[en] The integrated intensities measured in X-ray single-crystal high-pressure structural studies using a diamond-anvil cell are shown to be reduced substantially when the diamonds diffract at the same setting as the sample - by as much as 50% in some cases. The pressure and wavelength dependence of this process have been studied and also the effect of changing the beam divergence by the use of a synchrotron beam. The consequences for the accuracy of structural information derived from data sets collected at high pressure are considered and a data-collection strategy for detecting and avoiding the effects of diamond diffraction is proposed. (orig.)

[en] The crystal structure has been determined for NH₄H₂PO₄ and ND₄D₂PO₄ at atmospheric pressure, and for NH₄H₂PO₄ at a hydrostatic pressure of 8.9 kbar. In each case the temperature was set to just above the antiferroelectric-to-paraelectric phase transition. The relationship between the transition temperature and the O-H-O and O-D-O bond dimensions is discussed in comparison to the isomorphous but ferroelectrically ordering KH₂PO₄ and KD₂PO₄. (orig.)

[en] The sample volume in diamond-anvil pressure cells suitable for X-ray powder diffraction studies is very small (≤ 100 μm across). The resulting low signal-to-noise ratio has made it very difficult to obtain useful results with monochromatic angle-dispersive techniques, and the alternative white-beam energy-dispersive techniques have limited resolution and generally give unreliable peak intensities. The situation has been transformed recently by the introduction of the image-plate two-dimensional detector, which allows angle-dispersive methods to be used with a greatly increased signal and improved powder averaging. A short review is given of this development, the experimental techniques, and the principal advantages, particularly as found in results obtained at SRS Daresbury over the past two or three years. (au) (18 refs.)

[en] The phases of cerium above 5GPa have been studied using angle-dispersive powder diffraction techniques. The phase obtained between 5 and 12GPa at room temperature with filings of Ce has a monoclinic, distorted-fcc structure with four atoms in a C-face-centered unit cell. Heating to 373K for many hours yields the known α-U phase. With a single cut piece as a sample, the α-U phase is obtained at room temperature. copyright 1997 The American Physical Society

[en] Short communication

[en] It has long been known that the substitution of deuterium for hydrogen in hydrogen-bonded materials can lead to a change in the geometry of the hydrogen bonds - the so-called Ubbelohde effect. There can also be significant accompanying changes in the physical properties of the material. Among the most striking examples is the increase in the transition temperature, T_c, of hydrogen-ordering systems of the KH₂PO₄ type by >100 K on full deuteration. In all of these systems, T_c decreases under pressure, making it possible in principle to compare protonated and deuterated forms at the same T_c by applying pressure to the latter. Here we report the results of a high-pressure neutron diffraction study of the deuterated H(D)-ordering material PbDPO₄. We find that much, and possibly all, of the increase in T_c on deuteration is attributable to the accompanying changes in the hydrogen-bond dimensions. This suggests that purely mass-dependent effects on the quantum-mechanical tunnelling between the equivalent H(D) potential minima - an explanation that has been favoured previously -have little or no direct influence on T_c. Substantial revisions to the theory of this type of ordering transition are apparently required. (author)

[en] High-resolution neutron-diffraction techniques have been used to determine the crystal structure of deuterated squaric acid, D₂C₄O₄, in its ordered phase at 298 K. Comparison with H₂C₄O₄ at the same temperature reveals the effects of deuteration alone on the crystal structure. The main changes are seen in the geometry of the O-H(D)...O bonds; the overall O...O bondlength increases by ≅ 0.020 A on deuteration, and the O-D bond in D₂C₄O₄ is ≅ 0.015 A shorter than O-H in H₂C₄O₄. These changes probably account for much of the increase in the H(D)-ordering temperature, Tc, in this system, from 371 K in H₂C₄O₄ to 516 K in D₂C₄O₄. (orig.)

[en] 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. Initial results from studies of III--V semiconductors and La₂CuO₄ are given

[en] Unlike their trivalent lanthanide neighbours, europium (Eu) and ytterbium (Yb) are both divalent at ambient conditions, and both exhibit a significantly larger atomic volume than would be expected to be consistent with the general trend observed within the lanthanide series. Under pressure, the valence of Eu increases, and reaches +2.5 at a pressure of 12.1 GPa. Previous X-ray diffraction studies of Eu at high pressure are limited but have found that it exhibits a body-centred cubic (bcc) structure at ambient pressure, which transforms to a hexagonal close-packed (hcp) structure at 12.5 GPa and then to a new phase (Eu-III) at 18 GPa, which was initially and tentatively assigned a hexagonal structure closely related to hcp. Despite this interesting behaviour, very little further attention has been paid to the high-pressure behaviour of Eu until this year, when a study by Bi et al. reported results from an X-ray diffraction study up to 92 GPa. They attributed the complex diffraction patterns observed above 18 GPa to arise from a mixture of hcp and monoclinic phases, followed by a transition to a mixture of monoclinic and orthorhombic phases at 40 GPa. Single-phase patterns from the orthorhombic phase were reported to have been observed at 66 GPa. These diffraction results were supported by ab initio calculations. However, trial diffraction data collected by us at the SRS synchrotron contains several diffraction features not explained by these structures. We have therefore made a more detailed diffraction and computational study of Eu with the aim of determining the structural behaviour to 70 GPa

[en] The evolution of PbTe with pressure has been reexamined using synchrotron x-ray diffraction. The phase transition at 6 GPa is not to the GeS (B16) or TlI (B33) type structures, as previously reported, but to an orthorhombic Pnma structure, with cell parameters a=8.157(1), b=4.492(1), and c=6.294(1)A at 6.7 GPa. This structure corresponds to a distortion of the low-pressure NaCl structure with a coordination intermediate between the sixfold B1 (NaCl) and the eightfold B2 (CsCl) structure. We discuss the stability of this new structure with respect to other proposed phases using numerical methods. These results may modify the admitted paths of phase transitions between the B1 and B2 structures