Silicon dioxide is one of the most abundant natural compounds. Polymorphs of SiO₂ and their phase transitions have long been a focus of great interest and intense theoretical and experimental ...pursuits. Here, compressing single-crystal coesite SiO₂ under hydrostatic pressures of 26-53 GPa at room temperature, we discover a new polymorphic phase transition mechanism of coesite to post-stishovite, by means of single-crystal synchrotron X-ray diffraction experiment and first-principles computational modelling. The transition features the formation of multiple previously unknown triclinic phases of SiO₂ on the transition pathway as structural intermediates. Coexistence of the low-symmetry phases results in extensive splitting of the original coesite X-ray diffraction peaks that appear as dramatic peak broadening and weakening, resembling an amorphous material. This work sheds light on the long-debated pressure-induced amorphization phenomenon of SiO₂, but also provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature.
Bonding Changes in Compressed Superhard Graphite Mao, Wendy L.; Mao, Ho-kwang; Eng, Peter J. ...
Science (American Association for the Advancement of Science),
10/2003, Letnik:
302, Številka:
5644
Journal Article
Recenzirano
Compressed under ambient temperature, graphite undergoes a transition at ~17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the ...π-bonds between graphite layers convert to σ-bonds, whereas the other half remain as π-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form σ-bonds, whereas the nonbridging carbon atoms remain unpaired with π-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.
Hydrogen Clusters in Clathrate Hydrate Mao, Wendy L.; Mao, Ho-Kwang; Goncharov, Alexander F. ...
Science (American Association for the Advancement of Science),
09/2002, Letnik:
297, Številka:
5590
Journal Article
Recenzirano
High-pressure Raman, infrared, x-ray, and neutron studies show that H2and H2O mixtures crystallize into the sll clathrate structure with an approximate$H_{2}/H_{2}O$molar ratio of 1:2. The clathrate ...cages are multiply occupied, with a cluster of two H2molecules in the small cage and four in the large cage. Substantial softening and splitting of hydrogen vibrons indicate increased intermolecular interactions. The quenched clathrate is stable up to 145 kelvin at ambient pressure. Retention of hydrogen at such high temperatures could help its condensation in planetary nebulae and may play a key role in the evolution of icy bodies.
Recent theoretical and experimental studies have produced several unusual and interesting results on dense lithium, the first metal in the periodic table. These include the deviation from simple ...metal behaviour, superconductivity at 17 K, and a metal to semiconductor transition. Despite these efforts, at present there is no agreement on the location of the high-pressure solid phases and melting curve of Li, and there is no clear picture of its phase diagram above 50 GPa (refs 4, 5, 6, 7). Using powder and single-crystal high-pressure diffraction techniques, we have mapped out the lithium phase diagram up to 130 GPa over a wide temperature range between 77 and 300 K. Whereas the melting temperatures of materials usually rise under pressure, and even the lightest condensed gases, hydrogen and helium, melt at temperatures of the order of 103 K at 50 GPa (refs 8, 9), we find that at these pressures lithium remains a liquid at temperatures as low as 190 K, by far the lowest melting temperature observed for any material at such pressure. We also find that in its solid state above 60 GPa, lithium adopts three novel and complex crystal structures not previously observed in any element. Estimates of the zero-point energy suggest that quantum effects play a significant role in shaping the lithium phase diagram.
Minichromosome maintenance (MCM) proteins 2-7 are important in DNA replication licensing. Functional roles beyond licensing are speculated. In addition, significances in medulloblastoma (MB) remain ...unclear. In this study, we showed the frequent deregulation of MCM2 and MCM3 expression in 7 MB cell lines and 31 clinical samples. Moreover, DAOY and ONS76 and the clinical samples expressed elevated MCM7 transcripts with genomic gain of the gene. Immunopositivity restricted to tumor cells was found in 41, 37 and 53 out of 73 MB cases for MCM2, MCM3 and MCM7, respectively. High-MCM3 expression was associated with poor prognosis. Knockdowns of these MCMs significantly inhibited anchorage-dependent and -independent MB cell growth. The inhibition of MCM3 expression by small interfering RNA knockdown was related to G1 arrest with reduced cyclin A expression, whereas the MCM2- and MCM7-knocked-down cells arrested at G2/M with increased cyclin A expression. Interestingly, we demonstrated the links of these MCMs with cell migration and invasion using wound-healing and Transwell migration/invasion assays. Exogenous overexpression of MCM2, MCM3 and MCM7 increased anchorage-independent cell growth, and also cell migration and invasion capabilities in MB cells. The knockdown reduced the number of filopodial cells and the cells with intense stress fibers by blocking cdc42 and Rho activation. Taken together, deregulation of MCM2, MCM3 and MCM7 expression might be involved in MB tumorigenesis and we revealed undefined roles of these MCMs in control of MB cell migration and invasion.
Objective
This study examined the differences in 14-year outcomes of persons with schizophrenia with and without family caregiver(s) in a rural community in China.
Methods
All participants with ...schizophrenia (
n
= 510) aged 15 years and older were identified in a 1994 epidemiological investigation of 123,572 people and followed up in 2004 and 2008 in Xinjin County, Chengdu, China.
Results
Individuals without family caregiver in 1994 had significantly higher rate of homelessness (23.8 %) and lower rate of survival (47.5 %) in 2008 than those with family caregivers (5.1 and 70.9 %). Compared with individuals with family caregivers, those without family caregivers were more likely to be male, live alone, have fewer family members, lower family economic status, lower rates of marriage and complete remission, higher mean scores on PANSS and lower mean score on GAF in 2008. The predictors of participants without family caregiver in 2008 included having a small number of family members at baseline and being male.
Conclusions
The absence of a family caregiver is a predictive factor of poorer long-term outcome of persons with schizophrenia in the rural community. The critical role of family caregiving should be incorporated in the planning and delivering of mental health policies and community-based mental health services.
Using density functional theory, we show that the long-believed transition-metal tetraborides (TB(4)) of tungsten and molybdenum are in fact triborides (TB(3)). This finding is supported by ...thermodynamic, mechanical, and phonon instabilities of TB(4), and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB(3) structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB(3) cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB(3) under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.
Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. ...Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.
The first natural-occurring quasicrystal, icosahedrite, was recently discovered in the Khatyrka meteorite, a new CV3 carbonaceous chondrite. Its finding raised fundamental questions regarding the ...effects of pressure and temperature on the kinetic and thermodynamic stability of the quasicrystal structure relative to possible isochemical crystalline or amorphous phases. Although several studies showed the stability at ambient temperature of synthetic icosahedral AlCuFe up to ~35 GPa, the simultaneous effect of temperature and pressure relevant for the formation of icosahedrite has been never investigated so far. Here we present in situ synchrotron X-ray diffraction experiments on synthetic icosahedral AlCuFe using multianvil device to explore possible temperature-induced phase transformations at pressures of 5 GPa and temperature up to 1773 K. Results show the structural stability of i-AlCuFe phase with a negligible effect of pressure on the volumetric thermal expansion properties. In addition, the structural analysis of the recovered sample excludes the transformation of AlCuFe quasicrystalline phase to possible approximant phases, which is in contrast with previous predictions at ambient pressure. Results from this study extend our knowledge on the stability of icosahedral AlCuFe at higher temperature and pressure than previously examined, and provide a new constraint on the stability of icosahedrite.
PDF
