Shuai Yan

Low-density to high-density transition in Ce75Al23Si2 metallic glass

Using in situ high-pressure x-ray diffraction (XRD), we observed a pressure-induced polyamorphic transition from the low-density amorphous (LDA) state to the high-density amorphous (HDA) state in Ce(75)Al(23)Si(2) metallic glass at about 2 GPa and 300 K. The thermal stabilities of both LDA and HDA metallic glasses were further investigated using in situ high-temperature and high-pressure XRD, which revealed different pressure dependences of the onset crystallization temperature (T(x)) between them with a turning point at about 2 GPa. Compared with Ce(75)Al(25) metallic glass, minor Si doping shifts the onset polyamorphic transition pressure from 1.5 to 2 GPa and obviously stabilizes both LDA and HDA metallic glasses with higher T(x) and changes their slopes dT(x)/dP. The results obtained in this work reveal another polyamorphous metallic glass system by minor alloying (e.g. Si), which could modify the transition pressure and also properties of LDA and HDA metallic glasses. The minor alloying effect reported here is valuable for the development of more polyamorphous metallic glasses, even multicomponent bulk metallic glasses with modified properties, which will trigger more investigations in this field and improve our understanding of polyamorphism and metallic glasses.

Structural phase transitions in Bi2Se3 under high pressure

Raman spectroscopy and angle dispersive X-ray diffraction (XRD) experiments of bismuth selenide (Bi2Se3) have been carried out to pressures of 35.6 and 81.2u2009GPa, respectively, to explore its pressure-induced phase transformation. The experiments indicate that a progressive structural evolution occurs from an ambient rhombohedra phase (Space group (SG): R-3m) to monoclinic phase (SG: C2/m) and eventually to a high pressure body-centered tetragonal phase (SG: I4/mmm). Evidenced by our XRD data up to 81.2u2009GPa, the Bi2Se3 crystallizes into body-centered tetragonal structures rather than the recently reported disordered body-centered cubic (BCC) phase. Furthermore, first principles theoretical calculations favor the viewpoint that the I4/mmm phase Bi2Se3 can be stabilized under high pressure (>30u2009GPa). Remarkably, the Raman spectra of Bi2Se3 from this work (two independent runs) are still Raman active up to ~35u2009GPa. It is worthy to note that the disordered BCC phase at 27.8u2009GPa is not observed here. The remarkable difference in atomic radii of Bi and Se in Bi2Se3 may explain why Bi2Se3 shows different structural behavior than isocompounds Bi2Te3 and Sb2Te3.

Systematic investigation on the influence of the As4 flux on the magnetic property of (In,Cr)As quantum dots

We study the structure, optical and magnetic characteristics of self-assembled (In,Cr) As diluted magnetic semiconductor quantum dots as a function of the As-4 flux. Increasing the surface energy by increasing the As4 pressure leads to a smaller number of larger dots for a higher As-4 flux. The remanent magnetization measured at 5K also increases with increasing As-4 flux, which is attributed to the enhancement of the effective Cr content due to the As-4-rich condition. We explore the possibility of tailoring magnetism by controlling the As-4/In flux ratio without changing the Cr concentration. Furthermore, extremely low-density QDs have also been successfully grown. Copyright (C) EPLA, 2008

Pressure-Induced Crystallization and Phase Transformation of Para-xylene.

Static pressure is an alternative method to chemical pressure for tuning the crystal structure, bonds, and physical properties of materials, and is a significant technique for the synthesis of novel materials and fundamental research. In this letter, we report the crystallization and phase transformation of p-xylene under high pressure. Our optical micrographic observations and the appearance of lattice modes in the Raman and infrared (IR) spectra indicated that p-xylene crystallizes at ∼0.1u2009GPa. The X-ray diffraction (XRD) pattern at 0.84u2009GPa suggests that the crystallized p-xylene had a monoclinic phase with the Cc(9) space group. The sharp shrinkage of the lattice at ~13u2009GPa and the solid state of the decompressed sample we observed suggests a new crystalline phase of p-xylene. The in situ XRD showed that the new crystalline phase was still a monoclinic structure but with a different space group of C2(5), indicating that a phase transition occurred during further compression. The mass spectrometry experiment confirmed phase transition polymerization, with mainly trimer and tetramer polymers. Our findings suggest an easy and efficient method for crystallizing and polymerizing p-xylene under high pressure.

