Diffusion of Long Chain n-Alkylammonium Cations in Layer Compounds (n-C_nH_2n+1NH₃)₂MCl₄

Abstract

Evidence for long range, anisotropic diffusion of the long chain n-alkylammonium cations in layer compounds (n-C_nH_2n+1NH₃)₂MCl₄ is presented. Annealing mechanical mixtures of such compounds with M = Mn, Zn; n = 12, 18 results in the formation of mixed crystals of composition (n-C₁₂H₂₅NH₃) _{x 12} (n-C₁₈H₃₇NH₃)_{2-2x 12} MCl₄ in which bilayers of n-alkylammonium groups with differing chain lengths, positionally and conformationally disordered, alternate with crystalline metal halide layers (modification β').

When the ionic layers are polymeric (M = Mn), solid solutions are obtained on cooling from modification β′ in which the polymethylenic chains of the alkylammonium groups, while maintaining positional disorder, are partly cocrystallized in spite of the high difference in chain length (modification α′). Demixing into the pure components, on the other hand, is observed on cooling for nonmacroanionic systems (M = Zn).

Possible structural models of the mixed compounds in modifications α′, β′ are described, and the stability of such modifications relative to the unmixed components evaluated on the basis of a simple thermodynamical treatment.