The contributions of the O2- and K+ ions arise overwhelmingly from the polarizable parts of the electronic structure, namely, from the valence bands of the O-2p nonbonding states and from the conduction bands of the K-3d states.

Folate deficiency induced H2A ubiquitination to lead to downregulated expression of genes involved in neural tube defects.

BACKGROUND: Neural tube defects (NTDs) are common congenital malformations resulting in failure of the neural tube closure during early embryonic development. Although it is known that maternal folate deficiency increases the risk of NTDs, the mechanism remains elusive. RESULTS: Herein, we report that histone H2A monoubiquitination (H2AK119ub1) plays a role in neural tube closure. We found that the folate antagonist methotrexate induced H2AK119ub1 in mouse embryonic stem cells. We demonstrated that an increase in H2AK119ub1 downregulated expression of the neural tube closure-related genes Cdx2, Nes, Pax6, and Gata4 in mouse embryonic stem cells under folate deficiency conditions. We also determined that the E3 ligase Mdm2 was responsible for the methotrexate-induced increase in H2AK119ub1 and downregulation of neural tube closure-related genes. Surprisingly, we found that Mdm2 is required for MTX-induced H2A ubiquitination and is recruited to the sites of DSB, which is dependent on DNA damage signaling kinase ATM. Furthermore, folic acid supplementation restored H2AK119ub1 binding to neural tube closure-related genes. Downregulation of these genes was also observed in both brain tissue of mouse and human NTD cases, and high levels of H2AK119ub1 were found in the corresponding NTDs samples with their maternal serum folate under low levels. Pearson correlation analysis showed a significant negative correlation between expression of the neural precursor genes and H2AK119ub1. CONCLUSION: Our results indicate that folate deficiency contributes to the onset of NTDs by altering H2AK119ub1 and subsequently affecting expression of neural tube closure-related genes. This may be a potential risk factor for NTDs in response to folate deficiency.

F-box protein FBXO30 mediates retinoic acid receptor γ ubiquitination and regulates BMP signaling in neural tube defects.

Retinoic acid (RA), an active derivative of vitamin A, is critical for the neural system development. During the neural development, the RA/RA receptor (RAR) pathway suppresses BMP signaling-mediated proliferation and differentiation of neural progenitor cells. However, how the stability of RAR is regulated during neural system development and how BMP pathway genes expression in neural tissue from human fetuses affected with neural tube defects (NTDs) remain elusive. Here, we report that FBXO30 acts as an E3 ubiquitin ligase and targets RARγ for ubiquitination and proteasomal degradation. In this way, FBXO30 positively regulates BMP signaling in mammalian cells. Moreover, RA treatment leads to suppression of BMP signaling by reducing the level of FBXO30 in mammalian cells and in mouse embryos with NTDs. In samples from human NTDs with high levels of retinol, downregulation of BMP target genes was observed, along with aberrant FBXO30 levels. Collectively, our results demonstrate that RARγ levels are controlled by FBXO30-mediated ubiquitination and that FBXO30 is a key regulator of BMP signaling. Furthermore, we suggest a novel mechanism by which high-retinol levels affect the level of FBXO30, which antagonizes BMP signaling during early stage development.

Dependence of the Second-Harmonic Generation Response on the Cell Volume to Band-Gap Ratio.

The second-harmonic generation (SHG) coefficients of 12 nonlinear optical chalcopyrites, ABC2 (A = Zn, Cd; B = Si, Ge, Sn; C = P, As) were calculated by first-principles methods to find that given the primitive cell volume V and the band gap Eg of ABC2, the SHG coefficients dav of ABC2 increase almost linearly with increasing value of V/Eg. This suggests that a noncentrosymmetric crystal has a large SHG response when it consists of large atoms, leading to a small band gap.

Electronic Structure and Lithium Diffusion in LiAl2(OH)6Cl Studied by First Principle Calculations.

First-principles calculations based on the density functional theory (DFT) were carried out to study the atomic structure and electronic structure of LiAl2(OH)6Cl, the only material in the layered double hydroxide family in which delithiation was found to occur. Ab initio molecular dynamics (AIMD) simulations were used to explore the evolution of the structure of LiAl2(OH)6Cl during a thermally induced delithiation process. The simulations show that this process occurs due to the drastic dynamics of Li+ at temperatures higher than ~450 K, in which the [Al2(OH)6] host layers remain stable up to 1100 K. The calculated large value of the Li+ diffusion coefficient D, ~ 3.13 × 10 - 5 c m 2 / s , at 500 K and the high stability of the [Al2(OH)6] framework suggest a potential technical application of the partially-delithiated Li1-xAl2(OH)6Cl1-x (0 < x < 1) as a superionic conductor at high temperatures.

