Self-diffusion of methylammonium cations in the high-temperature solid phase of CH3NH3NO3

Abstract

The high-temperature solid phase of CH3NH3NO3, which is stable above 352 K, has been studied by differential thermal analysis, differential scanning calorimetry, X-ray powder diffraction, 1H nuclear magnetic resonance spectroscopy and electrical-conductivity experiments. X-ray powder patterns taken at 370 K reveal that CH3NH3NO3 forms a CsCl-type b.c.c. crystal with a= 4.91 A. From 1H n.m.r. measurements it is concluded that methylammonium cations undergo overall reorientation and self-diffusion in the crystal. The activation energy for the self-diffusion is evaluated as 29 kJ mol–1. However, electrical-conductivity experiments yield an activation energy of 45 kJ mol–1 for the same motional process. The diffusion constants are evaluated from both n.m.r. and electrical-conductivity measurements. The mechanism of the self-diffusion in the solid phase of CH3NH3NO3 is discussed. Comparisons are made of the dynamical properties of the cation in this phase with those of globular molecules in the plastic phase of various molecular crystals.