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 Å. 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.