Large Second Harmonic Generation (SHG) Effect and High Laser-Induced Damage Threshold (LIDT) Observed Coexisting in Gallium Selenide.

A big challenge for nonlinear optical (NLO) materials is the application in high power lasers, which needs the simultaneous occurrence of large second harmonic generation (SHG) and high laser induced damage threshold (LIDT). Herein we report the preparation of a new Ga2 Se3 phase, which shows the SHG intensities of around 2.3 times and the LIDT of around 16.7 times those of AgGaS2 (AGS), respectively. In addition, its IR transparent window ca. 0.59-25 µm is also significantly wider than that of AGS (ca. 0.48-≈11.4 µm). The occurrence of the strong SHG responses and good phase-matching indicate that the structure of the new Ga2 Se3 phase can only be non-centrosymmetric and have a lower symmetry than the cubic γ-phase. The observed excellent SHG and phase-matching properties are consistent with our diffraction experiments and can be well explained by using the orthorhombic models obtained through our high throughput simulations.

Nonequivalent Spin Exchanges of the Hexagonal Spin Lattice Affecting the Low-Temperature Magnetic Properties of RInO3 (R = Gd, Tb, Dy): Importance of Spin-Orbit Coupling for Spin Exchanges between Rare-Earth Cations with Nonzero Orbital Moments.

Rare-earth indium oxides RInO3 (R = Gd, Tb, Dy) consist of spin-frustrated hexagonal spin lattices made up of rare-earth ions R3+, where R3+ = Gd3+ (f7, L = 0), Tb3+ (f8, L = 3), and Dy3+ (f9, L = 5). We carried out DFT calculations for RInO3, including on-site repulsion U with/without spin-orbit coupling (SOC), to explore if their low-temperature magnetic properties are related to the two nonequivalent nearest-neighbor (NN) spin exchanges of their hexagonal spin lattices. Our DFT + U + SOC calculations predict that the orbital moments of the Tb3+ and Dy3+ ions are smaller than their free-ion values by ∼2µB while the Tb3+ spins have an in-plane magnetic anisotropy, in agreement with the experiments. This suggests that the f orbitals of each R3+ (R = Tb, Dy) ion are engaged, though weakly, in bonding with the surrounding ligand atoms. The magnetic properties of GdInO3 with the zero orbital moment are adequately described by the spin exchanges extracted by DFT + U calculations. In describing the magnetic properties of TbInO3 and DyInO3 with nonzero orbital moments, however, the spin exchanges extracted by DFT + U + SOC calculations are necessary. The spin exchanges of RInO3 (R = Gd, Tb, Dy) are dominated by the two NN spin exchanges J1 and J2 of their hexagonal spin lattice, in which the honeycomb lattice of J2 forms spin-frustrated ( J1, J1, J2) triangles. The J2/ J1 ratios are calculated to be ∼3, ∼1.7, and ∼1 for GdInO3, TbInO3, and DyInO3, respectively. This suggests that the antiferromagnetic (AFM) ordering of GdInO3 below 1.8 K is most likely an AFM ordering of its honeycomb spin lattice and that TbInO3 would exhibit low-temperature magnetic properties similar to those of GdInO3 while DyInO3 would not.

The Large Second-Harmonic Generation of LiCs2 PO4 is caused by the Metal-Cation-Centered Groups.

We evaluated the individual atom contributions to the second harmonic generation (SHG) coefficients of LiCs2 PO4 (LCPO) by introducing the partial response functionals on the basis of first principles calculations. The SHG response of LCPO is dominated by the metal-cation-centered groups CsO6 and LiO4 , not by the nonmetal-cation-centered groups PO4 expected from the existing models and theories. The SHG coefficients of LCPO are determined mainly by the occupied orbitals O 2p and Cs 5p as well as by the unoccupied orbitals Cs 5d and Li 2p. For the SHG response of a material, the polarizable atomic orbitals of the occupied and the unoccupied states are both important.

Superconductivity Induced by Oxygen Doping in Y2 O2 Bi.

When doped with oxygen, the layered Y2 O2 Bi phase becomes a superconductor. This finding raises questions about the sites for doped oxygen, the mechanism of superconductivity, and practical guidelines for discovering new superconductors. We probed these questions in terms of first-principles calculations for undoped and O-doped Y2 O2 Bi. The preferred sites for doped O atoms are the centers of Bi4 squares in the Bi square net. Several Bi 6p x/y bands of Y2 O2 Bi are raised in energy by oxygen doping because the 2p x/y orbitals of the doped oxygen make antibonding possible with the 6p x/y orbitals of surrounding Bi atoms. Consequently, the condition necessary for the "flat/steep" band model for superconductivity is satisfied in O-doped Y2 O2 Bi.