Multi-anvil apparatus enables us to study the behavior of materials in a quantitative manner but the pressure range is rather limited. On the other hand, development of laser-heated diamond anvil apparatus extended the pressure range to above 100 GPa and provides us many interesting features of materials under extreme conditions.
In this paper, a technique of single-crystal X-ray diffraction measurements under high pressures, using a diamond anvil cell with helium gas as an inert and hydrostatic pressuretransmitting medium, is described. The technique has been applied to a three-dimensional halogen-bridged mixed-valence gold complex, CS2 [AuICl2] [AuIIICl4] to investigate the mixedvalence state and the crystal structure under high pressures up to 18 GPa.
Lattice instability due to plastic and elastic deformation under pressure induces many types of structure transformations, including changes of coordination number, cation ordering, electronic state such as charge disproportionation, charge transfer and electron density of state, and spin-lattice interaction. Nonhydrostaticity of compression brings metastable states or intermediate states, which are different from the thermo-dynamically stable phases, because they are temporary states in the dynamical process. Kinetic study of the pressure-induced transition is discussed in accordance with the in situ observation of time-resolved diffraction measurement. Besides the stress-strain relation, the crystallite size is an effective parameter of phase transition under pressure. Molecular dynamical calculation simulates the high-pressure phase transition.
Recent technique and private history of low temperature X-ray structure analysis are described. Some examples taken at low temperature as well as low temperature and high pressure are shown. Special emphasis is given for the importance of synchrotron radiation experiments.
Various ground states such as superconductivity, Mott insulator, CDW and charge ordering for low-dimensional organic complexes, are studied using X-ray. Techniques including homemade cameras and know-hows for low temperature and high pressure X-ray measurements are presented.
Cryogenic protein crystallography is now the indispensable method in structural biology. In this method, protein crystals are rapidly cooled by low temperature nitrogen gas or liquid ethane, and diffraction intensity data are collected at cryogenic temperature. By applying this method, the X-radiation damage of protein crystals are drastically decreased, and we can easily obtain diffraction data even for highly radiation-sensitive protein crystals. This method has been applied to investigate the hydration structures around protein molecules and to analyze the structure of reaction-intermediates of proteins. Here, we briefly report the procedures and the experimental techniques using the newly developed devices and discuss the advantages and the disadvantages of this method. In addition, we describe the application of this method to crystallographically investigate the mechanism of the glassy transition observed in the vibrational states of protein molecules around 200 K.
The present authors have investigated the density fluctuation for supercritical fluids by means of small-angle X-ray scattering, interested in the inhomogeneity of the distribution of molecules. For supercritical CO2 and supercritical CF3H, similar curves of density fluctuations were obtained independent of the substances, which shows that these variables may be universal ones to describe the state of supercritical region. To study whether or not the similarities hold true for other substances with different intermolecular interactions, experiments for supercritical water have been also tried. Supercritical water is in hard condition in regard to its porosity, in addition to the high temperature and high pressure. The design of the sample holder for supercritical water and the results of small-angle X-ray scattering measurements are presented in the present paper.
Crystallographic studies of metastable and excited state molecular crystals provide direct geometrical information for those of light-induced structure changes or transition states in solid-state chemical reactions. Such geometry changes will give only very small difference in X-ray diffraction intensities. Special instrumentation and data acquisition technique must be required in order to detect small changes of intensity. This review describes a few examples of experimental and analysis technique applying to photo-excited crystallography.
The rapid X-ray diffraction data collection method has advantages for the time-resolved observation of solid-state reactions in crystal and structure analysis of unstable crystals. The new diffractometers for rapid data collection using two-dimensional detector have been developed to achieve both high speed measurement and high data accuracy. Step-by-step molecular structure changes of cobaloxime complex, distibene molecule are observed by using the new diffractometer first time.
With the use of extremely intense synchrotron radiation sources it has become possible to collect good quality x-ray diffraction data for protein crystals on a subsecond time-scale. This has led to the development of time-resolved protein crystallography, where the main objective is to determine the structures of short-lived intermediates during enzymatic reactions by monitoring the time dependence of the x-ray intensities. Furthermore, utilization of singlebunch mode at the third generation synchrotron sources enables us to shorten the time-scale of the time-resolved protein crystallography down to nanosecond order. Feasibility of singlepulse Laue diffraction experiment on protein crystals is discussed.