In condensed phosphates, the crystal packing of the whole structure often requires phosphate ions to be on the inversion center. However, due to the instability of the linear P-O-P bonds, the oxygen atoms on the inversion center are displaced, leading to the lowering of the crystal symmetry. If the coupled displacements of the oxygen atoms required by different mechanical paths of the MO6octahedra and PO4tetrahedra are contradictory, superstructures with large cells are resulted. Similar deformations of the structures are observed also in silicates and other oxometalates, but the oxygen atoms in these oxides have lower resistance against the linear X-O-X bonds.
FixL is a sensor histidine kinase of the two component system, which regulates the expression of the genes related to nitrogen fixation in the root nodule in response to the O2 levels. The crystal structure of the sensor domain of FixL, which contains a heme as a sensing site, was determined at 1.4 Å resolution. Based on the structure, we propose the O2 sensing mechanism: conformational changes in the distal side of the heme, which are induced by steric repulsion between the bent-bound O2 and the Ile209 side chain, would be transmitted to the histidine kinase domain. Our mechanism permits FixL to serve a real oxygen sensor, which can distinguish bent-bound O2 to other liner-bound ligands.
The structures of light-induced unstable species of photo/thermo/piezochromic compounds, hexaarylbiimidazolyl derivatives (o-Cl-HABI and HABI), cryotrapped in the single crystals were characterized by in situ X-ray crystallography. Photolysis of a crystal of o-Cl-HABI at low temperatures produced radical pairs with retaining the crystallinity. On the other hand, a pair of HABI was transformed to a complex composed of two lophyl radicals and a piezodimer on irradiation at low temperatures. The piezodimer, the existence of which has been assumed spectroscopically as an unstable isomer, was produced via molecular swapping of two photoinduced lophyl radicals with drastic conformational changes. These solid-state photoreactions were confirmed by spectroscopic methods and theoretical calculations as well.
Resonant x-ray scattering has recently been developed as an experimental probe to observe directly the orbital degree of freedom in correlated electron systems. We review recent experimental and theoretical researches in the resonant x-ray scattering in perovskite manganites. A future direction in this research field is also introduced.
This paper reviews structural phase transitions of BaSi2 at high pressures and high temperatures, and electrical resistivity measurements of two quenched high-pressure, high-temperature phases and a stable phase at ambient conditions. In situ x-ray diffraction measurements reveal that BaSi2 has three high pressure, high temperature phases: trigonal, cubic and an additional phase, at pressures up to 7 GPa and temperatures up to 1300 K. All of the high pressure, high temperature phases can be quenched at ambient conditions. The electrical resistivity measurements of trigonal, cubic and orthorhombic BaSi2, a stable phase at ambient conditions, at 1 atm show that the electrical resistivity strongly depends on their crystal structure: orthorhombic and cubic BaSi2 are n-type semiconductors, and trigonal BaSi2 is a hole metal that shows superconductivity with an onset temperature of 6.8 K.
High-pressure and high-temperature behavior of FeS was investigated up to 16 GPa and 750°C . A temperature-induced phase transition was observed at 4.6 GPa. The NiAs type phase was stable above 420°C, and it transformed to its super-structured phase with a hexagonal cell below 420°C in a temperature cooling process. The phase transition could be explained as a second-order transition, based on the analysis of X-ray diffraction intensities from the super-lattice.
X-ray fluorescence spectra have a tail in their low-energy side of characteristic discrete lines. Though the origin of the tailing was believed to be a response of electron gas in the conduction band to the creation of a core hole, the author proposes a new interpretation; the radiative Auger effect is an additional origin of the tailing. Information contained in the shape of tailing is discussed.