The refinement of electron-density distributions for noncentrosymmetric crystals from X-ray diffraction data may lead to a very good fit between model and data but to totally meaningless electron densities. This is to a large extent because varying certain parameters, or combination of parameters, in the model mainly leads to a change in the phases of the structure factors. A formal analysis of why this happens, when using multipole models, is given as well as specific examples using real data: the contributions of odd-order multipoles, which are invariant under crystal-class symmetry operations, are poorly determined. The importance of applying constraints on the models is stressed. The conclusions of this analysis can be carried over to refinements of anharmonic atomic vibrations.

No abstract

Dc conductivity of the whole series of homoionic alkali exchanged montmorillonites is investigated by means of Complex Impedance Spectroscopy. Conductivity of the samples is measured at the dry state and at various water loadings between 0 and 6 absorbed water molecules per cation. Dc conductivity of all the dehydrated samples follows an Arrhenius behavior. In contrast, the hydrated samples exhibit a non-Arrhenius temperature dependence of dc conductivity that is fruitfully fitted by using the VTF's empirical law. It is then shown that the critical temperature, T VTF , increases with water loading until the later equals approximately 3 for the Li + and Na + samples and higher values for the K + , Rb + , and Cs + samples. So, it appears that the departure from the Arrhenius behavior is directly related to the number of water molecules in interaction with the alkali extra-framework cations. For water loadings higher than 3, activation energy for dc conductivity tends to values of about 0.2-0.3 eV for all samples independently on the alkali extra-framework cations. In contrast, activation energy appears to be very sensitive to the considered alkali sample for water loadings lower than 1 and, in that case, closely related to the dehydration enthalpy of the samples.