Molecular motions of decylammonium chains in the perovskite type layered compound (C₁₀H₂₁NH₃)₂MnCl₄: An incoherent neutron scattering study

Incoherent neutron scattering (INS) experiments have been performed on various polycrystalline derivatives of decylammonium manganese tetrachloride. This compound displays a solid state phase transition, often referenced as a ‘chain melting process’, quite similar to that observed in lipid membranes. The quasi-elastic profiles obtained on high and medium resolution spectrometers (1-600 ps time scale) were analysed in the 240-370 K range, in particular at near, below and above the phase transition temperature (308 K). In the high temperature phase, the results clearly indicate that there is a gradient in the dynamical disorder of the -CH₂- units in going from the NH3-polar heads anchored to the inorganic matrix to the methyl end groups. Because of the crystalline modifications, the chains can adopt a stabilized ' kink' defect (t₄gtg't) located near the methyl end and perform fast motions related to both conformational interconversions and cooperative torsions running along the main molecular axis. We then discuss the molecular motions involved in the 'chain melting process' on the basis of the best fit parameters from a model of diffusion within a volume of anisotropic shape. These results are compared with those obtained by Raman spectroscopy on the related cadmium derivatives and by molecular dynamics simulations in lipid membranes.