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

摘要

Quasielastic and inelastic incoherent neutron scattering (QENS and IINS) and the analysis of the second moment (M-2) of proton magnetic resonance (H-1 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 degrees 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: T-C1 approximate to 271 K and T-C2 approximate to 164 K. However, it undergoes a very small change of the activation energy E-a (120 degrees) value for such a motion at T-C2. The estimated E-a value equals 2.5 kJ mol(-1) for high and intermediate temperature phases and 1.7 kJ mol(-1) for the low temperature phase. H-1 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 M-2 (reorientational correlation time is less than reciprocal of the line width). The estimated E-a(120 degrees) approximate to 3 kJ mol(-1) is in quite good agreement with that calculated from the QENS measurements. Above the phase transition at T-C2, 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 H-1 NMR line width). (C) 2007 Elsevier B.V. All rights reserved.