Titanium silicalite-1 (TS-1) has been shown to be a heterogeneous catalyst with remarkable efficiency and selectivity; however, the nature of the active Ti site in the MFI framework remains elusive. Here we report combined experimental and theoretical research on Ti distribution in the 12 crystallographically distinct T sites of the MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2). Using a multishell fit to extended X-ray absorption fine structure, we show that T4 is the most populated site, in marked contrast to the preferential substitution sites and the definitely excluded sites assumed hitherto by diffraction studies. The identification is supported by a good agreement between calculated and experimental X-ray absorption near-edge structure studies and by full periodic density functional theory (DFT) computation. In spite of having the identical most favored site, the preference order for the remaining sites predicted by DFT does not fully match the experimental results. This suggests that Ti distribution in the resulting TS-1 framework is positively correlated with the thermodynamic stability of pure material but can be affected by other factors such as interdefects. These new insights may facilitate the bottom-up design of new zeolites with tailored catalytic performance and studies on mechanisms of various oxidation reactions.

No abstract

Microscopy techniques using visible photons, x-rays, neutrons, and electrons have made remarkable impact in many scientific disciplines. The microscopic data can often be expressed as the convolution of the spatial distribution of certain properties of the specimens and the inherent response function of the imaging system. The x-ray grating interferometer (XGI), which is sensitive to the deviation angle of the incoming x-rays, has attracted significant attention in the past years due to its capability in achieving x-ray phase contrast imaging with low brilliance source. However, the comprehensive and analytical theoretical framework is yet to be presented. Herein, we propose a theoretical framework termed angular signal radiography (ASR) to describe the imaging process of the XGI system in a classical, comprehensive and analytical manner. We demonstrated, by means of theoretical deduction and synchrotron based experiments, that the spatial distribution of specimens' physical properties, including absorption, refraction and scattering, can be extracted by ASR in XGI. Implementation of ASR in XGI offers advantages such as simplified phase retrieval algorithm, reduced overall radiation dose, and improved image acquisition speed. These advantages, as well as the limitations of the proposed method, are systematically investigated in this paper.

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a markedly decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO2 glass.

Te 3 ) have been reported to exhibit a pressure-induced isostructural phase transition (IPT) in the low-pressure region (<5 GPa); however, the underlying mechanism is not fully understood yet. Here, we comparatively investigated the IPTs in a-Bi 2 Se 3 and a-Bi 2 Te 3 using high-pressure Bi L 3 -edge X-ray absorption fine structure spectroscopy, complemented with density functional theory (DFT) calculation. Near the IPT, the slope of the Bi L 3 -edge absorption energy shows a sudden decrease. In contrast to the monotonous shortening of the Bi-Bi bond-distance under compression, it is moreover rather surprising that the Bi-Se2 (Te2) and Bi-Se1 (Te1) bonddistances exhibit abnormal elongations around the IPT, which is synchronized with abrupt increases for their Debye-Waller factor. DFT calculation results show that near the IPT a structural distortion may appear for extremely small energy barrier. Those results indicate that each quintuple layer unit shrinks along the layer direction but elongates perpendicular to the layers at the onset of the IPT, which would be associated with a remarkable increase for the local structural disorder. Therefore, our findings suggest that the IPTs in a-Bi 2 Se 3 and a-Bi 2 Te 3 are coupled with a simultaneous charge redistribution and structural distortion accompanied with a static structural disorder increase.Sketch of the changes of local structure of centric Bi and lattice structure around the IPT.

In this paper, we present a comparative study of the vibrational properties of bulk MoS2 and its exfoliated nanosheets under high pressure by resonant Raman spectroscopy. The X‐ray photoelectron spectroscopy measurements demonstrate that the nanosheets made by chemical lithium insertion method are mainly consisted of metallic 1T phase. The wavenumbers of the Ebold2boldg1 and Alg Raman modes for the few layer nanosheets show a linear relationship with pressure which is different from the multilayer MoS2. This phenomenon is due to the differences in their electronic band structure. The systematic research about the 2LA (M) in bulk 2H–MoS2 suggests that the high pressure behaviour of this double resonance Raman mode arises from the pressure‐induced metallization and band gap closure. These difference vibrational properties for few and multilayer MoS2 can be applied for exploring pressure‐modulated advanced devices. Copyright © 2017 John Wiley & Sons, Ltd.

No abstract

The pressure-induced phase-transition sequences and structural evolution across the insulatormetal transition (IMT) in multiferroic BiFeO 3 still remain unclear. Here we use a combination of high-pressure XRD, XAFS experiment and first principle calculation to investigate the pressure-derived structural transformations and structure-related properties in bulk and nanoscale BiFeO 3 up to 55 GPa. A new Imma structure of BiFeO 3 has been discovered in the pressure range of 48-52 GPa, which presents ferromagnetic (FM) metallic properties and therefore plays a key role in the IMT. Local structure study reveals that the Bi 3+ cation gradually shifts toward the centrosymmetric position in BiO 12 cluster during IMT. Besides, the detailed structural information of post-perovskite Cmcm phase has also been determined and thus the complete phase sequence up to 60 GPa is obtained. Our research provides a structural origin of the IMT and a new way to understand the FM release in BiFeO 3 system.