Abstract
Neutron powder diffraction provides insight into the crystal and magnetic structures of materials. This insight can be achieved on pure materials under ambient conditions, but for greater understanding of the relationship between structure and function, crystallographic information during a process can prove powerful for rational material design strategies. These processes can involve temperature, applied magnetic or electric fields, applied pressure, gas dosing or devices such as electrochemical cells. These experiments are often referred to as a variable process or in situ or operando depending on the conditions applied to the materials or devices. Sample environments to undertake such experiments are accessible at a range of neutron scattering facilities around world. This work provides details on the theory behind diffraction, a snapshot of neutron diffractometers that are suited to such experiments and recent studies exploring the power of neutron powder diffraction in elucidating crystal and magnetic structures under various external conditions.