Phase transitions and NH3 motions in [Zn(NH3)4](ClO4)2 studied by incoherent neutron scattering and 1H NMR methods

Quasielastic and inelastic incoherent neutron scattering (QENS and IINS) and the analysis of the second moment (M2) of proton magnetic resonance (1H NMR) line for [Zn(NH3)4](ClO4)2 crystals in the function of temperature are reported. The QENS peak registered at 75 K and also at higher temperatures shows distinct broadening, which is typical for dynamically, orientationally disordered crystals (ODIC). Reorientational motion of NH3 ligands can be well described by a model of 120° proton jumps around a 3-fold axis of Zn-N bonds on a picoseconds time scale. The NH3 do not drastically change either the jump rate or the character of their reorientational motion at the phase transitions at: TC1 ≈ 271 K and TC2 ≈ 164 K. However, it undergoes a very small change of the activation energy Ea (120°) value for such a motion at TC2. The estimated Ea value equals 2.5 kJ mol−1 for high and intermediate temperature phases and 1.7 kJ mol−1 for the low temperature phase. 1H NMR studies revealed that NH3 ligands perform reorientation around the 3-fold symmetry axis close to 90 K when the second moment of the H NMR line exhibits the plateau of M2 (reorientational correlation time is less than reciprocal of the line width). The estimated Ea(120°) ≈ 3 kJ mol−1 is in quite good agreement with that calculated from the QENS measurements. Above the phase transition at TC2, the anisotropic reorientation of the whole [Zn(NH3)4]2+ cation around the 3-fold symmetry axis is set in motion with a frequency in the order of several kHz (the 1H NMR line width).