In crystallographic studies sampling is over the instantaneous configurations in time and space provided that the interaction time of the radiation is short compared to the atomic motion and provided that the coherence length of the radiation is shorter than the size of the coherently scattering sample. However, the atomic configurations in one specific unit cell do not obey the symmetry governing the time and space average. It is difficult to deduce from the time- and space averaged picture the local atomic arrangements. Unravelling atomic disorder very often requires independent information from techniques which can probe local arrangements. Certainly the most useful crystallographic experiment on ice-Ih is performed with neutron radiation.
Neutrons interact strongly with hydrogen, deuterium and oxygen atoms.
The scattering power in the neutron case is a constant as a function of the scattering angle and similar for all three elements, whereas in the X-ray case it is proportional to the number of electrons and decreases with increasing scattering angle. X-ray experiments are less informative and provide useful data mainly on the lattice constants and the distribution of electrons in the vicimity of the oxygen atoms and between oxygen and hydrgen atoms.