[en] Highlights: • Co"2"+/Cr"3"+ co-doped MgGa_2O_4 nanoparticles were prepared by three different methods. • The effect of synthesis method on the distribution of the cations was studied by XPS. • The samples prepared by sol–gel method have the largest inversion degree. • The synthesis method has also effect on the optical absorption and emission properties. • Optical properties of the nanoparticles are discussed on the basis of the structural results. - Abstract: Co"2"+/Cr"3"+ co-doped MgGa_2O_4 nanoparticles were prepared by three different methods (sol–gel, co-precipitation and solid state reaction). The effect of synthesis method on distribution of cations (Ga"3"+, Mg"2"+ and Cr"3"+ ions) was studied using X-ray photoelectron spectroscopy (XPS). These cations occupied octahedral sites as well as tetrahedral sites of the spinel structure in all samples. However, the inversion degree of these samples is greatly influenced by the synthesis method. The Co"2"+/Cr"3"+ co-doped MgGa_2O_4 nanoparticles prepared by the sol–gel method a larger inversion degree than those prepared by other methods. The effect of the synthesis method on absorption spectra and emission spectra was also studied. Samples obtained by sol–gel and co-precipitation methods exhibit broad absorption bands in two wavelength ranges: 300–500 nm and 500–700 nm, while the sample synthesized by solid state reaction only has an absorption peak at 430 nm in the range 300–500 nm. The difference in absorption spectra can be related to the distribution of Co"2"+ and Cr"3"+ ions. Emission spectra for samples prepared by sol–gel and co-precipitation methods show a broad emission band peaking at 700 and 680 nm, combining the emission characteristic of octahedral Cr"3"+ and tetrahedral Co"2"+ ions

[en] Co-doped MgGa₂O₄/SiO₂ nano-glass-ceramic composites with the composition of (1 − y) Co_xMg_1−xGa₂O₄-ySiO₂ (0 ≤ x, y ≤ 1) were synthesized by the sol–gel method and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy was used to study the chemical environments of cations and oxygen in the nanocomposites as a function of annealing temperature, cobalt content and SiO₂ content. The results show that Ga³⁺ and Mg²⁺ ions occupy both the tetrahedral and octahedral sites of spinel structure in the samples. The inversion parameter (two times the fraction of Ga³⁺ ions in the tetrahedral sites) of the nanocomposites is relatively larger in comparison with that of ZnGa₂O₄, and the value decreases with the increase of annealing temperature and silica concentration, while increases with cobalt-enrichment. All the Co-doped samples exhibit the intense absorption peak at ∼600 nm, which is characteristic of tetrahedral Co²⁺. But only in the composites of Co_0.3Mg_0.7Ga₂O₄ and 0.6Co_0.6Mg_0.4Ga₂O₄-0.4SiO₂, the absorption band at 300–500 nm characteristic of octahedral Co²⁺ ions is observed. The intensity of the absorption peak corresponding to octahedral Co²⁺ ions increases with decreasing annealing temperature for the Co_0.3Mg_0.7Ga₂O₄ nanocrystals.

[en] This report discusses the preparation and microstructure of Co/Ni co-doped MgGa₂O₄ nanoparticles. The nanoparticles with the size of 20–55 nm were synthesized by sol-gel method. The phase and crystallinity were confirmed by X-ray powder diffraction (XRD) pattern. The particle size was estimated according to XRD data and transmission electron microscopy. The electronic structure was studied using X-ray photoelectron spectroscopy (XPS). The XPS studies showed that Ga³⁺ ions possess tetrahedral and octahedral sites of spinel structure and the inverse degree (two times of the fraction of tetrahedral Ga³⁺ ions) has increased with the increase of the doping concentration of Co²⁺ and Ni²⁺ ions. For Co/Ni co-doped MgGa₂O₄, two broad absorption bands of 350~500 and 550~700 nm were observed in the absorption spectra. The broad band at 350~500 nm was assigned to the combination of the absorption of octahedral Co²⁺ and Ni²⁺ ions, whereas the absorption band at 550~700 nm is mainly due to tetrahedrally coordinated Co²⁺ ions and octahedrally coordinated Ni²⁺ ions.

