International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.7, p. 166

Section 2.7.13. High-pressure neutron diffraction

A. Katrusiaka*

aFaculty of Chemistry, Adam Mickiewicz University, Poznań, Poland
Correspondence e-mail: katran@amu.edu.pl

2.7.13. High-pressure neutron diffraction

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Neutron scattering is an indispensable and complementary technique in materials research (see Chapter 2.3[link] ), particularly for compounds containing heavy elements that strongly absorb X-rays, or light-atom weak X-ray scatterers (e.g. see Goncharenko & Loubeyre, 2005[link]). However, the flux of neutron sources, both reactors and spallation targets, is several orders of magnitude lower than that of X-rays, even from traditional sealed X-ray tubes. Moreover, the scattering cross sections of neutrons are on average two orders of magnitude smaller than for X-rays (Bacon, 1975[link]). These two considerations conflict with the requirement of small sample volume preferred for high-pressure devices. Consequently, a prohibitively long measurement time would be required to obtain meaningful neutron diffraction data from the DAC in its original form and size-optimized for X-ray studies. Therefore, initially, the designs of high-pressure devices for neutron scattering studies were based on typical large-volume presses: gas bombs with external multi-stage pressure generators, and piston-and-cylinder, multi-anvil and belt presses (Worlton & Decker, 1968[link]; Bloch et al., 1976[link]; McWhan et al., 1974[link]; Srinivasa et al., 1977[link]; Besson, 1997[link]; Klotz, 2012[link]). The sample volume in the Bridgman-type opposed-anvil press, with flat anvils separated by a gasket of pipestone, was increased severalfold by making a recess at the centre of the pressure chamber of the so-called Chechevitsa anvils (Stishov & Popova, 1961a[link],b[link]). The sample volume was further increased in toroid anvils by grooves supporting the gasket around the central recess (Khvostantsev et al., 1977[link]). This made them ideal for powder diffraction neutron measurements on samples of about 100 mm3 to above 10 GPa in a Paris–Edinburgh hydraulic press (Besson et al., 1992[link]; Besson, 1997[link]). The application of sintered diamond anvils increased this pressure range. High-pressure cells in a form optimized for neutron diffraction can contain between several cubic millimetres and a few cubic centimetres of sample volume. Such a large sample volume naturally limits the pressure range of cells used for neutron diffraction, compared with the DAC used for X-rays. However, the pressure range has increased considerably for neutron diffraction experiments during recent decades, to over 20 GPa in a moissanite anvil cell (Xu et al., 2004[link]; Dinga et al., 2005[link]), and to 40 GPa in a high-pressure cell capable of operating in helium cryostats at 0.1 K and in magnetic fields up to 7.5 T (Goncharenko, 2006[link]; Goncharenko et al., 1995[link]). High-pressure high-temperature cells for neutron diffraction are usually equipped with internal heaters capable of exceeding 1500 K (Zhao et al., 1999[link], 2000[link]; Le Godec et al., 2001[link], 2002[link]).

It is particularly advantageous for the construction of large presses for neutron studies that most of the materials used have very low absorption of neutrons. There are also metals (vanadium, aluminium) with very low scattering lengths, and it is possible to obtain alloys (Ti66Zr34) with the scattering length scaled to zero. This allows access of the neutron beam to the sample and exit of reflections. In a Paris–Edinburgh cell operating in the time-of-flight mode, the incident beam enters the pressure chamber through the tungsten carbide anvil, along its axis, and the reflections leave the chamber through the gasket along the slit between the anvils with approximately ±6° opening (Besson et al., 1992[link]; Takahashi et al., 1996[link]). In this operation mode, highly neutron-absorbing anvils made of sintered cubic boron nitride (cBN) can also be used (Klotz, 2012[link]). Alternatively, the monochromatic angle-dispersive mode of operation, with the incident and diffracted beams passing through the slit between the anvils, is possible but it is less efficient with regard to the use of the full spectrum of neutrons. The application of a focused neutron beam and the time-of-flight technique allow the use of small sample volumes of a fraction of a cubic millimetre in compact opposed-anvil high-pressure cells (Okuchi et al., 2012[link]). Two DACs were recently optimized for neutron diffraction on single crystals. Owing to the application of a white neutron beam, the structure of a crystal 0.005 mm3 in volume was determined. All reflections could be recorded because of the smaller Merrill & Bassett (1974[link]) design made of neutron-transparent beryllium–copper alloy (Binns et al., 2016[link]). Another design with a wide access to the sample for the primary and diffracted beams has been successfully used at a hot-neutron source (Grzechnik et al., 2018[link]).

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