Image-plate Area Detector Research Articles

Pressure dependence of the Imma phase of silicon.

A detailed structural study of the newly observed orthorhombic phase of silicon (space group Imma) has been made using angle-dispersive powder-diffraction techniques and an image-plate area detector. The Imma phase is found to be stable between 13.2(3) and 15.6(3) GPa, and both the \ensuremath{\beta}-tin-to-Imma and Imma-to-simple-hexagonal transitions are found to be first order with volume changes (\ensuremath{\Delta}V/${\mathit{V}}_{0}$) of 0.2(1)% and 0.5(1)%, respectively. The volume discontinuities at the transitions are accompanied by pronounced discontinuities in \ensuremath{\Delta}, the atomic coordinate of the Imma phase, which is found to vary from \ensuremath{\sim}0.3 to 0.4 over the stability range of the Imma phase.

イメージングプレート迅速Ｘ線回折法による高温高圧水の構造解析

A novel X-ray diffraction method for liquids using an imaging plate area detector has been successfully applied to constant-density measurements of water at high temperatures and high pressures covering the supercrital state. The radial distribution functions have shown that the local tetrahedral ice-like structure of water is ruptured above 416K and that the short-range O-O interactions at -2.9A still exist by -40% to the coordination number of 3.9, however, with large fluctuation and distorted hydrogen bonding in dense supercritical water at 649K.

Phase transitions in CdTe to 5 GPa.

Angle-dispersive powder-diffraction techniques using an image-plate area detector have been used to study the high-pressure phases of CdTe to 5 GPa. The recently discovered cinnabar phase between the zinc-blende and NaCl phases is found to be stable on both pressure increase and decrease. No pressure range of single-phase cinnabar has been found on pressure increase, but a single phase is obtained on pressure decrease, between 3.6 and 2.7 GPa. The cinnabar phase has an average bulk modulus of 32(1) GPa and is significantly more compressible than the zinc-blende and NaCl phases. All three phases are site ordered. Each atom in the cinnabar structure has two nearest-neighbor distances of \ensuremath{\sim}2.74 \AA{} (a bonds) and two of \ensuremath{\sim}2.94 \AA{} (b bonds), and the structure is thus close to fourfold coordinated. The next-nearest-neighbor distances are two of \ensuremath{\sim}3.5 \AA{} (c bonds). The a bonds are very similar in length to the nearest-neighbor distance in the zinc-blende phase, and the b bonds increase with pressure towards the nearest-neighbor distance in the NaCl phase. The c-bond distance and the interbond angles also become closer to the geometry of the NaCl structure with increasing pressure, but there are still large discontinuities in bond lengths and angles at the cinnabar-to-NaCl transition.

Crystal structure of the cinnabar phase of HgTe.

Angle-dispersive powder-diffraction techniques with an image-plate area detector and synchrotron radiation have been used to perform a structural refinement of mercury telluride in its cinnabar phase. We find that the structure is site-ordered with fractional coordinates of (u=0.641(1),0,1/3) and (v=0.562(1),0,5/6) for Hg and Te, respectively, at 3.6 GPa. The structure is substantially different from that of HgS cinnabar, with coordination much closer to fourfold.

Observation of a high-pressure cinnabar phase in CdTe.

Angle-dispersive powder-diffraction techniques with an image-plate area detector and synchrotron radiation have been used to reexamine the high-pressure behavior of cadmium telluride. We find that the well-known structural phase transition at \ensuremath{\sim}3.5 GPa from the zinc-blende to NaCl structure actually involves two closely spaced transitions---zinc blende to cinnabar, and cinnabar to NaCl. This is the example of the cinnabar structure outside the mercury chalcogenides. The Cd and Te atoms are site ordered, and the coordination of the structure is closer to fourfold than is found in HgS cinnabar.

New high-pressure phase of Si.

Angle-dispersive powder-diffraction techniques utilizing an image-plate area detector have been used to reexamine the high-pressure behavior of silicon to \ensuremath{\sim}18 GPa. We have observed a new intermediate body-centered orthorhombic structure between the well-known Si II (\ensuremath{\beta}-tin) and Si V (simple-hexagonal) structures. The existence of such a phase has been considered previously in energy calculations, and may pertain to the observed pressure dependence of the superconducting transition temperature of Si in the 13--18 GPa range.

Structural and electronic properties of InSb under pressure.

We have investigated the structural and electronic properties of the III-V semiconductor InSb under pressure by means of first-principles density-functional total-energy calculation using the all-electron full potential linear augmented plane-wave method. We find that in the high-pressure region, the \ensuremath{\beta}-Sn structure is unstable and a body-centered orthorhombic structure (space group Imm2) is energetically more favorable. Calculated structural parameters (a/b and a/c ratios, and atomic positions) are in good agreement with a recent highly accurate x-ray-diffraction experiment using an image plate area detector. Theoretical lattice constant and bulk modulus for the normal-pressure zincblende structure is also in very good agreement with experiments. We present calculated structural properties as well as band structures and charge densities for the various structures studied. We discuss the processes of the phase transformation, and also the bonding and structural stabilities in terms of the calculated electronic properties.

