Christopher J. Ridley and Konstantin V. Kamenev High pressure neutron and X-ray diffraction at low temperatures Abstract: This paper presents a review of techniques and considerations in the design and construction of high pressure, low temperature diffraction experiments. Also intended as an introductory text to new high pressure users, the crucial aspects of pressure cell design are cov- ered. The general classification of common designs, and a discussion into the key beam interaction, mechanical, and thermal properties of commonly used materials is given. The advantages of different materials and high pressure cell classifications are discussed, and examples of designs developed for low temperature diffraction studies are presented, and compared. Keywords: cryogenic instruments, high pressure, neu- tron diffraction, X-ray diffraction *Corresponding Author: Christopher J. Ridley, School of Engineering and the Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, The King’s Buildings, EH9 3JZ, UK Phone: þ44-1316-517238, e-mail: c.ridley@ed.ac.uk Konstantin V. Kamenev: School of Engineering and the Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, The King’s Buildings, EH9 3JZ, UK 1 Introduction The use of X-rays and neutrons for scattering experiments provides an essential tool for the study of matter. Informa- tion such as lattice spacings, atomic dynamics during ma- terial synthesis, and magnetic ordering can be gained from the sample. Much can be learned from the study of materials in their ambient pressure phase, recent exam- ples including the study of residual stress patterns in cast materials such as magnesium [1], used to improve the method of production, and time resolved studies of com- pounds such as in metal-organic frameworks (MOFs) [2], which are of interest to renewable energy research. Tech- nology making it possible to vary the thermodynamic parameters of the sample environment is becoming in- creasingly diverse, both for experiments at large scale fa- cilities [3] and for use with laboratory-based equipment [4]. This has opened up a new range of possible studies using temperature and pressure together as tuning para- meters. For example, the study of magnetic phases in so- lids frequently uses pressure as a tool to vary the interac- tion between magnetic sites through varying their separation, in an attempt to determine the magnetic order- ing of a system, and compare it to current models [5]. Further examples of the use of high pressures in neutron scattering are given in a review paper by Somenkov [6]. As cryogenic technologies have developed, lower temperatures have become attainable, and the demand for versatile instrumentation for use at temperatures close to absolute zero has increased. Pressure cells are perhaps the most diverse example of this. These devices, used for the application of pressure to the sample, vary in design depending on the desired pressure range, the required temperature stability and the properties which are in- tended to be measured. The first example of coupling pressure control with crystallographic measurements can be found in early X-ray experiments [7, 8], where pressure was generated on the sample using compressed gas. These studies were not able to reach the pressures cur- rently possible (achieving a maximum of 0:5 GPa), pri- marily due to the lack of suitable materials available to contain higher stresses. The size of these pressure instru- ments, and early cryostat restrictions, limited the base sample temperatures achievable, often to the temperature of the coldest substance the sample could be immersed in, typically liquid nitrogen (70 K). More recently, instru- mentation has been developed to allow the generation of high pressures on a larger sample size, making high pres- sure neutron studies possible. X-rays and neutrons scatter through entirely different interactions, meaning that there are few materials that scatter both sources equally (certain Fe–Ni compounds may be an exception to this). This pro- vides a useful contrast between the two sources, depend- ing what sample is studied; for example hydrogen is es- sentially invisible to X-rays, but scatters neutrons strongly. Neutron diffraction provides complementary in- formation describing the bulk properties of materials, due to the high penetrating power of neutrons, and magnetic properties, due to their charge neutrality and intrinsic magnetic moment. X-rays interact with the electronic structure of materials, meaning that low energy (or ‘soft’) DOI 10.1515/zkri-2013-1673  Z. Kristallogr. 2014; 229(3): 171 – 199 Brought to you by | University of Edinburgh Authenticated | 129.215.5.255 Download Date | 6/27/14 6:15 PM