14N and 35Cl double resonance study of the low-temperature phase transition in CH3NH3ClO4

A ¹⁴N and ³⁵Cl double resonance study of CH₃NH₃ClO₄ shows that, contrary to the high-temperature phase transition being due to a change of molecular motion of the cations, the low-temperature phase transition is triggered by a change of molecular dynamics of the anions. This shows the importance of measuring the chlorine quadrupole frequency of perchlorate ions in the solid state which has not been detected so far.