Structural evolution behavior of manganese monophosphide under high pressure: experimental and theoretical study

The influence of external pressure on the structural properties of manganese monophosphides (MnP) at room temperature has been studied using in situ angle dispersive synchrotron x-ray powder diffraction (AD-XRD) with a diamond anvil cell. The crystal structure of MnP is stable between 0 to 15 GPa. However, the compressibility of b-axis is much larger than those of a- and c-axes. From this result we suggested that the occurrence of superconductivity in MnP was induced by suppression of the long-range antiferromagnetically ordered state rather than a structural phase transition. Furthermore, the present experimental results show that the Pnma phase of MnP undergoes a pressure-induced structural phase transition at ~15.0u2009GPa. This finding lighted up-to-date understanding of the common prototype B31 structure (Strukturbericht Designation: B31) in transition metal monophosphides. No additional structural phase transition was observed up to 35.1u2009GPa (Run 1) and 40.2u2009GPa (Run 2) from the present AD-XRD results. With an extensive crystal structure searching and ab initio calculations, we predict that MnP underwent two pressure-induced structural phase transitions of Pnmau2009u2009→u2009u2009P213 and P213u2009u2009→u2009u2009Pm-3m (CsCl-type) at 55.0 and 92.0u2009GPa, respectively. The structural stability and the electronic structures of manganese monophosphides under high pressure are also briefly discussed.

Pressure-induced phase transitions of exposed curved surface nano-TiO2 with high photocatalytic activity

We report a unique phase transition in compressed exposed curved surface nano-TiO2 with high photocatalytic activity using in situ synchrotron X-ray diffraction and Raman Spectroscopy. High-pressure studies indicate that the anatase phase starts to transform into baddeleyite phase upon compression at 19.4u2009GPa, and completely transforms into the baddeleyite phase above 24.6u2009GPa. Upon decompression, the baddeleyite phase was maintained until the pressure was released to 6.4u2009GPa and then transformed into the α-PbO2 phase at 2.7u2009GPa. Together with the results of high-resolution transmission electron microscopy and the pressure-volume relationship, this phase transitions characteristics during the compression-decompression cycle demonstrate that the truncated biconic morphology possessed excellent stability. This study may provide an insight to the mechanisms of stability for high photocatalytic activity of nano-TiO2.

Author Correction: Pressure-Induced Crystallization and Phase Transformation of Para-xylene

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Isothermal pressure-derived metastable states in 2D hybrid perovskites showing enduring bandgap narrowing

Significance Metastable materials often exhibit unexpected striking properties that are not available in stable state. While metastable states are generally achieved by rapid cooling of materials from high temperature, it is imperative to explore other nonthermal routes to access metastable states, especially for heat-vulnerable materials. Here, we report that work by pressure, namely, a compression−decompression cycle under ambient temperature, can drive thermosusceptible organic−inorganic hybrid perovskites to their metastable state, in which the perovskites show enduring bandgap narrowing for significantly broadened solar absorption. This pressure-derived route provides a fundamental path to obtain metastable materials with unprecedented performance. Materials in metastable states, such as amorphous ice and supercooled condensed matter, often exhibit exotic phenomena. To date, achieving metastability is usually accomplished by rapid quenching through a thermodynamic path function, namely, heating−cooling cycles. However, heat can be detrimental to organic-containing materials because it can induce degradation. Alternatively, the application of pressure can be used to achieve metastable states that are inaccessible via heating−cooling cycles. Here we report metastable states of 2D organic−inorganic hybrid perovskites reached through structural amorphization under compression followed by recrystallization via decompression. Remarkably, such pressure-derived metastable states in 2D hybrid perovskites exhibit enduring bandgap narrowing by as much as 8.2% with stability under ambient conditions. The achieved metastable states in 2D hybrid perovskites via compression−decompression cycles offer an alternative pathway toward manipulating the properties of these “soft” materials.

Single crystal growth, crystalline structure investigation and high-pressure behavior of impurity-free siderite (FeCO3)

Single crystals of impurity-free siderite were grown successfully using high-temperature–pressure annealing. The size of crystals ranged up to 100xa0µm, and they exhibited a rhomboid shape upon cleavage along the (101) plane. The composition of Fe0.9988±0.0011CO3 was quantified using electron probe analysis. Accurate crystalline structural data were investigated by means of single crystal X-ray diffraction (XRD) and the unit cell dimensions obtained in the rhombohedral symmetry of the