A basic introduction to thermal motions of atoms in crystal structures, the underlying potentials and the physical information available from their analysis†

A teaching review is presented, introducing the basic principles of thermal motions of atoms in crystals: their definition as average displacements from equilibrium positions; their representation as ellipsoids; and the way in which their analysis can yield physical information. Thermal motion is placed in the context of ‘disorder’ in crystal structures and its effect on diffraction patterns. The treatment is qualitative and illustrative, rather than rigorous. †Delivered as ‘Beans, Sausages and Pancakes, a Recipe for Understanding Thermal Motion in Crystals’, a Plenary Teaching lecture at the BCA Annual Spring Meeting, Canterbury, 2007. Dedicated to the Memory of Professor Durward W. J. Cruickshank FRS, whose seminal contributions to the treatment of thermal motions of atoms in crystal structures began more than 50 years ago [see, for example, (1, 2)] and still influences our treatment of the subject today.