Neutron Scattering

fenoelectric phase of orthorhombic symmetiy (space group : Fdd2) in which the protons order
in short asymmetric O-H . ..0 bonds [3] . The contour plots of the proton distribution at
différent temperatures are shown in Fig . 6.
(a) TC + 2K (b) Te - 1 .3K
,~r
(c)
Tç - 10K
Fig . 6 . Contour plots ofthé refined proton distributions in KH2P04 at :
(a)Tc +2K,(b)Tc -1.3K,(c)T c -10K,(d)Tc -2OK.
12.51VIolecular disorder
Disordered structures and pseudosymmetries related to dynamical reorientation and/or
structural phase transitions are of great current interest . In principal, the dynamical disorder of
molecules is due to the fact that the intermolecular bonds are very much stronger than the
external ones between the molecular groups and the surrounding crystalline frame . It is
obvious that the chemical bonding scheme predicts the symmetry of a crystal structure, and
not the other way around . We can state, however, that in the case of an incompatible pointgroup
symmetiy of a molecule with respect to its site symmetiy in the crystal structure,
molecular disorder is the necessary consequence. In order to modellize the atomic density
distributions conectly in a way to obtain physically meaningful potentials, very accurate
Bragg intensities over a large sinO / ;, range are required. X-ray experiments are generally more
restricted than neutron studies because of the sinO /;, dependence of the atomic scattering
factor f .
12.5 .1 Example of molecular disorder :
Almost free rotation of NH 3 groups in the crystal structure ofNi(NH3)612
As an example, related to the H/D problem, the dynamical disorder of the NH3 group in the
cubic high temperature phase of the metal hexamine halide Ni(NH3)6I2 (space group : Fm3m)
is presented . Tbc corresponding crystal structure is shown in Fig . 7 . With the NH3 tetrahedra
12- 1 3