Phase-transition-induced defect formation in III-V semiconductors.

We present experimental and theoretical evidence for the creation of inversion domain boundaries (IDB's) at structural phase transitions in III-V semiconductors. A novel use of anomalous high-pressure powder x-ray diffraction with an image plate area detector allows for the study of weak difference scattering, the absence of which is attributed to IDB's. As a test of this, ab initio total energy pseudopotential calculations, including both ionic relaxation and boundary layer expansion, have been performed on a particular type of possible defect---a (110) IDB in InSb.

Deviatoric stress in a diamond anvil cell using synchrotron radiation with two diffraction geometries

Deviatoric stress in a diamond anvil cell with gold as a pressure and stress indicator is measured by two complementary techniques using synchrotron radiation. The first method employs a white X-ray beam using energy dispersive X-ray diffraction. The incident X-ray beam is parallel to the load axis and the diffraction pattern is recorded at a low two-theta angle. Using powder diffraction patterns of polycrystalline gold, we measured the elastic strain of two crystal planes oriented normal to the diffraction vector. Stresses nearly parallel and perpendicular to the load axis can be calculated by stress-strain tensor relationship. The other method uses a monochromatic wiggler X-ray beam. In this case, the diamond cell is oriented so that the incident beam is perpendicular to the load axis. The diffraction pattern is recorded on an image plate area detector. Elastic strains responding to stresses perpendicular and parallel to the load axis can be measured and stresses of the same orientations can be calculated from the strain data. These measurements provide a lower bound of the actual differential stresses in a diamond cell. With these techniques, we can measure stress distribution in a less deviatoric gasketted sample and determine yield strength of mantle materials at high pressures and temperatures.

Phase transitions in InSb at pressures up to 5 GPa.

The structural phase transitions in InSb at pressures up to 5 GPa at room temperature have been reexamined by angle-dispersive powder-diffraction techniques on a synchrotron source using an image-plate area detector. Two distinct behaviors have been found: (1) the cubic zinc-blende phase (P1) transforms at \ensuremath{\sim}2.1 GPa to a mixture of a tetragonal phase (P2) and an orthorhombic phase (P3), which then transforms to nearly single-phase P3 before recrystallizing to another orthorhombic phase (P4) at the same or only a slightly higher pressure; or (2) P1 transforms directly to P4 at \ensuremath{\sim}3.0 GPa. P2 is previously unobserved at room temperature. It has a \ensuremath{\beta}-tin structure (but it is not the InSb-II phase) and appears not to be long-range site ordered. P3 is the InSb-II phase, which is now shown to be orthorhombic and site ordered. P4 is the InSb-IV phase, but it is shown to have many previously unobserved superlattice reflections. P4 is also site ordered. The three phases P2, P3, and P4 appear to account for all previous clear results on InSb at room temperature in this pressure range, and it is shown that the long-accepted P-T phase diagram is incorrect.

Image Plate Studies of Tetrahedral Semiconductors Under Pressure: The Implication for Experiment and Theory

A series of high-pressure synchrotron x-ray powder diffraction experiments on tetrahedral semiconductors have been performed using an image plate area detector system in conjunction with a standard diamond anvil cell (DAC). Initial studies have focused on pressure-induced structural transitions in zincblende semiconductors with emphasis on the previously well-studied InSb compound. Site ordering of the nearly isoelectronic constituents is confirmed in some of the high pressure phases for the first time. Structural solutions are proposed which differ in essential features from previously suggested structures. The results obtained are discussed in the context of present theoretical understanding and data from new total energy calculations will be presented. The experimental evidence for phase transition-induced defects will be presented along with complimentary total energy pseudopotential calculations of their formation energies

Initiating a crystallographic study of a class II fructose-1,6-bisphosphate aldolase

We have reproducibly crystallized the metal-dependent Class II fructose-1,6-bisphosphate aldolase from Escherichia coli. Crystals in the shape of truncated hexagonal bipyramids have unit cell dimensions of a = b = 78.4 A ̊ , c = 290.6 A ̊ and are suitable for a detailed structural analysis. The space group has been identified as P6 122 or enantiomorph. Data sets to approximately 2.9 Å resolution have been recorded using both the Rigaku R-AXIS IIc image plate area detector coupled to a copper target rotating anode X-ray source and using the MAR image plate systems with synchrotron radiation at the EMBL outstation DESY in Hamburg, and at S.R.S. Daresbury. Diffraction beyond 2.5 Å has been observed when large freshly grown crystals are used with the synchrotron beam. A data set to this resolution has been collected. Several putative heavy-atom derivative data sets have also been measured using synchrotron radiation facilities and analysis of these data sets is in progress.

Angle-dispersive powder diffraction at high pressure using an image-plate area detector

Abstract An imaging-plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks and to a general method of processing the diffraction data. The advantages of using pressure cells that allow full diffraction patterns to be collected are also described. Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991