[en] Biominerals in the hard tissues of many organisms exhibit superior mechanical properties due to their unique hierarchical nanostructures. In this article, we show the microstructure of human tooth enamel examined by position-resolved small-angle x-ray scattering and electron microscopy. It is found that the degree of ordering of the biominerals varies strikingly within the dental sample. Combined with nanoindentation, our results show that both the hardness and the elastic modulus increase predominantly with the ordering of the biomineral crystallites. This can be attributed to the fact that the ordered structure helps sustain a more complex mechanical stress

[en] The emergence of ultra-intense, ultrafast, and coherent X-ray free electron lasers (FELs) offers opportunities for ultrafast time-scale studies and ultra-small structure determination, making it possible to acquire images and movies of single-molecules and single-particles at atomic resolution. With the completion and use of the FLASH, FERMI, LCLS, and SACLA facilities, X-ray FELs have entered a stage of rapid development, and a series of cutting-edge research results in the fields of physics, chemistry, biology, and materials science are emerging. To realize single particle imaging, research institutions are collaborating worldwide to overcome the technical barriers in experimental techniques, X-ray instrumentation and data analysis. They aim to achieve imaging of nanoparticles, bacteria, cells, viruses, clusters and biological macromolecules on the atomic scale. In this review we will give a brief introduction to the background, scientific significance, objectives, research roadmap, current status, and prospective future development of single particle imaging. (authors)

[en] For the first time to our knowledge, positron annihilation spectroscopy (PAS) was used to study vacancy defects in KTiOPO₄ (KTP) single crystals. Positron annihilation lifetime spectroscopy combined with dielectric measurements identified the existence of oxygen vacancies and reflected the concentration of vacancy defects in three samples. The vacancy defects in KTP do not consist of monovacancies, but rather vacancy complexes. Doppler broadening indicates that the vacancy defects are distributed uniformly. A relationship is established where a crystal with a low oxygen vacancy concentration and a highly balanced stoichiometry has a higher resistance to gray track formation

[en] Three-dimensional X-ray imaging of living specimens is challenging due to the limited resolution of conventional absorption contrast X-ray imaging and potential irradiation damage of biological specimens. In this letter, we present microtomography of a living specimen combining phase-contrast imaging and a Fourier-based iterative algorithm termed equally sloped tomography. Non-destructive 3D imaging of an anesthetized living yellow mealworm Tenebrio molitor was demonstrated with a relatively low dose using synchrotron generated X-rays. Based on the high-quality 3D images, branching tracheoles and different tissues of the insect in a natural state were identified and analyzed, demonstrating a significant advantage of the technique over conventional X-ray radiography or histotomy. Additionally, the insect survived without problem after a 1.92-s X-ray exposure and subsequent absorbed radiation dose of ∼1.2 Gy. No notable physiological effects were observed after reviving the insect from anesthesia. The improved static tomographic method demonstrated in this letter shows advantage in the non-destructive structural investigation of living insects in three dimensions due to the low radiation dose and high resolution capability, and offers many potential applications in biological science.

[en] We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of ∼15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M₅ edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens

[en] Highlights: • A heavily Yb³⁺-substituted langbeinite-type single crystal has been successfully grown by high temperature solution method. • Dual-color upconversion photoluminescence was observed in the highly stable Rb₂Ti_0.80Yb_1.20(PO₄)₃ single crystal. • Disordered Yb³⁺-Ti⁴⁺ pairs and crystal defect were confirmed by X-ray structural analyses. • Blue and red emissions are ascribed to the cooperative luminescence and intrinsic defect. -- Abstract: Inorganic upconversion materials show significant importance in both basic research and applied area, and searching for suitable host material and understanding of fundamental photophysical process are particularly worth studying in upconversion application. Herein, a heavily Yb³⁺-substituted langbeinite-type Rb₂Ti_0.80Yb_1.20(PO₄)₃ (RTYP) millimeter-sized single crystal, which has been successfully grown by high temperature solution method, exhibits efficient dual-color upconversion photoluminescence under 980 nm excitation. The lifetimes of two-photon blue emission and one-photon red emission are 4.1 μs and 17.1 μs, respectively. X-ray structural analyses reveal that the dual-color emissions are ascribed to disordered Yb³⁺-Ti⁴⁺ pairs and crystal defect, and the higher intensity of red emission is attributed to Yb³⁺-defect energy transfer activated by Ti⁴⁺. Notably, the blue emission originates from cooperative luminescence of Yb³⁺ dimer, and the red emission originates dominantly from the cooperative sensitization of the intrinsic defect caused by excited Yb³⁺ dimer. Moreover, RTYP with disordered structure shows high thermal stability up to 1,150 °C. This work not only demonstrates the promising potential of langbeinite-type materials in multicolor upconversion application, but also sheds light on the photophysical process associated with disordered crystal structure.

[en] We report the recording and reconstruction of x-ray diffraction patterns from single, unstained viruses, for the first time. By separating the diffraction pattern of the virus particles from that of their surroundings, we performed quantitative and high-contrast imaging of a single virion. The structure of the viral capsid inside a virion was visualized. This work opens the door for quantitative x-ray imaging of a broad range of specimens from protein machineries and viruses to cellular organelles. Moreover, our experiment is directly transferable to the use of x-ray free electron lasers, and represents an experimental milestone towards the x-ray imaging of large protein complexes