Techniques that probe the average structure, such as X-ray diffraction, can not distinguish between these two types of behaviour. Due to the mechanical softness, interesting dynamics can be observed already at low temperatures, with possible consequences for the material properties and potential applications. Examples include the ferroelectric ordering in some organic-inorganic perovskites, which is lost at higher temperatures due to the thermal motion of the atoms, and the negative thermal expansion observed for a number of MOFs. In experimentally determined crystal structures the structural disorder is evidenced by the existence of partial occupancy and large thermal displacement parameters. As an example the biphenyl-4,4'-dicarboxylate (BPDC, see Figure ) organic linker in crystal structures of the metal-organic framework UiO-67 4 can be represented in a planar conformation with a dihedral angle (↵ in Fig. ) of 0 between the two phenyl rings and large anisotropic thermal ellipsoids, or in a nonplanar conformation with two partially occupied sites with ↵ 6 = 0.