Sample environment equipment, cryostats, furnaces, pressure cells etc. are an essential adjunct to most neutron scattering experiments to induce the sample being studied into a phase or state of particular interest. He we give a brief overview of the diverse range of Sample Environment equipment that is currently in use, a background to its development and also some recent trends.

A review of sample environments in neutron scattering

It is of vital importance for the study of phase diagrams to investigate in situ diffusionless phase transition points at high temperatures. We have developed high-temperature techniques to measure in situ neutron and synchrotron X-ray powder diffractometry profiles. This paper reviews our recent studies obtained using these methods. The new furnace for high-temperature neutron diffraction study yields no extra peaks caused by heaters and refractory; thus, we have been able to measure very small neutron signals from the sample. Quality neutron diffraction data at high temperatures up to ∼1590°C have been analyzed using the Rietveld method for various materials, such as zirconia solid solutions and calcium titanate. High-resolution synchrotron X-ray diffractometry enables exact determination of the phase transition temperatures of perovskite-related materials.

In Situ observations of phase transition using high-temperature neutron and synchrotron X-ray powder diffractometry

Extensive neutron diffraction and Rietveld studies of dense, hot pressed mullite (3Al 2 O 3 ·2SiO 2 ) have been conducted up to 1650 °C in air, yielding a complete set of lattice parameters and axial thermal expansion coefficients. Unconstrained powders of the same stoichiometric composition were also analyzed by X-ray diffraction and Rietveld techniques up to 900 °C in air, from which lattice parameters and thermal expansion coefficients were obtained. An earlier reported structural discontinuity was confirmed by XRD to lie in the temperature range 425 to 450 °C. Single-crystalline mullite fibers of composition 2·5Al 2 O 3 ·SiO 2 were grown from the melt by a laser-heated, float zone method. A partial set of the single-crystal elastic moduli were determined from various sections of fiber, by Brillouin spectroscopy, from room temperature up to 1400 °C. They indicated a roughly 10% reduction in stiffness over that temperature range.

High temperature single crystal properties of mullite

Structural thermal expansions of undoped and Cr-doped 3/2 mullites (10.8 wt.% Cr2O3) were measured with in situ neutron (25–1600°C) and X-ray synchrotron (25–1000°C) diffraction techniques. Heat treatments between 25 and 300°C cause little and non-linear structural expansions. Above 300°C and up to 1000°C the mullites display linear and low increase of the lattice constants. The mean values of undoped mullite are slightly higher ( α av ≈ 5.45×10−6) than those of Cr-doped mullite ( α av ≈ 4.95×10−6), thus confirming earlier in situ high-temperature diffraction data. Above about 1000°C the expansion behaves discontinuously. Structural expansions of undoped and Cr-doped mullite strongly increase yielding similar mean expansion coefficients ( α av ≈ 7.50×10−6) for both materials. The highest increase of lattice constants has been determined in b axis direction throughout 1600°C in undoped and Cr-doped mullite. High temperature diffraction studies under heating-up and cooling-down conditions yielded reversible expansion effects, including the anomalous behaviour at about 1000°C.

High temperature thermal expansion of mullite: an in situ neutron diffraction study up to 1600°C

X-ray powder diffraction was performed on f3-Ag2Te at room temperature, on ion conducting a-Ag2Te at 523 K, 723 K and 923 K respec- tively and on y-Ag2Te at 1123 K. Rietveld profile refinement of f3-Ag2Te confirmed the space group P2dc and the positional parameters of the model of Frueh (Z. Kristallogr. 112 (1959) 44-52). For the a-modification in space group Fm5m, best results were obtained for a model with a statistical distribution of Ag ions: about 60% of the available Ag ions were placed into 32f(xxx) positions around the tetra- hedral 8c(%%%) sites and the remainder on the 48i(xx}'2) positions. The probability density function (PDF) calculated with these formal structure parameters suggests next neighbour jumps of Ag ions via the 48i sites. The first structure refinement of y-Ag2Te showed it to crystalize in space group Im5m and to be iso-structural to a-AgI. Third order anharmonic coefficients of the temperature factor of the Ag ions could be refined from our X-ray powder data. Corresponding PDF maps show Ag-densities bridging next nearest as well as second next nearest neighbours.

X-ray powder diffraction of Ag2Te at temperatures up to 1123 K

This paper describes a mirror furnace that has been developed for neutron diffraction work at temperatures up to 2300 K. It is based on a reflecting rotational ellipsoid, in which the heating element, a halogen lamp, is placed at one focus and the sample at the other. It works in a normal, i.e. oxidizing, atmosphere, but can also be used in a vacuum. It has been developed for experiments of long duration. The stability and reproducibility of the temperature are better than 1\% of the setting temperature. Further main characteristics are applicability to single-crystal and powder work, very low background, low power consumption and very easy and cheap handling. Several experiments have been carried out with the furnace.

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G. Lorenz

Papers

A mirror furnace for neutron diffraction up to 2300 K

Structural investigations up to 1300 K and ionic conductivity in Ca-stabilized zirconia