A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P 4 /nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T = 50 K) and tetragonal (T = 190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a bulk modulus ≈ 31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the c-axis. Key bond angles of the SeFe 4 pyramids and FeSe 4 tetrahedra are also determined as a function of pressure.

A facility is described which has been developed for the rapid acquisition of structural information through the use of heterochromatic synchrotron radiation from a sample pressurized in a diamond-anvil cell and simultaneously cooled to cryogenic temperature. The system employs a closed-cycle He refrigerator, which can be continuously operated, independent of any liquid cryogens, from a remote station. The compressive contact force between the diamonds, and hence the sample pressure, is also externally controlled, thereby providing remote control capabilities for both the pressure and temperature. NaCl has been used as an internal pressure calibrant and existing empirical equation-of-state calculations for NaCl have been extended to reduced temperatures for this purpose. Preliminary data on the lower pressure critical point associated with the isomorphic phase transition in Ce0.8Th0.1La0.1 are presented.

Electrochemical cell for neutron reflectometry studies of the structure of ionic liquids at electrified interface Rev. Sci. Instrum. 81, 074101 (2010) Surface force confinement cell for neutron reflectometry studies of complex fluids under nanoconfinement Rev. Sci. Instrum. 79, 103908 (2008) Millisecond time resolution neutron reflection from a nematic liquid crystal Rev. Sci. Instrum. 75, 2955(2004 Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. I. Nature of the noncritical adsorption profiles J. Chem. Phys. 119, 11917 (2003) Local membrane ordering of sponge phases at a solid-solution interface J. Chem. Phys. 116, 8533 (2002) Additional information on J. Appl. Phys. Specular neutron reflectometry has become an established probe of the nanometer scale structure of materials in thin film and multilayered form. It has contributed especially to our understanding of soft condensed matter of interest in polymer science, organic chemistry, and biology and of magnetic hard condensed matter systems. In this paper we examine a number of key factors which have emerged that can limit the sensitivity of neutron reflection as such a probe. Among these is loss of phase information, and we discuss how knowledge about material surrounding a film of interest can be applied to help resolve the problem. In this context we also consider what role the quantum phenomenon of interaction-free measurement might play in enhancing the statistical efficiency for obtaining reflectivity or transmission data.

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