No abstract

We report on the synthesis of superconducting single crystals of FeSe and their characterization by x-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that T(C) increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure x-ray diffraction study up to 12 GPa on a powder sample, and we find that, between 8.5 and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16% and is accompanied by the appearance of an intermediate, probably orthorhombic, phase.

We present a systematic pressure study of poly-and single crystalline SrFe2As2 by electrical resistivity and X-ray diffraction measurements. SrFe2As2 exhibits a structural phase transition from a tetragonal to an orthorhombic phase at T0 = 205 K. The structural phase transition is intimately linked to a spin-density-wave transition taking place at the same temperature. Our pressure experiments show that T0 shifts to lower temperatures with increasing pressure. We can estimate a critical pressure of 4 to 5 GPa for the suppression of T0 to zero temperature. At pressures above 2.5 GPa the resistivity decreases significantly below Tx ≈ 40 K hinting at the emergence of superconductivity but no zero-resistance state is observed up to 3 GPa.

High-energy X-ray focusing with bent-crystal monochromators is known to be hampered by so-called depth or crystal-thickness aberrations. A theoretical model of focus broadening based on the geometrical theory of X-ray diffraction in slightly deformed crystals is presented and compared with experimental data. First, it is shown that depth broadening can be avoided in the Laue geometry by an appropriate choice of asymmetry angle. Based on this finding, a monochromator for high-pressure diffraction experiments has been designed and a source-size-limited focal spot below 10 microns is observed. As a consequence of the box-shaped rocking curve of bent Laue crystals, the focus is free of long-ranging tails. Diffraction patterns of standard powder samples were recorded on imaging plates and a theoretical description of the energy-dispersion-related peak broadening is given. Finally, diffraction patterns of N2 at 180 kbar demonstrate the excellent data quality achievable with this monochromator.

Room-temperature high-pressure behaviour of SrWO 4 scheelite (I4 1 /a, Z = 4) has been studied to 20.7 GPa in a diamond anvil cell using synchrotron angle-dispersive X-ray powder diffraction. Above 10 GPa, it transforms to the fergusonite structure (I2/a, Z = 4). Both scheelite and fergusonite types are ordered superstructures of fluorite (Fm3m, Z = 4). There is no significant volume collapse at the scheelitefergusonite phase transition. However, the compression data including both phases of strontium tungstate cannot be fitted by a common Birch -Murnaghan equation of state. An onset of decomposition into component oxides occurs at about 15 GPa. The pressure-induced transformations are irreversible.