Zakład Fizyki Wysokich Ciśnień - publikacje

Abstract: 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). © 2007 Elsevier B.V. All rights reserved.