Development of high pressure and cryogenic techniques, and their application to neutron diffraction
Ridley, Christopher James Taylor
Neutron diffraction is an extremely powerful technique in condensed matter research; it can be used to measure crystallographic structures, including some of those undeterminable using X-rays. It is also perhaps the most powerful technique for determining magnetic structures, and for probing the strength of magnetic interactions, revealing information beyond that extractable from a magnetometer. High pressure is used by many condensed matter researchers as an additional thermodynamic variable, or tool to perturb otherwise stable systems, and has been used with neutron diffraction for many years. When coupled with low temperatures, this has led to the discovery of an enormous range of non-ambient phases of matter, with a range of exotic properties, some of which are discussed in this thesis. Pressure has a very strong effect on the magnetic properties of a material, with many of the most unusual magnetic phases existing only at extremely low temperatures, or pressures which can only be reached on very small samples. The main topic for this thesis is the study, development, and implementation of new techniques to combine low temperatures, high pressures, and neutron diffraction measurements from micro sized samples. A new pressure cell has been designed, tested, and commissioned with neutron beam time on the WISH diffractometer at the ISIS neutron facility. The cell is compact, with a total mass of approximately 5 kg, and is capable of generating large loads in excess of 4.5 tonnes force. Depending on the sample size used with the cell, the opposed anvil system is capable of generating a range of different pressures beyond what is widely available for low temperature neutron diffraction measurements. To save wasted experimental time in cooling and warming the device, the cell is capable of varying the applied load continuously down to 5 K, whilst the sample pressure can also be measured in-situ using a compact spectrometer system. Obtaining refineable neutron diffraction data from the small samples (< 1mm3) possible in an opposed anvil pressure cell is challenging due to extremely low ratios of signal-to-background when compared with large volume pressure cells. Finite element analysis (FEA) was performed to minimise the mass of the cell, whilst also minimising the amount of supporting material in the beam. Despite this, the signal from the sample is typically very weak; to overcome this, a novel 3D printed device has been designed and tested to collimate extremely small samples, removing much of the background signal from the surrounding material. It has enabled neutron data to be collected from samples an order of magnitude smaller than previously measurable in the cell. To maximise the pressures achievable in the pressure cell, for a given sample volume, an extended FEA study was performed to understand the evolutions of stresses in the cell, and understand the limitations of using sapphire as an anvil material. To complement this work, a compact piston cylinder cell has also been designed for a combination of different measurements. One of the key challenges in high pressure research is in knowing, or ensuring, that the conditions the sample is under are approximately the same for a variety of different measurements. Since different instruments, and techniques, may not allow for the same apparatus to be used between them, this is not always possible. A compact clamped piston cylinder cell has been designed, suitable for in-situ electrical measurements, with additional potential for simultaneous neutron diffraction measurements. The device is demonstrated through an ultrasonic characterisation of the compound UGe2. In addition to the information obtainable from neutron diffraction, much can be learnt from studying the transport properties of a material. This information can be used alongside neutron data to provide a full understanding of how a material behaves. One technique of interest measures how the electrical properties of a material changes under applied magnetic field. This is difficult to achieve under pressure due to the often anisotropic construction of the pressure cell affecting the magnetic field on the sample in different orientations, and the challenge in getting wires to the sample under pressure. This thesis presents the design, and preliminary testing, of an ultra compact high symmetry piston cylinder cell designed to be taken to sub-Kelvin temperatures and rotationally oriented in applied magnetic field. The spherical construction of the cell means that the field on the sample position is, to a very close approximation, identical in all orientations. Finally, this thesis presents a study of the binary alloy Pd3Fe under pressure. Pd3Fe was recently reported to undergo a large-volume collapse under high pressure at room temperature, resulting in near zero thermal expansion]. There are several competing theories on the mechanism behind this process. To investigate further, a series of single crystal Pd3Fe samples were grown, cut, prepared, and extensively analysed. The results of this study suggest that the cause for the large volume collapse may not be magnetic in nature, as previously expected.