High-pressure cells for low-temperature neutron scattering at ISIS

We propose to combine high-pressure, low-temperature and neutron diffraction techniques for studies of materials, enabling ISIS to achieve new extremes of high-pressure and low-temperature. The ability to study materials at elevated pressures allows researchers to modify many of their properties and to recreate the conditions existing in deep earth or on other planets. Very low temperatures are needed to study magnetic and other quantum phenomena in solids. Neutron scattering is the ideal tool for magnetic and many structural ordering phenomena. The proposed research is focused on combining these three components by developing high-pressure cells suitable for neutron diffraction studies working at temperatures down to a few tenths of a degree above absolute zero while achieving pressures of 10 GPa or more. The design of the pressure cells will be based on the opposed-anvil principle, in which the studied sample is squeezed between two hard anvils while supported by a metal gasket on its sides. It is important for neutron scattering to have as much sample as possible in the neutron beam and so large anvils will be needed for these pressure cells. The hardest known material is diamond, but large diamonds are forbiddingly expensive and may not be available. We therefore propose to use in the initial stages relatively inexpensive large silicon carbide (moissanite) anvils. Silicon carbide has a Mohs hardness of 9.25 (diamond has 10 on the same scale) and moissanite pressure cells are reported to achieve pressures of up to 60 GPa (approximately 600,000 atm). To accomplish this, it is important that the shape of the anvils is optimised in order to achieve maximum possible pressures without breaking and that their alignment with respect to each other and the pressure cell is perfect. Therefore, part of the proposed research will be focused on using the powerful technique of finite element analysis (FEA) for optimising the shape of the anvils. Using FEA will not only help to speed up the project and find the right shape for the anvils but will minimise anvil breakages in trial experiments. FEA, combined with computer aided design (CAD), will also help to design the pressure cells themselves and make sure that they will fit into the cryogenic equipment and are aligned with the detectors. Another important feature of the proposed pressure cells is the ability of the operator to change the pressure while the cell is inside the cryostat and measure the pressure inside the cell at the same time via a fibre optic cable. This will be very useful for changing pressure at constant low temperature, which is not possible with present technologies. This saves time in not having to warm the cell up to room temperature to change pressure and, then having to realign the sample inside the cell at each new pressure. The cells will initially be used to study ordering transitions in model systems such as multiferroic materials, manganese oxides exhibiting colossal magnetoresistance and magnetic rare earth oxosalts with complex spin structures arising from frustration.